The influence of phosphane coligands on the nuclearity of rhodium(I) 4-thiolatobenzoic acid complexes

[RhCl(PR3)(3)] (R = Ph, Et) reacts with the potassium salt of 4-mercaptobenzoic acid to give a mixture of the monomeric and dimeric complexes, [Rh(SC6H4COOH)(PR3)(3)] and [{Rh(mu-SC6H4COOH)(PR3)(2)}(2)], respectively. With the labile PPh3 coligand, the dimer is the major product, while for the electron-richer coligand PEt3, the equilibrium is easily shifted to the monomer by the addition of excess PEt3. Phosphane dissociation and dimerization could be prevented by using the chelating coligand PPh(C2H4PPh2)(2). [{Rh(mu-SC6H4COOH)(PPh3)(2)}(2)] (2b), [Rh(SC6H4COOH)-(PEt3)(3)] (3a), and [Rh(SC6H4COOH){PPh(C2H4PPh2)(2)}](4) were fully characterized by nuclear magnetic resonance and infrared spectroscopy, mass spectrometry, and elemental analysis. The molecular structures of 2b and 4 were determined by X-ray structure analysis. In solution, the lability of the phosphane ligands leads to the decomposition of 2b. One of the decomposition products, namely, the mixed-valent complex [{(RhRhIII)-Rh-I(mu-SC6H4COO)(mu-SC6H4COOH)(SC6H4COOH)(PPh3)(3)}(2)] (5), was characterized by X-ray structural analysis. The dinuclear rhodium(III) complex [{Rh(mu-SC6H4COO)(SC6H4COOH)(PEt3)(2)}(2)] (6) was shown to be a byproduct in the synthesis of 3a, and this demonstrates the reactivity of the rhodium( I) complexes toward oxidative addition. The structurally characterized complexes 2b, 4, 5, and 6 show hydrogen bonding of the free carboxyl groups.