Rhodium(III) complexes with cyanide and sulfur-donor ligands: rhodium-103 nuclear magnetic resonance chemical shift correlations

Abstract

The ¹⁰³Rh NMR chemical shifts for hexa(cyano-κC)-, hexa(thiocyanato-κS)-rhodate(III) and tris(O,O′-diethyl dithiophosphato-κ²S,S′)rhodium(III) have been determined in aqueous solution. Two-bond spin–spin coupling is observed between ¹⁰³Rh and ³¹P in the ¹⁰³Rh NMR spectrum of [Rh{S₂P(OEt)₂}₃], ²J(Rh–P)= 13 Hz. A value for the nephelauxetic ratio (β= 0.29 ± 0.05) was obtained for the [Rh(SCN)₆]^3– ion from a correlation between the ¹⁰³Rh NMR chemical shifts of octahedral rhodium(III) complexes and the ligand-field parameter ratio, β/ΔE(¹A_1g–¹T_1g). Comparison with the nephelauxetic ratios of other S- and N-bonded complexes shows S-bonding between rhodium(III) and the thiocyanate ligand. An empirical correlation is demonstrated between the metal NMR chemical shift and the logarithm of the overall thermodynamic formation constant (log β₆), for a range of octahedral rhodium(III) and cobalt(III) complexes, thus allowing the formation constants log β₆= 47 ± 4 and 35 ± 3 to be estimated for [Rh(CN)₆]^3– and [Rh(SCN)₆]^3–, respectively. The stability of the aquafluororhodium(III) species is discussed. The thermodynamic basis for the dissolution of rhodium metal in aqueous solution using cyanide or thiocyanate is examined.