A reinvestigation of the isomerization of alkyne dicobalt hexacarbonyl complexes by the perpendicular-to-parallel twist of the alkyne: Influence of the induced carbonyl reorientations

Abstract

The isomerization of the (CH 2H 2)Co 2(CO) 6 complex by the perpendicular-to-parallel twist of the alkyne has been reinvestigated. The large monotonous increase of the energy found earlier by EH calculations turns out not to be due to the twisting process itself but rather to the fact that no allowance was made for relaxation of the carbonyl orientations; upon reorientation of the carbonyls into a trigonal bipyramid geometry about the cobalt centers the increase in energy disappears almost completely. A Mulliken population analysis does not reveal the expected analogy between the reaction with the isomerization of tetrahedrane into cyclobutadiene. Calculations on intermediate structures along a twisting reaction path in which the carbonyls undergo a continuous reorientation induced by the acetylene twist, lead to a state correlation diagram in which a level crossing is observed, allowing the reaction to be classified as symmetry forbidden. The same diagram indicates, however, that there is an upper limit for the activation energy of this isomerization of 117 kJ/mol, a value which can be regarded as within the range of allowed reactions. This result is considered in the light of the available kinetic data on various substitution reactions of (alkyne)Co 2(CO) 6 complexes.