The inverse-trans-influence in tetravalent lanthanide and actinide bis(carbene) complexes

Abstract

Across the periodic table the trans-influence operates, whereby tightly bonded ligands selectively lengthen mutually trans metal-ligand bonds. Conversely, in high oxidation state actinide complexes the inverse-trans-influence operates, where normally cis strongly donating ligands instead reside trans and actually reinforce each other. However, because the inverse-trans-influence is restricted to high-valent actinyls and a few uranium(V/VI) complexes, it has had limited scope in an area with few unifying rules. Here we report tetravalent cerium, uranium and thorium bis(carbene) complexes with trans C=M=C cores where experimental and theoretical data suggest the presence of an inverse-trans-influence. Studies of hypothetical praseodymium(IV) and terbium(IV) analogues suggest the inverse-trans-influence may extend to these ions but it also diminishes significantly as the 4f orbitals are populated. This work suggests that the inverse-trans-influence may occur beyond high oxidation state 5f metals and hence could encompass mid-range oxidation state actinides and lanthanides. Thus, the inverse-trans-influence might be a more general f-block principle.

ARTICLE

Received 16 Nov 2016 | Accepted 2 Dec 2016 | Published 3 Feb 2017

Matthew Gregson1,*, Erli Lu1,*, David P. Mills1, Floriana Tuna2, Eric J.L. McInnes2, Christoph Hennig3,4, Andreas C. Scheinost3,4, Jonathan McMaster5, William Lewis5, Alexander J. Blake5, Andrew Kerridge6 & Stephen T. Liddle1

Across the periodic table the trans-inuence operates, whereby tightly bonded ligands selectively lengthen mutually trans metalligand bonds. Conversely, in high oxidation state actinide complexes the inverse-trans-inuence operates, where normally cis strongly donating ligands instead reside trans and actually reinforce each other. However, because the inverse-trans-inuence is restricted to high-valent actinyls and a few uranium(V/VI) complexes, it has had limited scope in an area with few unifying rules. Here we report tetravalent cerium, uranium and thorium bis(carbene) complexes with trans C M C cores where experimental

and theoretical data suggest the presence of an inverse-trans-inuence. Studies of hypothetical praseodymium(IV) and terbium(IV) analogues suggest the inverse-trans-inuence may extend to these ions but it also diminishes signicantly as the 4f orbitals are populated. This work suggests that the inverse-trans-inuence may occur beyond high oxidation state 5f metals and hence could encompass mid-range oxidation state actinides and lanthanides. Thus, the inverse-trans-inuence might be a more general f-block principle.

1 School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK. 2 EPSRC National UK EPR Facility, School of Chemistry and Photon Science Institute, The University of Manchester, Oxford Road, Manchester M13 9PL, UK. 3 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Bautzner Landstrasse 400, D-01314 Dresden, Germany. 4 The Rossendorf Beamline, ESRF, BP 220, F-38043 Grenoble, France. 5 School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK. 6 Department of Chemistry, Lancaster University, Lancaster LA1 4YB, UK. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to A.K. (email: mailto:[email protected]

Web End [email protected] ) or to S.T.L. (email: mailto:[email protected]

Web End [email protected] ).

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DOI: 10.1038/ncomms14137 OPEN

The inverse-trans-inuence in tetravalent lanthanide and actinide bis(carbene) complexes

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14137

The trans-inuence is a long-established, well-documented concept of broad relevance across inorganic chemistry1,2. This thermodynamic ground-state phenomenon classically

occurs in square-planar and pseudo-octahedral d-block complexes where tightly bonded ligands selectively lengthen mutually trans metalligand bonds. The trans-inuence is fundamentally important and underpins the trans-effect3, a kinetic rate effect where the order of substitution of ligands at a metal centre can be controlled; this is a key parameter to control, for example, the syntheses of cis- or trans-[PtCl2(NH3)2], whose isomerism is important regarding cancer treatment4. Although the bonding of lanthanide(III) and low/mid oxidation state early actinide ions is considered more ionic than in the d-block, there are crystallographic, and in some instances computationally supported, examples of complexes where metrical parameters are consistent with the presence of the trans-inuence516.

In high oxidation state actinide complexes the opposite phenomenon of the inverse-trans-inuence (ITI) can be found1721. Here, strongly donating ligands that normally adopt cis orientations to avoid destabilizing the respective metalligand bonds via the trans-inuence in fact reside trans to one another and even mutually reinforce each other. The classical, dominant example of the ITI is the uranyl(VI) dication, {UO2}2, that adopts a trans-linear geometry and is chemically robust because of strong, ITI-strengthened uraniumoxygen bonds20. Indeed, linear trans-dioxo actinyls {AnO2}n (An U, Np, Pu) are

well known and prevalent, but trans-dioxos in the d-block are unusual and require strong equatorial s-donor ligands to weaken the metal-oxo linkages sufciently to enable them to reside mutually trans. Two isostructural complexes that demonstrate the trans-inuence and the ITI are [MoO2Cl2(OPPh3)2] (I)22 and [UO2Cl2(OPPh3)2] (II)23, respectively (Fig. 1); in the former the oxos are cis, whereas in the latter they are trans. The ITI often plays a structure-dictating role, but this is not a criterion that must be met to make invoking the ITI valid; there are examples of high valent uranium complexes where ligands are constrained by their own architecture such that they have no choice but to place donor groups trans to a strongly donating ligand like a nitride or oxo, but despite this they present very short metalligand distances despite their unfavourable bonding situation. For example, in the complexes [U(TrenTIPS)(E)] [TrenTIPS N(CH2CH2NSiPri3)3;

E N (III), O (IV)] (Fig. 1)24,25 the UN

amine distances are short at 2.465(5) and 2.482(6) , respectively, despite being trans to nitride and oxo ligands, whereas such UNamine distances

are normally 2.52.7 (ref. 26). In these systems there is no ITI structure-dictating role, and as part of a polydentate Trenligand with minimal reorganization energy27 the Tren amines are forced to be unfavourably trans to a nitride or oxo28, but the UNamine distances are short, not long, that credibly invokes the ITI.

The origin of the ITI is complex, but is in part rationalized on the basis that in high oxidation state early actinides 6p orbitals are semi-core and transfer electron density to vacant 5f orbitals, creating an electron hole that is compensated for by additional donation of electron density from trans ligands1721. For many years the ITI was limited to uranyl(VI) complexes20 or structurally analogous complexes such as [UOCl5] (ref. 29), but in recent years a limited number of uranium(V) and (VI)

ITI complexes have emerged24,25,3033. The unifying theme has been high oxidation state (V or VI) metal complexes combined with hard, polarizing, charge-loaded oxo, imide and nitride ligands. Because it is limited to high oxidation state early actinides, the question of whether the ITI is a niche concept or in fact has a broader underpinning role for the f-block has remained

unanswered for around a quarter of a century in an area with few unifying rules.

When considering if the ITI could have a broader basis, it would have to be demonstrated to operate over a larger range of oxidation states, and be expanded beyond actinides to include the lanthanides. The IV oxidation state is the logical next step to take in terms of the general synthetic availability of uranium and thorium complexes, as the only two actinides that can be routinely handled without specialist facilities, and also because a IV oxidation state opens the door to extend this concept to the lanthanides; cerium has an accessible IV oxidation state under normal conditions, presenting the opportunity to compare cerium, uranium and thorium together34,35. Although hard, formally di- and trianionic oxygen and nitrogen ligands have so far exclusively supported the ITI with high oxidation state metals, by moving to a mid-range oxidation state a softer, isoelectronic dianionic carbon-based ligand might be arguably desirable to approximately maintain the relative energy matching of frontier metal and ligand orbitals, and we note that the only examples of uranium and even thorium in the 2 oxidation state

under ambient conditions are stabilized by carbon-based ligands3638. Indeed, carbon should be a good ligand for the ITI more generally because of its generally high-lying frontier orbitals compared with uranium31. However, a paucity of synthetically accessible families of complexes where the metal can be varied in a common mid-range oxidation state has limited testing the above hypothesis.

