A 103Rh nuclear magnetic resonance study of rhodium(III) bromide complexes in aqueous solution

Abstract

The ¹⁰³Rh NMR signals of all ten monomeric aquabromorhodium(III) complexes, [RhBr_n(OH₂)_6–n]^3–n with n= 0–6, including the geometric isomers for n= 2–4, have been assigned. The chemical shifts δ_Rh 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 δ_trans > δ_cis and δ_mer > δ_fac. 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 (δ_Rh= 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, ¹A_1gâ†�¹T_1g. Linear regression gives an estimate of the core-electron diamagnetic shift to δ_Rh^d=–5000(1000).