Tetrahedral Cyclopentadienylmetal Carbonyl Clusters of Manganese and Chromium: A Theoretical Study.

Tetranuclear CpM(CO) clusters have been synthesized for iron and vanadium but not for the intermediate first-row transition metals manganese and chromium. All of the low-energy structures of these "missing" CpM(CO) (M = Mn, Cr) species are shown by density functional theory to consist of a central M tetrahedron with each of the four faces capped by a μ-CO group. The individual low-energy structures differ in their spin states and in their formal metal-metal bond orders along the six edges of their central M tetrahedra. The two low-energy CpMn(μ-CO) structures are a triplet structure with all Mn-Mn single bonds and a singlet structure with one Mn≡Mn triple bond and five Mn-Mn single bonds along the six tetrahedral edges. Related low-energy CpCr(μ-CO) structures include a quintet structure with all Cr-Cr single bonds and a singlet structure with two Cr≡Cr triple bonds and four Cr-Cr single bonds. However, the potential energy surface of the CpCr(CO) system is complicated by three other structures of comparable energies including two triplet structures and one quintet structure with various combinations of single, double, and triple chromium-chromium bonds in the central Cr tetrahedron.