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 (η5-C5H5)2Co2(µ-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 mol−1 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 (η5-C5H5)2Fe2(µ-NO)2, (η5-C5H5)2Co2(µ-CO)2, and (η5-C5H5)2Ni2(µ-CO)2 molecules. Detailed topological analysis of the electron density of (η5-C5H5)2Co2(µ-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 (η5-C5H5)2Co2(µ-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.
author list (cited authors)
Low, A. A., & Hall, M. B.