NATURE OF METAL-METAL INTERACTIONS IN SYSTEMS WITH BRIDGING LIGANDS .2. ELECTRONIC AND MOLECULAR-STRUCTURE OF THE CYCLOPENTADIENYLNITROSYLCOBALT DIMER AND RELATED MOLECULES
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Ab initio calculations on (5-C5H5)2Co2(-NO)2 show that the singlet ground state possesses a bent, butterfly core (Co2N2) in contrast to the published X-ray structure but in agreement with both the solution and solid-state IR measurements. This bent, singlet ground state lies nearly 100 kJ mol1 below the planar singlet or triplet states, which are nearly degenerate. Our results resolve the discrepancy between other recent theoretical results which predict a low-lying triplet state for the planar structure and the magnetic susceptibility measurements which prove that the molecule is diamagnetic with no low-lying paramagnetic states. Further evidence that our predicted structure is correct comes from parallel calculations, reported here, that correctly predict the structure of the related (5-C5H5)2Fe2(-NO)2, (5-C5H5)2Co2(-CO)2, and (5-C5H5)2Ni2(-CO)2 molecules. Detailed topological analysis of the electron density of (5-C5H5)2Co2(-NO)2 shows that at certain geometries and levels of electron correlation there exists a Co-Co bond but at other geometries and levels of electron correlation the Co-Co bond critical point vanishes, to be replaced by a Co2N2 ring critical point. Thus, we must conclude that, like Co2(CO)8, the Co-Co interaction is near the region of change from having to not having a bond. Orbital and deformation density analyses of the bent dimer support the traditional view of a Co-Co single bond, albeit a weak one. A redetermination of the structure of (5-C5H5)2Co2(-NO)2 might result in a different Co-Co bond length, which would help resolve the nature of the Co-Co interaction and finally resolve the longstanding dilemma of the lack of variation in Co-Co bond distances with bond order as the bridging ligands are changed from nitrosyl to carbonyl. 1993, American Chemical Society. All rights reserved.