Here, we report the realization of our aim by the synthesis of cerium, uranium and thorium bis(carbene) complexes that exhibit linear C M C cores supported by the BIPMTMS ligand

[BIPMTMS {C(Ph2PNSiMe3)2}2 ]. Although the C M C

units are trans and thus would conventionally be expected to present long M C distances, they in fact exhibit exceedingly

short M C distances, and for cerium among the shortest

experimental Ce-C distance on record. Theoretical calculations reveal that when the pseudo-core 5p (cerium) or 6p (uranium or thorium) orbitals are isolated from the valence manifold the M C distances increase. Taken together with the short

M C distances, and considering that they are disposed

trans, this suggests that the ITI may extend beyond high oxidation state 5f metals to operate in mid-range oxidation state

OPPh3

OPPh3

O

Cl

Cl

OPPh3

Mo U

U

O

Ph3POO O

Cl Cl

I

II

SiPri3

N

N N N N

N

N

SiPri3 SiPri3 SiPri3

SiPri3

U

N

O

N

SiPri3III IV

Figure 1 | Selected literature examples where the trans-inuence and inverse-trans-inuence are invoked. Complexes I and II are identical except for the metal ion and the molybdenum complex exhibits cis-oxos whereas the uranium complex has trans-oxos, consistent with the trans-inuence and inverse-trans-inuence, respectively. Complexes III and IV both exhibit short uraniumamine distances despite being trans to hard, charge-loaded nitride and oxo ligands that is not consistent with the trans-inuence but conversely invokes the inverse-trans-inuence.

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14137 ARTICLE

+

SiMe3

SiMe3

Ph

SiMe3

SiMe3

Ph

Ph

Ph

Ph Ph

SiMe3 SiMe3

Ph

N

Ph

P

N N

Ph

Ph

Ph Ph

THF [K(CH2Ph)]

18C6

P

P P

Ph

O O

O

O

O

O

O

Ph Ag

N N

M = Pr & Tb

P

P

P

Ph

N

O

P

Ph

III

AgBPh4

Ph

P

M M

M = Ce (1Ce)

Pr (1Pr) Tb (1Tb)

M = U (4U)

Th (4Th)

CH

Ph Ph Ph

III MIII

K

-PhMe

P

N

N

N

Ph

Ph

C C C C C

N

-[K(18C6)(THF)2] [BPh4]

Ph

P P

N N

Ph

SiMe3

SiMe3

SiMe3

SiMe3

SiMe3

SiMe3

M = Pr (6Pr)

M = Ce (2Ce)

Pr (2Pr) Tb (2Tb)

Tb (6Tb)

M = Ce AgBPh4

-Ag0

-[K(18C6)(THF)2][BPh4]

Ph Ph Ph

Ph

Ph

Ph

Ph

Ph

Ph

SiMe3 SiMe3 SiMe3SiMe3

SiMe3

SiMe3

SiMe3

Cl

Cl

Ph

P P

P P

N

N

IV IV IV

C C C C

M M M

Cl

P P

P

N

N

Ph

Ph

Ph

Ph

N

THF

2 LiCH2SiMe3 CH2SiMe3

CH2SiMe3 2 SiMe4

H2C(PNSiMe3)2

3 LiCl

P

N Li

N N

Ph

Ph

SiMe3

THF

M = U (5U)

Th (5Th)

M = Ce (3Ce)

U (3U) Th (3Th)

Figure 2 | Synthesis of bis(carbene) complexes. The cerium(III) carbenemethanide complex 1Ce reacts with benzyl potassium and 18C6 in THFto give the cerium(III) bis(carbene) separated ion pair complex 2Ce with elimination of toluene. Complex 2Ce is readily oxidized by silver tetraphenylborate to give the cerium(IV) bis(carbene) complex 3Ce with elimination of elemental silver and potassium 18-crown-6 ether bis(THF) tetraphenylborate.

The uranium and thorium carbene complexes 4U and 4Th are converted to the corresponding carbene dialkyl complexes 5U and 5Th by salt elimination with two equivalents of trimethylsilylmethyl lithium. Complexes 5U and 5Th react with BIPMTMSH2 by alkane elimination to give the uranium and thorium bis(carbene) complexes 3U and 3Th, respectively. Attempts to prepare the praseodymium(IV) and terbium(IV) analogues of 3Ce resulted in elimination of potassium 18-crown-6 bis(THF) tetraphenylborate and insertion of silver into the bis(carbene) complexes to give 6Pr and 6Tb.

f-element metal complexes with appropriate ligand matching. Investigations of hypothetical praseodymium(IV) and terbium(IV) analogues suggest similar but increasingly diminished ITI phenomena.

ResultsSynthesis. Treating the cerium(III) carbene-methanide complex [Ce(BIPMTMS)(BIPMTMSH)] (1Ce)39 with benzyl potassium and 18-crown-6 ether (18C6) in tetrahydrofuran (THF) gives the yellow cerium(III) bis(carbene) complex [Ce(BIPMTMS)2][K(18C6)(THF)2] (2Ce) in 52% yield (Fig. 2).

Although cerium(IV) is regarded as a difcult oxidation state to access in an organometallic context because cerium(IV) is oxidizing and organometallic ligands are reducing, we nd that oxidation of 2Ce can be straightforwardly accomplished by AgBPh4 to give the green cerium(IV) bis(carbene)

[Ce(BIPMTMS)2] (3Ce) in 43% yield after work-up and recrystallization (Fig. 2). The oxidation of 2Ce to 3Ce is so favourable that even small traces of dry air will effect oxidation. This suggests that the two carbenes together are well suited to stabilizing cerium(IV) and producing a robust C Ce C unit, cf, the stability of the ITI-stabilized uranyl

O U O dication. The corresponding uranium and thorium

bis(carbene) complexes [M(BIPMTMS)2] (M U, 3U; Th, 3Th)

were prepared by a different methodology (Fig. 2). The mono (carbene) dichloride complexes [M(BIPMTMS)Cl3Li(THF)2]

(M U, 4U; Th, 4Th)40,41 were converted to the corresponding

dialkyls [M(BIPMTMS)(CH2SiMe3)2] (M U, 5U; Th, 5Th)42;

subsequent thermolysis with BIPMTMSH2 gave 3U and 3Th in 75% and 52% yields as brown and colourless crystals, respectively, after work-up.

Characterization data. The 1H nuclear magnetic resonance (NMR) spectra of 3Ce and 3Th span 010 p.p.m. and are characteristic of diamagnetic complexes, whereas that of para-magnetic 3U spans 33 p.p.m. The 31P NMR spectra of 3Ce, 3U and 3Th exhibit resonances at 13.7, 219.7

and 6.3 p.p.m., respectively; the 31P NMR resonance for 3Ce compares well with that of [Ce(BIPMTMS)(ODipp)2]

( 10.2 p.p.m., Dipp 2,6-diisopropylphenyl)43, and those of

3U and 3Th are typical of such complexes41,44. The 13C{1H} NMR carbene resonances for 3U and 3Th could not be located, even utilizing 13C-31P 2D NMR techniques, but the equivalent carbenes in 3Ce were observed at 343.5 p.p.m. (JPC 170 Hz);

this is more deshielded than that of [Ce(BIPMTMS)(ODipp)2] (324.6 p.p.m., JPC 149 Hz)43, possibly suggesting that the

carbenes in 3Ce are donating more strongly to cerium than in [Ce(BIPMTMS)(ODipp)2] despite their trans arrangement.

This might be expected for an ITI, and is also within the 200400 p.p.m. range of covalent transition metal carbenes rather than that observed for ionic yttrium(III) analogues (1040 p.p.m.)44.

