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

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(1A1g–1T1g). 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.