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Pentacoordinate hydrogen atoms were identified by single-crystal neutron diffraction analysis of [N(CH)4]3[H2Rh,3(CO)24]. The hydrogen atoms are located in square pyramidal cavities of the Rhl3 cluster, in positions almost coplanar with the Rh4 faces on the surface of the cluster. They are slightly displaced inward, toward the central rhodium atom of the cluster, with average H-Rh(central) and H-Rh(surface) distances of 1.84(2) and 1.97(2) angstroms, respectively. This result shows that hydrogen, which normally forms only one bond, can be attached to five other atoms simultaneously in a large metal cluster.
In the late 1970s, it was shown by neutron diffraction (1-3) that H atoms could exist in the interstitial sites of metal cluster compounds such as [HCo6(CO)15], [HRu6(CO)is]-, [HNi12(CO)21]3-, and [H2Ni12 (CO)zl]2-. The H atoms were found to be either six-coordinate (>6-H), located at the centers of metal octahedra (1, 2), or in triply bridging positions (>3-H) on the interior faces of metal octahedral cavities (3). Here we report an example of pentacoordinate hydrogen (mu5-H) found in the square pyramidal cavities of the cluster complex [H2Rhl3(CO)24]3
The [H2Rhl3(CO)24]3- anion, first reported in 1975 (4), is a member of a family of clusters, [HxRhl3(CO)24](5-x)- (x = 1 to 4), whose members are interconvertible by means of a series of acid-base equilibria (4-9). The Na salt of the title anion was prepared by refluxing a mixture of Rh4(CO)12 and NaOH in isopropanol under an atmosphere of H2, followed by addition of Na2CO3 (8). It was then converted to the tetramethylammonium salt by metathesis with [N(CH3)4]CI. Slow recrystallization from a mixture of isopropanol and methyl ethyl ketone yielded crystals of [N(CH3)4]3[H2Rhl3(CO)24].(CH3COC2H5) just large enough for neutron diffraction. A small crystal was first used for a preliminary x-ray diffraction analysis (10) to obtain the atomic positions of the non-H atoms. Neutron diffraction data were collected at 15 K on a crystal with volume 1.8 mm3 over an 11week period at the Brookhaven High-Flux Beam Reactor (11-13). Difference-Fourier maps, phased by the non-H positions, readily revealed the two hydridic peaks, as well as those corresponding to the H atoms of the tetramethylammonium ions and the methyl ethyl ketone solvate molecule. The structure was refined to a final agreement factor of R(F) = 0.12 for 2678 reflections with I > 3t(I) (...