As expected for a 1S0 4f 0 cerium(IV) ion, the ultraviolet/ visible/near-infrared (UV/Vis/NIR) spectrum of 3Ce exhibits no absorbances in the NIR region where ff or transitions associated with multicongurational character might occur (Supplementary Methods). Two absorptions (Fig. 3a), the broadness of which is a dening feature in many cerium(IV) complexes45, are observed in the visible region at 17,000 and 24,700 cm 1 (e 4,895 and

14,387 M1 cm1, respectively), and the latter absorbance is responsible for the green colour of 3Ce. In order to understand the electronic transitions responsible for the green colour of 3Ce we modelled the spectrum using time-dependent density functional theory (TD-DFT) calculations at the statistical

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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14137

a b

10

0

15,000

1 cm1 )

I(A)

(M

10,000

5,000

0

1.0 6

5

4

3

2

1

0

5,700 5,710 5,720 5,730 5,740 5,750 5,760

Energy (eV)

10

20 3 2.5 1.5 1 0.5

E (V versus Fc/Fc+)

25 mV s1

50 mV s1

100 mV s1

200 mV s1

300 mV s1

400 mV s1

500 mV s1

2

30 25 20 15 10

Wavenumber (103 cm1)

c d

0.8

0.6

3 K mol1 )

0.4

0.2

0.0

0 50 100 150

U Ce Th

Normalized absorption

[afii9851]T(cm

200 250 300

T (K)

Figure 3 | Spectroscopic and magnetic characterization data. (a) The UV/Vis/NIR spectrum of 3Ce, illustrating the broad nature of the two principal transitions in the visible region. (b) Cyclic voltammogram of 0.2 mM 3Ce in THF at selected sweep rates (0.1 M [N(Pr)4][BArF4] supporting electrolyte,

BArF4 tetrakis(3,5-triuoromethylphenyl)borate) versus [Fe(Cp)2]0/ showing a single quasi-reversible redox process assigned to the CeIV/CeIII redox

couple. (c) Variable temperature magnetic susceptibility data for 3Ce, 3U and 3Th indicative of populated low-lying paramagnetic states for 3U and diamagnetic, closed-shell assignments for 3Ce and 3Th. (d) Cerium LIII-edge XANES spectrum of the cerium(IV) complex 3Ce (red trace) in comparison with its cerium(III) precursor 2Ce (black trace). As references, spectra of 0.01 M cerium(III) nitrate in water (green trace) and of cerium(IV) dioxide (blue trace) are given. The XANES spectra of 2Ce and 3Ce were recorded at 15 K and the references were recorded at 298 K.

average of orbital potentials (SAOP)/zeroth order regular approximation (ZORA)/triple zeta basis set with one polarization function (TZP) level and the prole of the experimental spectrum is reproduced well by these calculations (Supplementary Fig. 1); the calculated absorption bands at 17,000 and 24,700 cm 1 are principally composed of ligand-to-metal charge-transfer from Ce C p- and s-combinations to

cerium(IV) 4f orbitals. The UV/Vis/NIR spectrum of 3U is characterized by weak (eo80 M1 cm1) absorptions over the range 5,00020,000 cm 1 that are characteristic of the 3H4 electronic manifold of the 5f 2 uranium ion (Supplementary

Fig. 2), whereas for 3Th the spectrum is featureless in the visible and NIR regions as expected for its 6d05f 0 nature.

The room temperature cyclic voltammogram of 3Ce reveals a well-resolved, quasi-reversible single redox process at E1/2 1.63 V versus [Fe(Z5-C5H5)2]0/ assigned as the

CeIV/CeIII redox couple (Fig. 3b; for full scan see Supplementary Fig. 3). The CeIV/CeIII redox couple is known to vary widely as a function of ligand environment46, and the reduction potential observed for 3Ce is towards the negative end of reported values, suggesting that the bis(carbene) environment stabilizes cerium(IV), reecting the facile oxidation of 2Ce to 3Ce. Under the same conditions, no redox processes were observed for 3U or 3Th in the accessible solvent window.

Powdered samples of 3Ce, 3U and 3Th were studied by variable temperature SQUID (superconducting quantum interference device) magnetometry (Fig. 3c and Supplementary Fig. 4). Complex 3U has a wT value of 0.95 cm3 K mol 1 at 298 K (equivalent to 2.77 mB, in agreement with an Evans method

solution magnetic moment of 2.61 mB) that decreases below30 K to a value of 0.4 cm3 K mol 1 at 2 K. Unlike typical magnetization behaviour of uranium(IV) complexes generally (Supplementary Fig. 5), complex 3U retains a high wT value over most of the temperature range and the low-temperature wT value is much higher than expected from temperature-independent paramagnetism alone. This suggests that the crystal eld of two strongly donating axial ligands is sufcient to stabilize low-lying paramagnetic states (consistent with low-temperature magnetization data) separated widely from higher energy states (assuming a 3H4 ground term, a strongly axial eld would stabilize the |mJ| 4 non-Kramers doublet), and hence even at

low temperatures the complex is paramagnetic. This phenomenon has been observed before in uranium(IV) complexes with strongly donating, multiply bonded axial ligands4752. Complexes 3Ce and 3Th are diamagnetic, and the essentially nil or small negative wT slope for the latter rule out any temperature-independent paramagnetism behaviour, suggesting there is no multicongurational ground character in 3Ce, and this is also consistent with the absence of low energy absorptions in the optical spectrum. At low temperatures, complex 3Ce has an insignicant wT value (0.003 cm3 K mol 1 at 2 K) and X- and

Q-band electron paramagnetic resonance (EPR) spectra are completely silent that contrasts with 4f 1 2Ce that exhibits strong EPR features characteristic of cerium(III) (Supplementary Figs 6 and 7).

The characterization data for 3Ce, 3U and 3Th support their IV oxidation state formulations. To unequivocally conrm that 3Ce is a 1S0 complex we subjected it to X-ray absorption

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14137 ARTICLE

Si1

Si3

N3 P3

C32

P4

N1 P1

C1 Ce1

P2 N2

N4 Si4

Si2

Figure 4 | Molecular structure of 3Ce at 150 K with displacement ellipsoids set to 30%. Hydrogen atoms and lattice solvent are omitted for clarity. The structures of 3U and 3Th are very similar. Selected bond lengths () and angles (): C1-P1 1.664(2), C1-P2 1.664(2), C32-P31.665(5), C32-P4 1.663(4), P1-N1 1.6128(18), P2-N2 1.6174(19), P3-N31.6247(18), P4-N4 1.6202(18), Ce1-C1 2.385(2), Ce1-C32 2.399(3), Ce1-N1 2.4766(17), Ce1-N2 2.5122(17), Ce1-N3 2.4726(18), Ce1-N42.4966(16), P1-C1-P2 164.31(15), P3-C32-P4 163.61(14), N1-Ce1-N2 127.16(6), N3-Ce1-N4 127.36(6), C1-Ce1-C32 176.98(7).

spectroscopy techniques. The X-ray photoemission spectroscopy (XPS) of 3Ce is weak because the cerium is a small component of the 151-atom structure, but it exhibits a spectrum characteristic of cerium(IV) in the energy range 870930 eV (Supplementary Fig. 8). The X-ray absorption near-edge spectroscopy (XANES) of 3Ce reveals two absorptions characteristic of cerium(IV)53, (Fig. 3d), and when all characterization data are taken together the self-consistent picture that emerges is that 3Ce is a closed-shell singlet cerium(IV).

Solid-state structures. The solid-state structures of 3Ce, 3U and 3Th were determined by single crystal X-ray diffraction and were found to be essentially isostructural. Complex 3Ce is illustrated in Fig. 4 and further details of that and all other structurally determined complexes in this study can be found in Supplementary Figs 914 and Supplementary Tables 1 and 2. In each case, pseudo-octahedral metal centres present with mutually trans carbenes (3Ce, 176.98(7); 3U, 177.5(2); 3Th, 176.21(8)) with deviations from the octahedral ideal because of the BIPMTMS N-M-N bite angles resulting in the imino nitrogen atoms residing above and below the hypothetical equatorial plane. The carbenes adopt essentially planar T-shaped geometries with sum of angles spanning the range 357.23(18) to 360.00(15) and the metal-BIPMTMS 6-membered chelate rings are essentially planar contrasting with some BIPMTMS complexes where the carbene can be distinctly pyramidal44, including [Ce(BIPMTMS)(ODipp)2]43. In 3Ce, 3U and 3Th, the two

BIPMTMS ligands are disposed essentially orthogonally to one another, with dihedral angles between the two N-M-N planes of 92.5(2), 92.2(4) and 91.4(2), respectively. The Ce1-C1 and Ce1-C32 distances in 3Ce (2.385(2) and 2.399(3), respectively) are exceedingly short, 40.2 shorter than the corresponding

distances in 2Ce, and B0.04 shorter than the Ce C distances

in [Ce(BIPMTMS)(ODipp)2]43 and the special case of cerium conned within an endohedral fullerene54. Signicantly, considering they are trans, the Ce-C bond distances in 3Ce are the shortest experimentally determined Ce-C distances to date in discrete molecular compounds, being surpassed only by short (2.247(17)-2.334(15) ) ceriumcarbon distances in periodic ethynediidehalide clusters5559. Short molecular Ce-C distances have been found in theoretical models of experimentally unknown CeCH2 and [Cp2CeCH2] that are sterically unimpeded and, in the case of the former, benet from the reduced electronic repulsion from a net positive charge60,61.

