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 -C 6 H 4 C(CF 3 ) 2 O), was obtained by the reaction of Rf 2 P − Li + generated in situ with [Cp*RhCl 2 ] 2 . X-ray structural analysis of 4 showed that the hypervalent phosphorus ligand Rf 2 P acted as a bidentate ligand coordinating to the Rh center through the phosphorus and one oxygen atom of a Martin ligand. The Rh center became coordinatively saturated and was stabilized by the additional coordination of the oxygen atom. However, in solution the equilibrium between the bidentate ( 4A ) and the monodentate ( 4B ) species was observed ( 4A : 4B =6:1 in C 6 H 6 , 4:1 in CHCl 3 , 2:1 in THF, 1.6:1 in CH 2 Cl 2 ), and the monodentate species 4B became predominant in polar solvents ( 4A : 4B =1:2 in CH 3 CN, 1:2.5 in CH 3 NO 2 , 1:3 in DMSO). Therefore, the strong Lewis acidic Rh center can be regarded as masked in the solid state but in solution it becomes unmasked to 4B , in which a solvent is coordinating to the Rh center.