Axial-ligand substitution reactions of dirhodium(II) tetraacetate with phosphines and phosphites in acetonitrile

Abstract

The kinetic and thermodynamic parameters for the axial-ligand substitution reactions of the bis(acetonitrile) adduct of dirhodium(II) tetraacetate with a variety of phosphines and phosphites in acetonitrile have been measured. The rate-determining formation of the Rh/sub 2/(O/sub 2/CCH/sub 3/)/sub 4/(CH/sub 2/CN)PR/sub 3/ adduct occurs with a rate constant that is independent of the nature of the ligand: k/sub 1/(25.0/sup 0/C) = (1.05 +/- 0.05) x 10/sup 5/ M/sup -1/ s/sup -1/, ..delta..H/sub 1//sup ++/ = 10.9 +/- 0.6 kcal mol/sup -1/, and ..delta..S/sub 1//sup ++/ = 1 +/- 2 cal K/sup -1/ mol/sup -1/. Slightly higher rate constants (1.6 x 10/sup 5/ M/sup -1/ s/sup -1/ at 25.0/sup 0/C) were found for several diphosphines (PPh/sub 2/(CH/sub 2/)/sub n/PPh/sub 2/, n = 2-4) and are attributed to the presence of two ligand coordination sites. A dissociative mechanism is proposed for substitution of the axially coordinated acetonitrile, and the rate parameters are compared with values reported for substitutions of oxygen-donor solvent molecules on the dirhodium(II) complex. The stability constants for the mono- and bisadducts have been measured at 25/sup 0/C with K/sub 1//K/sub 2/ approx. 20. The dependence of K/sub 1/ and K/sub 2/ on the nature of the phosphine or phosphite correlatesmore » well with the sigma-donor strength of the ligand, with ..pi..-acceptor strengths and steric effects being relatively insignificant.« less