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Abstract
Chapter 1. The first cyclopentadienylnickel amido complexes have been isolated and characterized as (($\eta$-$\rm C\sb5Me\sb4R\sp\prime)Ni(\mu$-NHR)) $\sb2.$ These complexes are dimers in solution and in the solid state, as shown by the synthesis of mixed amido complexes, by NMR spectroscopy, and by crystallographic studies on three such compounds. Resonances in the $\sp1$H NMR spectra of these diamagnetic dimers display unusual chemical shifts that are explained on the basis of ring-current anisotropy and inductive effects. The dimers undergo reversible cis/trans isomerization at elevated temperatures; mechanistic studies indicate that this process proceeds through cleavage of one dative nitrogen-nickel bond, rate-limiting rotation of the amido group, and recoordination to regenerate the bridge. The dimer (Cp*Ni($\mu$-NH(p-tol))) $\sb2$ reacts with CO and with $\rm\sp{t}$BuNC to give the insertion products Cp*Ni(CO)(C(O)NH(p-tol)) and Cp*Ni(CN$\rm\sp{t}Bu)(C(N\sp{t}Bu)NH$(p-tol)) (7), respectively, and with PMe$\sb3$ to give the unstable monomeric amido complex Cp*Ni(PMe$\sb3)$-(NH(p-tol)) (5).
Chapter 2. Reactive monomeric amido and methoxo complexes, Cp*Ni(PEt$\sb3)$NHTol and Cp*Ni(PEt$\sb3)$OMe, have been synthesized and fully characterized. The former is the first monomeric 18-electron nickel amide to be synthesized and the latter is the first structurally characterized monomeric nickel methoxide complex. The amido complex Cp*Ni(PEt$\sb3)$NHTol reacts with various Bronsted acids (HX) to produce complexes of the type Cp*Ni(PEt$\sb3)$X (X = NHAr, OR, Osilica, SR), and compounds with hydridic hydrogens to give the hydridonickel complex Cp*Ni(PEt$\sb3)$H. The polarity of Ni-N and Ni-O bonds is also demonstrated by reactions with alkali metal salts and trimethylsilyl chloride, and by the crystallographic and NMR characterization of phenol adducts of Cp*Ni(PEt$\sb3)$OTol.
Chapter 3. The phosphine ligands in Cp*Ni(PEt$\sb3)$X (X = OTol, SAr) exchange with PMe$\sb3$ through an associative mechanism; the rate increases with the electronegativity of X. The thermodynamics of equilibrium reactions interconverting Cp*Ni(PEt$\sb3)$X + HX$\sp\prime$ and Cp*Ni(PEt$\rm\sb3)X\sp\prime$ + HX have been analyzed. Instead of following a 1:1 correlation, the correlation between H-X and M-X bond energies shows a preference for nickel binding to more electronegative ligands. Examples of similar thermodynamic preferences occur throughout transition metal chemistry. The evidence suggests that these results are attributable to a large electrostatic component in the bonding between Ni and X. This qualitative E-C model explains the reactivity, thermodynamics, and phosphine exchange rates of this series of nickel complexes, and may be general to many metal-ligand bonds.
Chapter 4. The syntheses of Cp*Ni(PEt$\sb3)$Me and Cp*Ni(PEt$\sb3)$Br have been accomplished in high yield. The X-ray crystal structures of several Cp*Ni(PEt$\sb3)$X complexes provide a large sample of similar structures for the analysis of ring distortions as well as a systematic variation of X in order to evaluate the trans influence felt by opposite sides of the cyclopentadienyl ring. Major advances include demonstration that Cp can reduce its electron donation to a metal without "slipping," the rational analysis of Cp distortions, and a well-supported trans-influence series for cyclopentadienylmetal complexes.
