Molecular orbital calculations at the extended Hückel level are used to rationalize the barriers to vertex rotation in the tetrahedral metal cluster complexes FeCo2(CO)9S, 2, and (C5H5)MoCo2(CO)8CH, 3. It is shown that, in accord with experimental observations on 2, rotation of an Fe(CO)3 fragment through 60° brings about a weakening of the metal–metal bonding interactions within the FeCo2 triangle. In the MoCo2 cluster, 3, rotation of the CpMo(CO)2 fragment about an axis joining the molybdenum to a central point within the tetrahedron gives 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 Cp2Mo2(CO)4(HC≡CH), 4, has been elucidated by means of a Bürgi–Dunitz analysis of the X-ray crystal structures of a series of related clusters in which the CpMo(CO)2 units exhibit a range of orientations. The calculations suggest that the barriers to vertex rotations in 4 are primarily of steric rather than electronic origin. Key words: metal clusters, vertex rotations, EHMO calculations.