H3Ni4

Neutron Diffraction Studies of Tetrahedral Cluster Transition Metal Hydride Complexes HFeCo3(CO)9-(P(OCH3)3)3 and H3Ni4(C5H5)4... 

One motivation to carry out a neutron diffraction investigation of H3Ni4(Cp)4 was to check the possibility of disorder of the hydride ligands over all four faces of the Ni4 tetrahedron. The hydrides were not located from the x-ray data (33, 34). Rather, their positions were inferred from the deviations of the structure from strict tetrahedral symmetry. The observed Cp(i)-Cn-Cp( ) angles (see Table IV) are distorted from the tetrahedral value such that Cp(2), Cp(3), and Cp(4) are bent away from Cp(l). The face defined by Ni(2), Ni(3), and Ni(4) therefore could be expected to be vacant. Our neutron results indicate, that this is indeed the case, with no evidence for disorder of the hydride ligands on the nuclear density maps. 

The metal clusters in HFeCo3(CO)g(P(OMe)3)3 and H3Ni4(Cp)4 contain different numbers of electrons. The former cluster is a closed-shell structure (60 electrons) while the latter contains 63 electrons and is paramagnetic with S... 

Mean cobalt-cobalt and nickel-nickel distances observed in these complexes are very close to interatomic distances determined at ambient temperatures in cobalt and nickel metals (Co-Co 2.489(7) A vs. 2.507 A in a-cobalt (33) Ni-Ni 2.469(6) A vs. 2.492 A in the metal (39)). The mean M-H bond lengths, as well as hydride displacements from M3 faces, are less for nickel in H3Ni4(Cp)4 than for cobalt in HFeCo3(CO)9(P(OMe)3)3. Although the differences are marginally significant within error limits (Ni-H 1.691(8) A vs. Co-H 1.734(4) A displacements from plane Ni3 0.90(3) A vs. Co3 0.978(3) A), they are in the expected direction since the covalent radius should vary inversely with atomic number within a transition series. However, other effects such as the number of electrons in the cluster also can influence these dimensions. 

The saturated (60 e-) tetrahedral cluster H4Co4(C5Hs)4 and the electron-excess (63 e ) tetrahedral cluster H3Ni4(CsHs)4 were analyzed with X-rays by Huttner and Lorenz230-231). In the former case the face-bridging H atoms were located and were found to be displaced an average of 0.8 A from the Co3 faces of the tetrahedron (Fig. 54). The average measured Co—H distance was 1.67(7) A. 

Fig- 55. a The structure of H3Ni4Cp4, determined by a neutron diffraction study (Ref. 232). One of the CsHs rings has been removed for clarity, b The H3Ni4 core of this molecule, which resembles a cube with a missing comer... 

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