The U1-C1 and U1-C32 distances in 3U (2.410(6) and2.421(6) , respectively) are statistically equivalent to the corresponding distances in 3Ce (ionic radii of CeIV 0.87

versus UIV 0.89 )62. However, the Th1-C1 and Th1-C32

distances in 3Th (2.514(3) and 2.516(3) , respectively) are B0.05 longer than would be predicted purely based on the increase in ionic radius of ThIV (0.94 )62. The short M C bond

lengths in 3Ce, and to some extent 3U, are all the more notable for the fact they are trans, and suggest that an ITI, rather than trans-inuence, may be operating.

Reactivity. To give experimental support to the formulation of 3Ce, 3U and 3Th as carbene complexes, we examined their reactivity towards PhCHO. In all cases the Wittig-alkene product PhC(H) C(PPh2NSiMe3)2 was formed in essentially

quantitative yield. Although 3Th reacts too quickly to be monitored (reaction complete in o5 min), and 3U reacts quickly (480% consumed in 15 min) and is paramagnetic, hence reliable data could not be extracted from questionable NMR integrations, was found to be amenable to a full study (Supplementary Figs 1517). The reaction of 3Ce with two equivalents of PhCHO was tted to second-order kinetics overall (rst order with respect to both 3Ce and PhCHO) with k (298 K)

1.28 10 40.255 10 4 mol 1 dm3 s 1. Eyring and Arrhe

nius analyses yielded DHz 37.22 kJ mol1 and DSz

194.4 6 J mol1 K1, affording DGz 95.22 kJ mol1

(298 K) and Ea 40.02 kJ mol1. These data are consistent

with the reaction conditions and the metallaoxetane intermediates formed from a double [2 2]-cycloaddition/bond

metathesis reaction to produce CeO2 and PhC(H)

C(PPh2SiMe3)2. This reactivity is characteristic of covalent early metal carbenes44 and contrasts with more ionic yttrium(III)-methanediide BIPMTMS complexes that activate aryl CH bonds of carbonyl compounds63. We note that the order of reactivity of 3Ce, 3U and 3Th with PhCHO is 3Th43U43Ce; this is principally consistent with the ionic radii of the metals but is also consistent with the increasing level of covalency in the M C

bonds of 3Ce43U43Th suggested by our calculations.

Theoretical calculations. To probe the electronic structures of 3Ce, 3U and 3Th, we undertook DFT calculations on their full structures. The geometry-optimized calculations match closely the experimentally determined structures within 0.05 and 2 (Supplementary Tables 311), and the TD-DFT calculations model the experimentally determined electronic absorption spectrum of 3Ce well. We thus conclude that the theoretical models provide a qualitative description of the electronic manifolds of 3Ce, 3U and 3Th. Although the analysis that follows shows that the dominant feature of the metalcarbon interactions in 3Ce, 3U and 3Th is electrostatic bonding, covalent contributions are present and the discussion focusses on this latter aspect. For 3Ce, 3U and 3Th, the calculated MDCq metal charges are1.99, 2.77 and 2.48, respectively, and the MDCq carbene charges

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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14137

3Ce: 1A1

Occ: 1.979

Occ: 1.982 Occ: 1.979 Occ: 1.979 Occ: 1.979

Occ: 0.599

Occ: 1.974

Occ: 1.977 Occ: 1.977

3U: 3B1

Occ: 0.401

Occ: 0.401

Occ: 0.599

3Th: 1A1

Occ: 1.982 Occ: 1.978 Occ: 1.980 Occ: 1.980

Figure 5 | Selected CASSCF-calculated natural orbitals of truncated models of bis(carbene) complexes. Only strongly occupied MC bonding orbitals and 5f orbitals are shown. Pronounced multicongurational character is present among the 5f orbitals in the truncated U complex, but the rest of the electronic manifold comprises orbitals with occupations close to integer values showing at best weak multicongurational character. All orbitals rendered at an isosurface of 0.02 atomic units (a.u.).

are 1.79/ 1.80, 2.03/ 2.06 and 1.95/ 2.01, respec

tively. Although care must be taken when analysing calculated charges, they are indicative of metal(IV) ions44,64 and that perhaps the cerium ion in 3Ce is better matched to receiving electron donation from the ligands than the uranium and thorium ions in 3U and 3Th. However, the calculated MDCm spin density of 2.27 for the uranium ion in 3U is certainly

consistent with charge donation from the BIPMTMS ligand to uranium and also with its 5f 2 uranium(IV) formulation.

Inspection of the Kohn Sham frontier orbitals of 3Ce, 3U and 3Th (Supplementary Figs 1820) clearly shows twofold bonding interactions between each carbene and the respective metal. Each M C bond is polarized, however, as evidenced by M C

NalewajskiMrozek bond orders averaging 1.10, 1.30 and 0.73 for 3Ce, 3U and 3Th, respectively. For comparison, the Ce C bond

order in [Ce(BIPMTMS)(ODipp)2] is also 1.1 (ref. 43), whereas uranium(IV)-BIPM complexes typically have U C double

and UC single bond orders of B1.4 and B0.5, respectively44,64, and thorium is polarized with Th C bond orders of

B0.7 (ref. 64). Therefore, the data presented here t the overall literature trends.

A clearcut view of the M C bonding in 3Ce, 3U and 3Th

from the Kohn Sham frontier orbitals is precluded because of the delocalized nature of molecular orbital calculations. We therefore turned to natural bond orbital (NBO) analysis (Supplementary Figs 21 and 22) that is suited to the treatment of chemical bonding in molecular complexes. For 3Ce, the Ce C s-bonds

are composed of B13% cerium and 87% carbon character; in each case the cerium contribution is B46% 5d and B53% 4f, with the remaining B1% being 6s. For the Ce C p-bonds,

cerium contributes only B8% to these bonds and the cerium component is predominantly 4f character (B80%) with a modest 5d component (B19%), with the remainder being of 5p character. These data compare well with those of [Ce(BIPMTMS)(ODipp)2]42, and indeed there is growing evidence that cerium(IV) is suited to utilizing its 4f orbitals in bonding to ligands43,53,65,66. This may be important, because a study on lanthanide(III) chalcogenide complexes found a trans-inuence where the lanthanide utilizes predominantly d orbitals12; in contrast the metal(IV) complexes, certainly for cerium and uranium, are deploying predominantly f orbitals, in line with the general theory of the ITI1721. For 3U, the U C s-bonds are composed of B14.5% uranium

character and B85.5% carbon character. As for 3Ce, the 5f:6d contributions of 3U are well balanced at B51:47%, with the remainder of 7s character. The U C, p-bond is B14:86%

uranium/carbon and, like for cerium, is within the uranium component principally 5f (B86%) with modest 6d (B13%), with the remaining B1% being 6p character. For 3Th, NBO does not return any Th C interactions, suggesting that the Th C

bonding is highly ionic.To better understand the nature of the M C interactions

in these compounds we turned to complete/restricted active space self-consistent eld (CASSCF/RASSCF) methodologies that allows us to consider electron correlation through a rigorous conguration-interaction-based approach to directly compare open- and closed-shell compounds. The computational cost of such calculations required truncation and symmetrization of the experimentally determined structures to produce tractable models, but by retaining key structural motifs near identical electronic structures are obtained (Supplementary Tables 1214).

