A¹⁰³Rh nuclear magnetic resonance study of rhodium(<scp>III</scp>) bromide complexes in aqueous solution

Abstract

The 103Rh NMR signals of all ten monomeric aquabromorhodium(III) complexes, [RhBrn(OH2)6–n]3–n with n= 0–6, including the geometric isomers for n= 2–4, have been assigned. The chemical shifts δRh of these complexes demonstrate a nephelauxetic dependence, namely a decrease in δ with an increase in the number of bromo ligands. For the geometrical isomers it has been shown that δtrans > δcis and δmer > δfac. Evidence is given for the existence of oligomeric aquabromorhodate(III) species and structural characteristics are proposed consistent with the observed chemical shifts. The 103Rh NMR chemical shift has been measured for the aqueous [Rh(NH3)6]3+ complex (δRh= 4766 at 276 K) and, together with other six-co-ordinated rhodium(III) complexes, correlated with the ligand-field parameter ratio β/ΔE, where β is the nephelauxetic ratio and ΔE the electronic transition energy for octahedral complexes, 1A1gâ†�1T1g. Linear regression gives an estimate of the core-electron diamagnetic shift to δRhd=–5000(1000).