Electronic structure of [(CpCr){(CO)(3)M}]mu-Cot (M = Cr, Fe): A theoretical study

The electronic structure of two cyclooctatetraene-bridged dinuclear first-r ow transition metal complexes of the type [(CpM){(CO)(3)M'}]mu-Cot (M = Cr; M' = Fe (1), Cr (2)) was investigated by complete active space self-consis tent field (CASSCF) calculations. In this context the differences in the bi nding capabilities of the complex fragments CpM and (CO)(3)M are discussed on the basis of extended Huckel molecular orbital (MO) calculations. The ge ometries used for the CASSCF calculations for complex 1 were obtained from the crystal structure. For 2 a model structure was established by geometry optimization using density functional methods. The CASSCF results agree wel l with the experimental findings and provide insight into the binding situa tion of the two compounds. Complex 1 can be regarded as being composed of a chromocene-like subunit CpCr(eta(5)-C5H5) and the fragment (CO)(3)Fe(eta(3 )-C3H3). A direct metal-metal bond is found, involving one initially singly occupied orbital of each fragment, leading to a doublet ground state for 1 with the remaining unpaired electron localized at the chromium center. For 2 no such direct metal-metal bond can be recognized. A very weak direct me tal-metal interaction is induced by electron donation from the Cot(2-) liga nd into a formally unoccupied metal-metal binding orbital combination. In t he quartet ground state all three unpaired electrons are localized at the c hromium center of the formally doubly positive charged CpCr unit, on which complex fragment [(CO)(3)Cr(eta(5)-Cot)](2-) acts like a cyclopentadienyl l igand. The coordination sphere of the chromium center of the CpCr unit rese mbles that of a metallocene metal center and its metal 3d occupation scheme corresponds to that of vanadocene.