STRUCTURE-REACTIVITY CORRELATIONS FOR THE DISSOCIATIVE UNCATALYZED ISOMERIZATION OF MONOALKYLBIS(PHOSPHINE)PLATINUM(II) SOLVENTO COMPLEXES

Complexes of the type cis-[PtL2Me2] (1-14) (L = an extended series of phosphines of widely different steric and electronic properties) were synthesized, characterized, and used as precursors for the formation o f cis-monoalkylplatinum(II) solvento species in methanol. The cleavage of the first platinum-alkyl bond by protonolysis is a fast process, b ut the sequent cis to trans isomerization of the cationic solvento spe cies [PtL2(Me)(MeOH)](+) is relatively slow and it can be monitored us ing P-31 NMR or conventional spectrophotometry. A large collection of H-1 and P-31 NMR data for cis-[PtL2Me2], cis-[PtL2Me(MeOH)](+), and tr ans-[PtL2Me(MeOH)](+) complexes showed interesting dependencies upon t he size, the sigma-donor capacity, and mutual position of the phosphin es in the coordination sphere of the metal. The rate constants for iso merization of cis-[PtL2Me(MeOH)](-) were resolved quantitatively into steric and electronic contributions of the phosphine ligands, by means of correlations with parameters which reflect their sigma-donor abili ty (chi values) and steric requirements (Tolman's cone angles, theta). The electronic and steric profiles obtained for these reactions are d iscussed within the framework of a mechanism which involves dissociati ve loss of the solvent molecule and interconversion of two geometrical ly distinct 3-coordinate T-shaped 14-electron intermediates. The facto rs controlling the stability of these coordinatively unsaturated speci es are discussed. The electronic and steric influences of phosphines a s ''spectator'' ligands in a dissociative process are compared with th ose shown by these ligands when used as nucleophiles in associative su bstitution processes. The activation parameters Delta H-not equal and Delta S-not equal were measured using both conventional isothermal and non-isothermal spectrophotometric kinetics.