Experimental Analysis of the Antibonding Trimetal Character of the HOMO in the Paramagnetic Fischer-Palm Ni3(η5-C5H5)3(μ3-CO)2 Cluster: Synthesis and Stereochemical Characterization of the Electronically Equivalent 5-C5Me5)CoNi2(η5-C2H5)2(μ3-CO)2>- Monoanion and ...

Article abstract of DOI:10.1021/ja00375a019

The preparation and characterization by X-ray diffraction and other physical methods are reported for three triangular metal cyclopentadienyl carbonyl clusters, viz., the monoanion (2), isolated as the ⁺ salt, from the one-electron reduction of (η⁵-C2Me5)CoNi2(η⁵-C2H5)2(μ₃-CO)2 (1) with potassium naphthalenide, the monoanion (4), isolated as the ⁺ salt, from the one-electron reduction of the Fischer-Palm (FP) Ni3(η⁵-C2H5)3(μ₃-CO)2 (3) with potassium acenaphthalenide, and Ni3(η⁵-C5Me5)3(μ₃-CO)2 (5), produced by the photochemical reaction of Ni2(4h⁵-C5Me5)2(μ₂-CO)2 and nickelocene.Comparative structural features of the monoanions 2 and 4 provide unambiguous experimental evidence for the Strouse-Dahl bonding model (1969), which postulated that the HOMO in the classical FP molecule 3 is primarily an in-plane, trimetal antibonding orbital of nondegenerate a₂' representation under D_3h symmetry.The most notable structural difference upon the formation of 2 from 1, both of whose central CoNi2(CO)2 cores closely conform to C_2υ-2mm symmetry, is that the Ni-Ni bond in the paramagnetic monoanion has enlarged by 0.062 Angstroem from 2.326 (2) Angstroem in 1 to 2.388 (2) Angstroem in 2 relative to the smaller but significant increase in the two equivalent Co-Ni bonds by 0.021 Angstroem from 2.371 Angstroem in 1 to 2.392 Angstroem in 2.These bond-length increases reflect the addition of the unpaired electron in 2 to an antibonding trimetal MO which for the CoNi2 cluster has substantially greater Ni 3d AO than Co 3d AO character.The resulting mean of 2.390 Angstroem for the virtually identical Co-Ni and Ni-Ni bond lengths in 2 is experimentally the same as that of 2.389 (2) Angstroem found forthe three crystallographically identical Ni-Ni bond lengths in the electronically equivalent 3.The observation that the monoanion 4 has an average Ni-Ni bond length of 2.421 Angstroem, which is 0.032 Angstroem larger than that in the neutral FP parent 3, provides further proof for the trimetal antibonding character of the HOMO containing one electron in 3 and two electrons in 4; its nondegeneracy is conclusively demonstrated from magnetic susceptibility measurements which show 4 to be diamagnetic at room temperature.The slight observed distortion in the Ni3(CO)2 core of 4 from a regular D_3h configuration toward C_s-?_υ symmetry may be attributed to electronic considerations, by which the HOMO of 4 can thereby acquire carbonyl character which is symmetry forbidden for the HOMO of 3 under threefold rotational site symmetry.The structural determination and refinement of 5, an unprecedented example (to our knowledge) of a triangularly bonded metal cluster with three pentamethylcyclopentadienyl ligands, were complicated by a crystal-twinning problem, which was overcome by the use of the least-squares program RAELS.This permethylated analogue of 3 possesses crystallographic C_3h-3/m site symmetry...