RASSCF calculations were employed in order to identify an appropriate active space for each system. Because of the computational expense of such calculations, RAS1, RAS2 and RAS3 subspaces were constrained to consist of 12, 7 and 12 orbitals, respectively. The 7 RAS2 orbitals comprise the 4f/5f manifold, whereas the RAS1 and RAS3 orbitals account for orbitals with signicant C/N 2s and 2p character and natural orbitals whose occupation numbers most deviate from integer values. This active space ensured that all M C and MN

interactions were accurately modelled. State-averaged RASSCF calculations indicated a degenerate 3B1/3B2 ground state in the 3U model complex, corresponding to a state of E symmetry in the full idealized D2d point group. The natural orbital occupation numbers resulting from these calculations allowed complete active spaces to be dened. Subsequent CASSCF calculations correlated 8 electrons in 8 orbitals in the cases of 3Ce and 3Th model complexes, and 10 electrons in 12 orbitals in the case of the U complex.

CASSCF calculations revealed that all complexes are dominated by M(IV) congurations, (Fig. 5), contributing96.0%, 96.0% and 95.4% to the ground state wavefunctions of the 3Ce, 3U and 3Th model complexes, respectively. Maximum deviations from integer values in natural orbital occupations were0.026, 0.021 and 0.026 for the 3Ce, 3U and 3Th model complexes, respectively, indicating at most weak multicongurational character. The resultant electronic structure is almost identical to that obtained from the RASSCF simulations, and the dominant M(IV) character in all systems is commensurate with all experimental measures. Notably, as shown in Fig. 5, 3Ce, 3U and 3Th show s-bonding combinations that are strongly reminiscent of uranyl, a feature that also emerges from the DFT analysis that is consistent with an ITI in these complexes.

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To further probe the covalent contribution to bonding in these compounds, we directly analyse the resultant electron densities via the quantum theory of atoms in molecules (QTAIM) and focus on two parameters: the delocalization index between two bonded atomic centres (d) and the magnitude of the electron density at the bond critical point between the centres (rBCP). The

delocalization index, formally a two-electron property of the system, is a measure of the number of electrons shared between two atoms, and is large when orbital mixing because of energetic near degeneracy between the two atoms is pronounced. In this sense, it probes similar properties to those given by orbital decomposition via NBO analysis or, experimentally, by X-ray absorption spectroscopy that probes orbital energy near degeneracy. On the other hand, rBCP quanties electronic charge concentration in the bond between two atoms. Combined, these measures give an indication as to whether orbital mixing leads to charge accumulation in the bonding region (and hence bond stabilization) and hence provide a more complete method for assessing bond covalency than orbital analysis alone (Supplementary Table 15)53.

QTAIM-derived atomic charges are more consistent than those obtained via NBO analysis, with q(Ce)oq(U)oq(Th). Carbene charges reect this trend that therefore provides some evidence for a greater covalent interaction in the 3Ce complex. Stronger evidence is provided by the delocalization indices, d(M,C), that are notably larger for 3Ce than either 3U or 3Th. Consideration of rBCP demonstrates that this electron sharing corresponds to charge accumulation in the M C bonding region: rBCP follows

a similar trend to d(M,C), with the Ce and U complexes exhibiting signicantly larger values than that of Th. Inspection of the ellipticity parameter e for the M C bonds reveals values

that are consistent with an asymmetric distribution of electron density around the M C bond comparable to those found

for alkenes and [M(BIPMTMS)(ODipp)2]43, conrming that in 3Ce, 3U and 3Th there is a M C bonding interaction involving

two electron pairs donated from a carbene to a metal.

Probing the inverse-trans-inuence. To probe whether the ITI is operating in 3Ce, 3U and 3Th, we adopted the method of OGrady and Kaltsoyannis29. Here, 3Ce, 3U and 3Th were geometry optimized with a frozen core, up to 4d for 3Ce and 5d for 3U and 3Th and with the pseudo-core 5 or 6p orbitals, respectively, either explicitly included as valence orbitals or placed in the frozen core. Although the ITI is a complex phenomenon that involves several factors, it is clear that p orbitals are involved in the ITI and that this method isolates the contributions that the pseudo-core 5/6p orbitals have on the bonding. Although the carbene ligands in 3Ce, 3U and 3Th clearly exhibit polarized M C bonds, the ITI is predominantly dependent on the

charge and polarizing nature of the coordinated ligands33. To rule out coincidental systematic errors from a particular method, we examined the effect of varying the functional (BP86 versus PBE) and the basis set (normal all-electron basis set versus a normal frozen core up to 4 or 5d for 3Ce, 3U, and 3Th, respectively) separately or simultaneously, and found no signicant changes in equilibrium geometries. However, when the 5 or 6p orbitals for 3Ce, 3U and 3Th, respectively, were additionally also placed in the frozen core, signicant changes to the equilibrium geometries were observed in all cases. Specically, the M C distances lengthen by B0.05 when the relevant

p orbitals are placed in the frozen core that are very similar shifts to those found previously for [MOX5] anions (M U, Np;

X F, Cl, Br)29, and this represents the p orbital contributions

to the ITI. It should be noted that, on inclusion of the relevant p orbitals into the frozen cores, the MNimino distances

also elongate; however, the latter lengthen by only B0.02 , less than half that of the change to the M C linkages.

As expected, with p orbitals in the frozen cores the metal and carbene charges increase, indicating more polarized and presumably weakened interactions. For example, in 3Ce the cerium and carbene charges rise from 1.83 and 1.71 when

5p orbitals are included in the valence region to 2.54 and

1.82 when 5p orbitals are placed in the frozen core. Inspection of the differences of the total energies of the geometry-optimized structures of 3Ce, 3U and 3Th with the p orbitals in the frozen core or in the valence region yields energy differences of12.5, 20.5 and 18.2 kcal mol 1 for 3Ce, 3U and 3Th, respectively. This provides a qualitative bracketing of the stabilizing energy that the inclusion of the pseudo-core p orbitals in the bonding to a bis(carbene) ligand set provides, and compares well with the ITI of 6 kcal mol 1 calculated for a uranium(VI)-mono(oxo)

unit in a tris(aryloxide) triazacyclononane complex30.

As 3Ce, 3U and 3Th all appear to exhibit the ITI, we investigated the synthesis of the analogous praseodymium(IV) and terbium(IV) bis(BIPM) complexes 3Pr and 3Tb, respectively. We targeted these complexes because after cerium they have the next two lowest fourth ionization energies of all lanthanides34,35. Nevertheless, the fourth ionization energies of these two elements are still considerable, and we could not access 3Pr and 3Tb experimentally. Attempting AgBPh4-mediated oxidations of 2Pr and 2Tb (Fig. 1) results not in oxidation to give 3Pr and 3Tb but instead elimination of [K(18C6)(THF)2][BPh4]

and isolation of [M(BIPMTMS)2Ag] (M Pr, 6Pr; M Tb,

6Tb); photolysis or electrochemistry experiments on 6Pr and 6Tb resulted in intractable decomposition products. However, although we could not prepare 3Pr and 3Tb, 3Ce, 3U, 3Th, 2Pr and 2Tb provide experimentally calibrated benchmarks with which to provide condence in the calculated hypothetical geometry-optimized structures of 3Pr and 3Tb. Inspection of the equilibrium geometries of 3Pr and 3Tb calculated with their respective 5p orbitals in-core and included in the valence regions reveals that the ITI persists but diminishes on moving from Ce to Pr to Tb (Supplementary Tables 1619). Specically, the Pr C

distances elongate by 0.02 when the 5p orbitals are placed in the frozen core, and the Pr-N distances elongate by only 0.006 . For Tb, the effect is signicantly reduced, with a 0.007 elongation of the Tb C distance when the 5p orbitals are placed in the frozen

core and the Tb-N distances elongate by 0.006 . We conclude from these data that the ITI may, in principle, apply across the lanthanide(IV) series, but as 4f electron occupancy increases the ITI diminishes. This is consistent with the theory of the ITI as donation of (n)p electron density into the (n 1)f orbital

manifold will become less favourable as the (n 1)f occupancy

increases because of interelectronic repulsion19. Furthermore, the appearance of a trend suggests greater levels of f-orbital covalency that is supported by the characterization data more widely, whereas for lanthanide(III) systems when the trans-inuence has been studied and principally d-orbital participation has been invoked then the trans-inuence trend is uniform, suggestive of mainly ionic bonding character12. Thus, the observations of uniform trans-inuence with d-orbital bonding for lanthanide(III) ions12 versus diminishing ITI for lanthanide(IV) ions where f-orbital bonding is principally invoked is internally consistent.

