Kinetics and mechanisms of the ligand substitution reactions of cyclopentadienyl M tetracarbonyl, indenyl M tetracarbonyl, M = niobium or tantalum, and molybdenum(2) (carbon monoxide)(6) [mu-eta(5),eta(5)-cyclopentadienylide-carbon alkyl(2)-cyclopentadien

Kinetic studies were performed for carbon monoxide substitution reactions of cyclopentadienyl niobium/tantalum tetracarbonyl (η5-C ₅H₅)M(CO)₄, M = Nb, Ta, and indenyl niobium/tantalum tetracarbonyl (η5-C₉H₇)M(CO) ₄, M = Nb, Ta, with phosphines to study the indenyl ligand effect in the fifth group transition metals. The (η5-C₅H ₅)M(CO)₄, M = Nb, Ta, has been found to react by the dissociative or dissociative-interchange mechanism, while (η5-C ₉H₇)M(CO)₄, M = Nb, Ta, has been found to react by the associative mechanism. This study concluded that the indenyl ligand effect in niobium and tantalum complexes is expressed by the increased rate of ligand substitution in indenyl complexes and by the change of the mechanism of ligand substitution from dissociative (in the cyclopentadienyl complexes) to the associative (in the indenyl complexes). The change of the mechanism in this case is attributed to the ability of the indenyl ligand to undergo so-called ring slippage from η5 to η3 configuration in the transition state preserving an 18-electron stable intermediate.; The photochemical carbonylation of hydrocarbons and aromatic compounds by Rh(I) or Ir(I) complexes of the type trans-MCl(CO)(PR ₃)₂ (M = Rh, Ir; R = alkyl) has been originally proposed by Bergman and first reported by Eisenberg. In the current investigation we used trans-RhCl(CO)(PMe₃)₂ to study carbonylation of supercritical ethane in the presence of carbon monoxide, and carbonylation of ethane in supercritical carbon dioxide in the presence of carbon monoxide. Attempts to carbonylate methane in a similar gas mixture failed but lead to new chemistry of carbon dioxide activation by the rhodium complex. Spectral evidence for transient formation of a neutral metalloannydride from reaction of rhodium complex with CO₂ are presented for the first time.; Kinetic studies were performed for carbon monoxide substitution by phosphine ligands in Mo₂(CO)₆[μ-η5, η 5-C₅H₄-CR₂-C₅H₄]. Reaction of ligand substitution in this bimetallic complex takes place by dissociative pathway due to ability of this molecule to form a carbonyl bridged stable intermediate upon dissociation of carbon monoxide. The dissociative nature of ligand substitution is supported by the large positive values of the entropy of activation and the independence of the rate constant of the reaction on the nature of phosphine ligand.