Reaction mechanisms of metal–metal bonded carbonyls. Part V. A kinetic study of the reaction of diphenylacetylene with hexacarbonylbis(tri-n-butylphosphine)dicobalt

Abstract

The kinetics of reaction of diphenylacetylene with hexacarbonylbis(tri-n-butylphosphine)dicobalt in decalin at 100 °C have been studied. The reaction proceeds in two stages, the initial product being the acetylene-bridged complex [(OC)₃Co(µ-C₂Ph₂)Co(CO)₂PBu₃]. This then undergoes a substitution reaction with tributylphosphine, released in the first stage, to form (µ-C₂Ph₂){Co(CO)₃PBu₃}₂. The first stage proceeds by two main paths, one of which rises to a limiting rate with increasing [C₂Ph₂], the other being first order in [C₂Ph₂]. Both paths are retarded by carbon monoxide and by tributylphosphine but quantitative study of the latter effect is made difficult by the occurrence of direct attack by phosphine on the complex, apparently to form a more highly substituted cobalt carbonyl. Nevertheless, it can be concluded that the two paths are probably distinguished by whether the acetylene attacks a reactive intermediate before or after reversible dissociation of both a phosphine and a carbon monoxide ligand. When dissociation occurs before attack by acetylene it appears that carbon-monoxide dissociation followed by phosphine dissociation occurs at least ten times more frequently than the reverse order. The reactive intermediate involved when attack by the acetylene occurs before dissociation can be formulated as a carbonyl-bridged complex, formed by metal migration, and the sequence of dissociation is again predominantly CO followed by phosphine.