The delocalized nature of the molecular orbital approach makes the identication of key molecular orbitals involved in the ITI difcult29,30. Thus, inspection of individual molecular orbitals would not be expected to provide clear-cut information, as has proven to be the case even in highly symmetric complexes29. However, an examination of the electronic manifolds of 3Ce, 3U and 3Th reveals a common molecular orbital that may

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be signicant. All three complexes exhibit a molecular orbital at B 16.4 eV (3Ce, 16.483; 3U, 16.485; 3Th, 16.494 eV)

B12 eV below the highest occupied molecular orbital ( 2 for 5f2

3U). In each case contributions from the 2s orbitals of each carbene (normalized to 34%) and 5 or 6p orbitals for cerium or uranium and thorium, respectively (normalized to 5%) are found. Interestingly, a similar orbital is computed for 3Pr at

16.476 eV, although here the carbon (normalized to 32%) and praseodymium (normalized to 2.5%) contributions are notably less well matched. In contrast, for 3Tb the closest match is now a molecular orbital at 20.455 eV with normalized

carbon and terbium contributions of 9% and 3%, respectively. We suggest that this molecular orbital that is common to 3Ce, 3U, 3Th and 3Pr but not 3Tb may represent a signature in this instance of the ITI19,20,29.

DiscussionWe have prepared three new metal bis(carbene) complexes that contain linear C M C cores and the characterization

data show these complexes to be unequivocally metal(IV) complexes and thus valid to compare with one another. For the cerium and uranium derivatives the M C bonds are short, and

for the former one of the shortest Ce-C bonds on record; that they are so short despite the fact that they are strongly donating dianions disposed trans with respect to one another suggests that the ITI is operating instead of the more classical trans inuence. Theoretical calculations suggest the presence of an ITI in cerium, uranium and thorium derivatives, as removal of pseudo-core (n)p orbitals from the valence region in calculations consistently results in elongation of the M C bonds by over twice that of the

elongation of the M-imino bonds. Interestingly, the characterization data and theoretical data taken together suggest a consistent trend of covalency of 3CeB3U43Th. This work suggests that the ITI concept, rst established a quarter of a century ago, may extend beyond high oxidation state 5f metals to now encompass mid-range oxidation state 5f actinides and 4f lanthanides. Calculations also suggest, however, that the ITI may diminish on moving from left to right in the lanthanide series and with increasing (n 1)f occupation number. Although an opposite

trend may operate for the actinides29, it may be that the more diffuse 5f orbitals could tolerate occupancy more; however, thus far the radioactivity of those elements has precluded any detailed body of experimental work from being compiled and hence further work will be required to provide the necessary benchmarks with which to investigate this. The observations of uniform trans-inuence with d-orbital bonding for lanthanide(III) ions12 versus the trend reported here of diminishing ITI for the lanthanide(IV) ions investigated, where f-orbital bonding is principally invoked, is gratifyingly internally consistent. Thus, the ITI might be a more general f-block principle.

Methods

General. Experiments were carried out under a dry, oxygen-free dinitrogen atmosphere using Schlenk-line and glove-box techniques. All solvents and reagents were rigorously dried and deoxygenated before use. Compounds were variously characterized by elemental analyses, electrochemistry, NMR, Fourier transform infrared spectroscopy (FTIR), EPR, XANES and UV/Vis/NIR electronic absorption spectroscopies, single crystal X-ray diffraction studies (Supplementary Data 111), Evans methods and SQUID magnetometry, and DFT, NBO, QTAIM, CASSCF and RASSCF computational methods.

Synthesis of [Ce(BIPMTMS)2][K(18C6)(THF)2] (2Ce). THF (15 ml) was added to a precooled ( 78 C) mixture of 1Ce (1.41 g, 1.17 mmol) and [K(CH2Ph)]

(0.15 g, 1.17 mmol). The resulting orange suspension was allowed to slowly warm to room temperature with stirring over 16 h to afford an orange solution.18C6 (0.31 g, 1.17 mmol) in THF was added and stirred for a further 2 h. The solvent was removed in vacuo to afford an orange solid. The solid was washed with

toluene to afford 2Ce as a yellow powder. Yield: 2.31 g, 52%. Recrystallization of a small portion from toluene afforded yellow crystals of 2Ce on storing at room temperature. Anal. Calcd for C82H116CeKN4O8P4Si4: C, 57.91; H, 6.87; N, 3.29%.

Found: C, 57.65; H, 6.89; N, 3.09%. 31P{1H} NMR (C6D6, 298 K): d 7.22 (CeCP2). FTIR v/cm 1 (Nujol): 1,350 (w) 1,302 (w), 1,077 (s), 961 (m), 846 (m), 771 (m), 743 (s), 695 (m), 633 (m), 522 (s). Magnetic moment (Evans method, THF, 298 K): meff

2.18 mB.

Synthesis of [Ce(BIPMTMS)2] (3Ce). Toluene (15 ml) was added to a precooled ( 78 C) mixture of 2Ce (1.64 g, 0.96 mmol) and [Ag(BPh4)] (0.41 g, 0.96 mmol).

The resulting yellow suspension was allowed to warm to room temperature with stirring over 16 h to afford a green suspension. The suspension was ltered and volatiles were removed in vacuo to afford a green solid. Recrystallization from toluene (2 ml) afforded 3Ce as green crystals. Yield: 0.52 g, 43%. Recrystallization of a small portion from pentane (3 ml) afforded green crystals suitable for single crystal X-ray diffraction analysis. Anal. Calcd for C62H76CeN4P4Si4: C, 59.42;

H, 6.11; N, 4.47%. Found: C, 59.63; H, 6.15; N, 4.43%. 1H NMR (C6D6, 298 K):d 0.95 (36H, s, NSi(CH3)3), 7.08 (24H, d, p/o-Ar-H), 7.24 (16H, m, o-Ar-H) p.p.m.

13C{1H} NMR (C6D6, 298 K): d 6.41 (NSi(CH3)3), 127.26 (m-Ar-C), 129.78 (o-Ar-C), 132.52 (p-Ar-C), 140.50 (i-Ar-C), 343.48 (t, JPC 170.22 Hz, CeCP2) p.p.m.

31P{1H} NMR (C6D6, 298 K): d 13.65 (CeCP2) p.p.m. 29Si{1H} NMR (C6D6,

298 K): d 2.17 (NSi(CH3)3) p.p.m. FTIR v/cm 1 (Nujol): 1,294 (w), 1,246 (w),

1,064 (s), 843 (m), 757 (w), 693 (w), 656 (m), 597 (w), 509 (m).

Preparation of [U(BIPMTMS)(CH2SiMe3)2] (5U). THF (10 ml) was added to a precooled ( 78 C) mixture of [U(BIPMTMS)(Cl)3(Li)(THF)2] (1.09 g, 1 mmol)

and LiCH2SiMe3 (0.19 g, 2 mmol). The mixture was then allowed to slowly warm to room temperature with stirring over 6 h to afford a brown solution. Volatiles were removed in vacuo and the resulting brown solid was extracted with toluene (20 ml). Volatiles were removed in vacuo and the resulting brown solid was recrystallized from hexane (3 ml) and stored at 30 C to afford 5U as brown crystals. Yield

0.67 g (69%). Anal Calcd for C38H60N2P2Si4U: C, 47.22; H, 6.00; N, 2.98. Found: C, 46.53; H, 5.92; N, 2.97. The carbon is consistently low over ve independently synthesized batches that we attribute to carbide formation. 1H NMR (C6D6, 298 K): d 7.76 (s, 18H, Si(CH3)3), 0.41 (s, 18H, Si(CH3)3), 5.21 (m, 4H, Ar-H),

5.49 (br, 8H, Ar-H), 5.85 (br, 8H, Ar-H), 7.15 (br, 4H). IR v/cm 1 (Nujol): 1,403 (w), 1,305 (w), 1,248 (m, br), 1,108 (m), 1,019 (m, br), 836 (s, br).

