Synthesis and crystal structure of a rhodium(III) complex bearing a hypervalent phosphorus(V) ligand as a bidentate ligand and equilibrium between bidentate and monodentate structures in solution

A new Rh(III) complex bearing a hypervalent phosphorus(V) ligand, Rf(2)PRh( Cp*)Cl (4) (Rf=Martin ligand; o-C6H4C(CF3)(2)O-), was obtained by the react ion of Rf(2)P - Li+ generated in situ with [Cp*RhCl2](2). X-ray structural analysis of 4 showed that the hypervalent phosphorus ligand Rf(2)P acted as a bidentate ligand coordinating to the Ph center through the phosphorus an d one oxygen atom of a Martin ligand. The Ph center became coordinatively s aturated and was stabilized by the additional coordination of the oxygen at om. However, in solution the equilibrium between the bidentate (4A) and the monodentate (4B) species was observed (4A:4B = 6:1 in C6H6, 4:1 in CHCl3, 2:1 in THF: 1.6:1 in CH2Cl2), and the monodentate species 4B became predomi nant in polar solvents (4A:4B = 1:2 in CH3CN, 1:2.5 in CH3NO2, 1:3 in DMSO) . Therefore, the strong Lewis acidic Ph center can be regarded as masked in the solid state but in solution it becomes unmasked to 4B, in which a solv ent is coordinating to the Rh center. (C) 1999 Published by Elsevier Scienc e S.A. All rights reserved.