Complexes of the platinum metals. Part 26. Multinuclear nuclear magnetic resonance studies on rhodium(II) carboxylate adducts

Abstract

A multinuclear (¹H, ^l3C, ³¹P, and ¹⁰³Rh), variable-temperature, n.m.r. study on some [Rh₂(O₂CR)₄]–P(OR′)₃ systems (R = Me, Et, Pr, or Ph; R′= Me, Et, or Ph) is reported. The ambient-temperature (298 K) spectra, which generally display broad unresolved signals with no discernible internuclear couplings, are consistent with the establishment in solution of dynamic equilibria of the form [{(R′O)₃P}Rh(O₂CR)₄Rh{P(OR′)₃}]⇌[{(R′O)₃P}Rh(O₂CR)₄Rh(solv)]+ P(OR′)₃(solv = solvent). Low-temperature (ca. 213 K) spectra of the same solutions show fully resolved resonances indicative of a frozen equilibrium mixture containing mono and bis adducts together with free phosphorus-donor ligands. The large ³¹P chemical shift difference (ca. 50–60 p.p.m.) between the mono and bis adducts points to the operation of a strong trans influence across the binuclear metal centre. This has been confirmed by a study of ca. 30 mixed adducts [{(MeO)₃P}Rh(O₂CMe)₄RhL] in which ³¹P chemical shifts and ²J(RhP) coupling constants have been employed to establish a trans-influence series for the ligands L.