(Evans method, C6D6, 298 K): 2.45 mB.

Preparation of [U(BIPMTMS)2] (3U). Toluene (15 ml) was added to a mixture of [U(BIPMTMS)(CH2SiMe3)2] (0.97 g, 1.00 mmol) and BIPMTMSH2(0.56 g, 1.00 mmol) at room temperature. The resulting brown solution was stirred at 90 C for 18 h, and then was allowed to cool to ambient temperature and ltered. All volatiles were removed from the ltrate to afford a brownish black solid that was washed by 5 ml of pentane at 0 C to afford the product as a brown solid. Yield0.81 g, 60%. Recrystallization of a small portion from a toluene/hexane mixture at 5 C afforded brown crystals suitable for a single crystal X-ray diffraction study. Anal. Calcd for C62H76N4P4Si4U C6H14: C 56.81; H 6.16; N 3.90. Found: C 56.44;

H 6.16; N 3.89. 1H NMR (C6D6, 298 K): d 33.88 (br, 36 H, NSi(CH3)3),

12.04 (s, 9 H, Ar-H), 13.76 (s, 17 H, Ar-H), 30.42 (br, 14 H, Ar-H). 31P{1H} NMR (C6D6, 298 K): d 219.70 (br, UCP2) p.p.m. FTIR n/cm 1 (Nujol): 1,958 (w),

1,450 (s), 1,334 (s), 1,281 (m), 1,245 (m), 1,178 (w), 1,155 (w), 1,103 (s), 1,035 (m), 745 (m), 669 (m), 628 (m), 514 (w), 489 (w). Magnetic moment (Evans method, C6H6 298 K): meff 2.61 mB.

Preparation of [Th(BIPMTMS)(CH2SiMe3)2] (5Th). Li2BIPM (1.71 g, 3 mmol) in THF (10 ml) was added to a precooled ( 78 C) suspension of ThCl4(THF)3.5

(1.88 g, 3 mmol) in THF (10 ml). The pale yellow reaction mixture was stirred at

78 C for 30 min and at room temperature for 2 h. After which the mixture was cooled to 78 C again, and LiCH2SiMe3 (0.56 g, 6 mmol) in THF (10 ml) was

added. The resulted pale yellow solution was kept at 35 C for 12 h and then

all volatiles were removed in vacuo to afford a yellow oil that was extracted

by i-hexane (3 10 ml). All volatiles were removed in vacuo from the ltrate to

afford the product as a pale yellow solid. Yield: 2.17 g, 75%. Recrystallization of a small portion from i-hexane (3 ml) at 30 C afforded colourless crystals suitable

for single crystal X-ray diffraction analysis. Anal. Calcd for C39H60N2P2Si4Th: C,

48.63; H, 6.28; N, 2.91%. Found: C, 48.70; H, 6.39; N, 2.65%. 1H NMR(C6D6, 298 K): d 0.01 (s, 4 H, ThCH2), 0.22 (s, 18 H, CH2SiMe3 or NSiMe3),

0.43 (s, 18 H, CH2SiMe3 or NSiMe3), 6.98 (br, 12 H, p- and o-Ar-H), 7.59 (br, 8 H, m-Ar-H) p.p.m. 13C{1H} NMR (C6D6, 298 K) d 2.74 (s, CH2SiMe3 or NSiMe3),4.54 (s, CH2SiMe3 or NSiMe3), 73.75 (t, 1JPC 159 Hz, ThCP2), 92.59 (s, ThCH2),

130.18 (s, o- and p-ArC), 131.13 (t, 3JPC 5.6 Hz, m-ArC), 137.79 (t, 1JPC 50.3

Hz, ipso-ArC) p.p.m. 31P NMR (C6D6, 298 K): d 5.81 (s) p.p.m. 29Si{1H} NMR (C6D6, 298 K): d 6.85 (t, 2JPSi 3.10 Hz, NSiMe3), 1.17 (s,

ThCH2SiMe3) p.p.m. FTIR v/cm 1 (Nujol): 1,591 (w), 1,403 (s), 1,302 (m), 861 (s), 695 (w), 608 (w), 588 (w), 551 (w).

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Preparation of [Th(BIPMTMS)2] (3Th). Toluene (10 ml) was added toa mixture of [Th(BIPMTMS)(CH2SiMe3)2] (0.87 g, 0.90 mmol) and BIPMTMSH2(0.51 g, 0.90 mmol). The resulting pale yellow solution was stirred at 50 C for 20 h, and then was dried in vacuo to afford sticky yellow solid. The crude product was washed with hexanes (2 10 ml) and then dried in vacuo to afford 3Th as a

colourless powder. Yield: 0.63 g, 52%. Recrystallization of a small portion from toluene at 5 C afforded colourless crystal suitable for single crystal X-ray diffraction. Anal. Calcd for C62H76N4P4Si4Th: C 55.34; H 5.69; N 4.16%.

Found: C 55.02; H 5.78; N 3.91%. 1H NMR (C6D6, 298 K): d 0.30 (s, 36 H, NSi(CH3)3), 7.037.12 (m, 25 H, Ar-H), 7.597.66 (m, 15 H, Ar-H). 13C{1H} NMR (C6D6, 298 K): d 5.25 (s, NSi(CH3)3), 127.37 (s, Ar-C), 129.75 (s, Ar-C), 132.31 (s, Ar-C), 141.27 (t, 3JPC 47.0 Hz, i-Ar-C). The carbene centre in 3Th was

not observed in the 13C{1H} NMR spectrum. 31P{1H} NMR (C6D6, 298 K): d 6.25 (ThCP2). 29Si{1H} NMR (C6D6, 298 K): d 6.83 (NSi(CH3)3).

FTIR n/cm 1 (Nujol): 1,305 (m), 1,244 (w), 1,154 (m), 1,051 (s), 804 (s), 741 (w), 722 (s), 696 (w), 634 (s), 492 (m).

Preparation of [Tb(BIPMTMS)(BIPMTMSH)] (1Tb). BIPMTMSH2 (4.47 g, 8 mmol) in toluene (10 ml) was added dropwise to a precooled ( 78 C)

suspension of [Tb(CH2Ph)3(THF)3] (2.32 g, 4 mmol) in toluene (15 ml). The resulting orange suspension was warmed to room temperature with stirring over 16 h and then reuxed for 10 min to afford a yellow solution. Volatiles were removed in vacuo and the resulting yellow residue recrystallized from hot toluene (4 ml) to afford colourless crystals of 1Tb on cooling to room temperature. Yield:1.81 g, 36%. Anal. Calcd for C62H77N4P4Si4Tb: C, 58.50; H, 6.10; N, 4.40%. Found: C, 58.61; H, 6.06; N, 4.33%. FTIR v/cm 1 (Nujol): 1,367 (s), 1,242 (m), 1,219 (m), 1,105 (s), 1,031 (m), 841 (s), 696 (m), 638 (w), 610 (w), 553 (m), 522 (m). Magnetic moment (Evans method, C6D6, 298 K): meff 9.90 mB.

Preparation of [Pr(BIPMTMS)2][K(18C6)(THF)2] (2Pr). THF (15 ml) was added to a precooled ( 78 C) mixture of 1Pr (5.25 g, 4.18 mmol) and

[K(CH2Ph)] (0.55 g, 4.18 mmol). The resulting orange suspension was allowed to slowly warm to room temperature with stirring over 16 h to afford an orange solution. 18C6 (1.11 g, 4.18 mmol) in THF was added and stirred for a further 2 h. The solvent was removed in vacuo to afford an orange solid. The solid was washed with toluene to afford 2Pr as a yellow powder. Yield: 4.50 g, 63%. Recrystallization of a small portion from toluene afforded yellow crystals of 2Pr on storing at room temperature. Anal. Calcd for C82H116KN4O8P4PrSi4: C 57.89; H, 6.87; N, 3.29%.

Found: C, 56.72; H, 6.67; N, 3.29%. FTIR v/cm 1 (Nujol): 1,352 (w), 1,302 (w), 1,105 (m), 964 (w), 826 (w), 695 (w), 634 (w), 522 (m). Magnetic moment (Evans method, THF, 298 K): meff 3.56 mB.

