DELOCALIZED BONDING IN TETRAHEDRAL ORGANOL-TRANSITION METAL-CLUSTERS - AN EHMO STUDY OF METAL VERTEX ROTATIONS IN CO2FE(CO)(9)S, CPMOCO(2)(CO)(8)CR, AND IN CP(2)MO(2)(CO)(4)(RC-CR)

Molecular orbital calculations at the extended Huckel level are used t o rationalize the barriers to vertex rotation in the tetrahedral metal cluster complexes FeCo2(CO)(9)S, 2, and (C5H5)MoCo2(CO)(8)CH, 3. It i s shown that, in accord with experimental observations on 2, rotation of an Fe(CO)(3) fragment through 60 degrees brings about a weakening o f the metal-metal bonding interactions within the FeCo2 triangle. In t he MoCo2 cluster, 3, rotation of the CpMo(CO)(2) fragment about an axi s joining the molybdenum to a central point within the tetrahedron giv es rise to three minima in which the cyclopentadienyl ring is oriented proximal or distal relative to the capping carbynyl moiety, or in the plane of the three metals. The rotation trajectory of the CpMo(CO)(2) vertices in Cp(2)Mo(2)(CO)(4)(HC=CH), 4, has been elucidated by means of a Burgi-Dunitz analysis of the X-ray crystal structures of a serie s of related clusters in which the CpMo(CO)(2) units exhibit a range o f orientations. The calculations suggest that the barriers to vertex r otations in 4 are primarily of steric rather than electronic origin.