Structure and solution behaviour of rhodium(I) Vaska-type complexes for correlation of steric and electronic properties of tertiary phosphine ligands

This paper deals with the synthesis of rhodium(I) Vaska-type complexes of the general form trans-[Rh(CO)X(PX3)] (X =halide, X aryl or alkyl substituent) when incorporating tertiary phosphine ligands and illustrates their simple application as probes to evaluate steric and electronic effects, specifically in tertiary phosphine ligands, analogous to the Tolman model. They are easy to prepare and do not exhibit the toxicity as the corresponding nickel complexes, while structural and infrared data can be conveniently utilised to estimate ligand properties with relative ease. These Vaska-type complexes are often obtained as by-products when reacting [Rh(mu-Cl)(CO)(2)](2) with bidentate ligands in non-stoichiometric amounts, thus yielding unreacted dimer, which, upon addition of the tertiary phosphine, quantitatively converts to the Vaska-type complex. A representative summary of rhodium(I) Vaska-type systems is reported and structures correlated with steric and electronic properties of related literature complexes. Selected aspects of reactions of the corresponding arsine and stibine analogues are illustrated and solvent effects on equilibrium behaviour as well as the iodomethane oxidative addition described. (C) 2003 Elsevier B.V. All rights reserved.