Preparation of [Tb(BIPMTMS)2][K(18C6)(THF)2] (2Tb). THF (15 ml) was added to a precooled ( 78 C) mixture of 1Tb (0.94 g, 0.74 mmol) and

[K(CH2Ph)] (0.096 g, 0.74 mmol). The resulting orange suspension was allowed to slowly warm to room temperature with stirring over 16 h to afford a yellow solution. 18C6 (0.31 g, 1.17 mmol) in THF was then added and the resulting yellow solution stirred for 2 h. The solution was then reduced in volume to ca. 2 ml that afforded colourless crystals of 2Tb on standing at room temperature. Yield: 0.26 g, 38%. Anal. Calcd for C82H116KN4O8P4Si4Tb: C, 57.27; H, 6.80; N, 3.26%.

Found: C, 56.67; H, 6.63; N, 3.34%. FTIR v/cm 1 (Nujol): 1,351 (w), 1,303 (w), 1,071 (s), 960 (m), 848 (m), 760 (m), 743 (s), 700 (m), 634 (m), 523 (s). Magnetic moment (Evans method, THF, 298 K): meff 10.55 mB.

Preparation of [Pr(BIPMTMS)2Ag] (6Pr). Toluene (15 ml) was added to a precooled ( 78 C) mixture of 2Pr (1.70 g, 1.00 mmol) and [Ag(BPh4)]

(0.43 g, 1.00 mmol). The resulting yellow suspension was allowed to slowly warm to room temperature with stirring over 16 h. The suspension was ltered and volatiles removed in vacuo and the resulting solid washed with hexanes (10 ml) to afford 6Pr as a colourless powder. Yield 1.18 g, 87%. Recrystallization of a small portion from toluene afforded colourless crystals of 6Pr. Anal. Calcd for C62H76AgN4P4PrSi4 1.25(C7H8): C 57.54; H, 5.87; N, 3.79%. Found: C, 57.44;

H, 5.84; N, 3.69%. 31P{1H} NMR (C6D6, 298 K): d 98.83 (1P, PrCP2), 83.25 (1P, PrCP2), 33.99 (2P, PrCP2) p.p.m. FTIR v/cm 1 (Nujol): 1,244 (w), 1,080 (s), 1,026 (s), 831 (m), 768 (m), 732 (m), 657 (m), 606 (w), 575 (w), 526 (w).

Magnetic moment (Evans method, d8-THF, 298 K): meff 2.93 mB.

Preparation of [Tb(BIPMTMS)2Ag] (6Tb). Toluene (15 ml) was added to a precooled ( 78 C) mixture of 2Tb (1.72 g, 1.00 mmol) and [Ag(BPh4)]

(0.43 g, 1.00 mmol). The resulting yellow suspension was allowed to slowly warm to room temperature with stirring over 16 h. The brown suspension was ltered and volatiles reduced in volume to 2 ml to afford colourless crystals of6Tb upon storage at room temperature. Yield 0.42 g, 31%. Anal. Calcd for C62H76AgN4P4Si4Tb 0.8(C7H8): C, 55.84; H, 5.71; N, 3.85%. Found: C, 55.85;

H, 5.81; N, 3.83%. FTIR v/cm 1 (Nujol): 1,435 (m), 1,243 (w), 1,167 (m), 1,106 (s), 1,060 (s), 833 (s), 769 (m), 715 (m), 693 (m), 611 (w), 527 (w). Magnetic moment (Evans method, d8-THF, 298 K): meff 9.12 mB.

Reaction of [Ce(BIPMTMS)2] with PhCHO. Benzaldehyde (2.5 mg, 24 mmol) was added to a solution of [Ce(BIPMTMS)2] (15 mg, 12 mmol) in d6-benzene (0.4 ml).

The reaction mixture was shaken vigorously, forming a green reaction mixture. The reaction mixture was stored at room temperature for 16 h and then analysed by multinuclear NMR that revealed a small amount of conversion to (Me3SiNPPh2)2C C(H)Ph. The reaction mixture was then heated to 60 C

for 48 h. Analysis of the crude mixture showed quantitative conversion to (Me3SiNPPh2)2C C(H)Ph. All spectroscopic data matched previously

reported data67.

Reaction of [Ce(BIPMTMS)2] with ArCHO. d6-benzene (0.4 ml) was addedto a mixture of 9-anthracene carboxaldehyde (4.9 mg, 24 mmol) and [Ce(BIPMTMS)2] (15 mg, 12 mmol). The reaction mixture was shaken vigorously, forming a green reaction mixture. The reaction mixture was stored at room temperature for 16 h, heated to 60 C for 48 h and heated to 80 C for 48 h. Analysis of the crude mixture showed no conversion to (Me3SiNPPh2)2C C(H)Ar.

Reaction of [U(BIPMTMS)2] with PhCHO. Benzaldehyde (6.4 mg, 60 mmol) was added to a solution of [U(BIPMTMS)2] (40.5 mg, 30 mmol) in d6-benzene(0.4 ml). The reaction mixture was shaken vigorously, forming a brown reaction mixture. The reaction mixture was stored at room temperature for 48 h and then analysed by multinuclear NMR that revealed 95% conversion to (Me3SiNPPh2)2C C(H)Ph. All spectroscopic data matched previously

reported data67.

Reaction of [U(BIPMTMS)2] with ArCHO. d6-benzene (0.4 ml) was added to a mixture of 9-anthracene carboxaldehyde (12.4 mg, 60 mmol) and [U(BIPMTMS)2]

(40.5 mg, 30 mmol). The reaction mixture was shaken vigorously, forming a brown reaction mixture. The reaction mixture was stored at room temperature for96 h and then analysed by multinuclear NMR that revealed 90% conversionto (Me3SiNPPh2)2C C(H)Ar. All spectroscopic data matched previously

reported data40.

Reaction of [Th(BIPMTMS)2] with PhCHO. Benzaldehyde (6.4 mg, 60 mmol) was added to a solution of [Th(BIPMTMS)2] (40.1 mg, 30 mmol) in d6-benzene(0.4 ml). The reaction mixture was shaken vigorously, forming a colourless reaction mixture. The reaction mixture was stored at room temperaturefor 36 h and then analysed by multinuclear NMR that revealed 95% conversion to (Me3SiNPPh2)2C C(H)Ph. All spectroscopic data matched previously

reported data67.

Reaction of [Th(BIPMTMS)2] with ArCHO. d6-benzene (0.4 ml) was added to a mixture of 9-anthracene carboxaldehyde (12.4 mg, 60 mmol) and [Th(BIPMTMS)2]

(40.1 mg, 30 mmol). The reaction mixture was shaken vigorously, forming a yellow reaction mixture. The reaction mixture was stored at room temperature for48 h and then analysed by multinuclear NMR that revealed 95% conversionto (Me3SiNPPh2)2C C(H)Ar. All spectroscopic data matched previously

reported data40.

Data availability. The X-ray crystallographic coordinates (cif format) for structures reported in this article have been deposited at the Cambridge Crystal-lographic Data Centre (CCDC), under deposition numbers 1500929-1500939. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif

Web End =www.ccdc.cam.ac.uk/data_request/cif . All other data are available from the corresponding authors on request.

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Acknowledgements

We are grateful to the Royal Society, the UK Engineering and Physical Sciences Research Council, the Marie Curie IIF Scheme, the European Research Council, The University of Nottingham, The University of Manchester, the UK National Nuclear Laboratory and COST Action CM1006 for generous support and funding, the National Service for

Computational Chemistry Software for access to the Columbus HPC facility and the UK EPSRC National EPR Facility. We thank Dr Emily Smith (University of Nottingham) for recording the XPS spectrum of 3Ce.

Author contributions

M.G., E.L. and D.P.M. synthesized and characterized the compounds. F.T. and E.J.L.M. recorded and analysed the EPR and SQUID data. C.H. and A.C.S. recorded and analysed the XANES data. J.M. and S.T.L. conducted and analysed the DFT and NBO calculations. W.L. and A.J.B. carried out the single crystal X-ray diffraction work. A.K. conducted and analysed the CASSCF/RASSCF and QTAIM data. S.T.L. originated the central idea, supervised the work, analysed the data and wrote the manuscript with contributions from all co-authors.

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