Delocalized bonding in tetrahedral organo-transition metal clusters: an EHMO study of metal vertex rotations in Co₂Fe(CO)₉S, CpMoCo₂(CO)₈CR, and in Cp₂Mo₂(CO)₄(RC≡CR)

Abstract

Molecular orbital calculations at the extended Huckel level are used to rationalize the baniers to vertex rotation in the tetrahedral metal cluster complexes F~CO,(CO)~S, 2, and (C5H5)MoCo,(CO),CH, 3. It is shown that, in accord with experimental observations on 2, rotation of an Fe(CO), fragment through 60' brings about a weakening of the metal-metal bonding interactions within the FeCo, triangle. In the MoCo, cluster, 3, rotation of the CpMo(CO), 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 orientedproximal or distal relative to the capping carbynyl moiety, or in the plane of the three metals. The rotation trajectory of the CpMo(CO), vertices in Cp,Mo,(CO),(HC=CH), 4, has been elucidated by means of a Burgi-Dunitz analysis of the X-ray crystal structures of a series of related clusters in which the CpMo(CO), 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.

Reaction of ruthenium and iron alkynyl complexes with [Mo2(CO)4(η-C5H5)2] ‡

“…The NMR fluxionality of alkyne adducts of the type reported here has long been known 31 and a recent crystallographic and theoretical study 32 has elegantly implicated Mo(CO) 2 (η-C 5 H 5 ) vertex rotation in the equilibration. In the present work the geometry of the Mo(CO) 2 (η-C 5 H 5 ) vertices is similar to that found in the preceding survey of 22 related vertices 32 where the Cp centroid ᎐Mo᎐CO angles were found to lie in a narrow range of 114.3 ± 3.7Њ. These workers applied a Bürgi-Dunitz analysis of the trajectory of the ring centroid associated with the molybdenum cyclopentadienyl fragment to a number of crystallographically characterised compounds and concluded that racemisation of these inherently chiral molecules proceeds via vertex rotation giving initially a molecule Complex 3a exhibits fluxional behaviour in solution, previously observed for simple alkyne adducts 31 and the complexes 5.…”

“…30-32 A number of processes are thought to occur: (1) a relatively low-energy process interchanges semibridging and terminal CO groups on the molybdenum vertices and (2) a higher-energy process equilibrates the C 5 H 5 groups on the Mo atoms, which is thought to occur with some degree of vertex rotation. 32 In the present work the alkyne is unsymmetrical and as such we have not reached the slow exchange limit at the lowest temperature measured, in spite of the steric demand of the alkyne. The Fig.…”

“…0.16 Å in 3b and by a similar amount in the structure of 4b. The geometry of the Table 1 Selected bond lengths (Å) and angles (Њ) for complexes 3 and 4 1)᎐C (11), C( 12) Mo( 2)᎐C (21), C( 22) Ru( 3)᎐C (31), C (32) Mo( 1)᎐Mo( 2)᎐C( 1) Mo( 1)᎐Mo( 2)᎐C( 2) Mo( 2)᎐Mo( 1)᎐C( 1) Mo( 2)᎐Mo( 1)᎐C( 2) Mo( 1)᎐C( 1)᎐Mo( 2) Mo( 1)᎐C( 2)᎐Mo( 2) C( 1)᎐Mo( 1)᎐C( 2) C( 1)᎐Mo( 2)᎐C( 2) Ru( 3)᎐C( 1)᎐C( 2) Mo( 1)᎐C( 11)᎐O( 11) Mo( 1)᎐C( 12)᎐O( 12) Mo( 2)᎐C( 21)᎐O( 21) Mo( 2)᎐C( 22)᎐O( 22) C( 12)᎐Mo( 1)᎐C( 11) C( 22)᎐Mo( 2)᎐C( 21) Mo( 1)᎐Mo( 2)᎐C(…”