Metal—Metal Bond Making and Breaking in Binuclear Complexes with Phosphine Bridging Ligands

Abstract

Polyfunctional phosphine ligands make useful bridging ligands for constructing binuclear metal complexes. This article is concerned with the use of two ligands -bis(diphenylphosphino)methane (dpm) and 2-diphenylphosphinopyridine (Ph 2 Ppy)-in promoting metal--metal bond forming and breaking reactions. Three specific topics will be covered. These are the ability of dpm to modify the stability of Rh-Rh bonds in interrelated compounds of Rh(I), Rh(II) and Rh(III), the utility of dpm bridging ligands in allowing novel metal-metal bond opening and closing in low valent rhodium and palladium complexes, and the usefulness of Ph 2 Ppy in the stepwise construction of binuclear complexes. Modification of Metal-Metal Bonding in RhodiumComplexes by a Bridging Diphosphine. The yellow, planar complexes, (RNC) 4 Rh + , undergo novel self-association reactions in concentrated solution to form the blue or violet dimers, (RNC) 8 Rh2 2+ , via reaction (1) (1,2). The equilibrium constants for this reaction are strongly L + (1) 2L-Rh-L L L = RNC influenced by solvent and by the size of the substituent on the isocyanide ligand. Not unexpectedly (t-BuNC) 4 Rh + with its bulky substituents is most reluctant to dimerize. Substitution of the bidentate dpm for half of the isocyanide ligands produces the blue, dimeric cation 1. (3) These dimers, which can be prepared with various isocyanide substituents including t-butyl, show no tendency to dissociate into monomers. Similar stabilization of the dimeric structure can be achieved by the use of bifunctional isocyanides like 1,3-diisocyanopropane. (4,5) These form qua-L L \ / Rh---/ \ / l _ 2 + 0097-6156/81/0155-0167$05.00/0