Reaction pathways in polynuclear metal carbonyls

Abstract

This study concentrates on the reactivity, characterization, and synthesis of transition metal clusters. The primary goals behind this investigation were: (1) to determine whether or not electronic information can be transmitted along the metal-metal bonds of a cluster; (2) to access the importance of metal-metal bond homolysis during substitution reactions; (3) to identify the reactive sites of a cluster; and (4) to develop a systematic route to the synthesis of new transition metal cluster compounds. A kinetic investigation of the ligand substitution processes of Co₄(CO)₉(tripod) {tripod = HC(PPh₂)₃} and its phosphine substituted derivatives is reported. In this cluster system, the tripod group coordinates to three different metal atoms, reducing the chance of cluster fragmentation. Rate constants and activation parameters for the CO displacement reactions of the cobalt-tripod clusters were found to be consistent with a dissociative process. The electronic and steric influences of the coordinated phosphine ligands are examined. In order to accurately access the kinetic information and determine exactly how the phosphine ligands affect the reaction rate, the molecular arrangement of these clusters must be firmly established. In this regard we report the X-ray structures of several key species: Co₄(CO)₉(tripod); Co₄(CO)₈(PMe₃)₂(tripod); Co₄(CO)₇(PMe₃)(,2)(tripod); and Co₄(CO)₇(dppm)(tripod) {dppm = H₂C(PPh₂)₂}. The ligand framework in the double-chelated cluster, Co₄(CO)₇(dppm)(tripod) hinders the intramolecular ligand migration process. Therefore, this cluster can be used to determine the exact site a carbon monoxide group dissociates from. Trinuclear {[M₃(CO)₁₂S][PPN₂)} and tetranuclear {[M₄(CO)₁₇S][PPN₂)} group six (M = Cr,Mo,W) clusters have been synthesized by utilizing a sulfur atom as a template to collect mononuclear metal fragments. This method allows the stepwise construction both homonuclear and heteronuclear clusters. Both the intramolecular and intermolecular carbon monoxide migration processes of these group 6 clusters are investigated.