The Nature of Halogen Dependence of ¹⁰³Rh NMR Chemical Shift in Complex Anions cis-[X1X2Rh(CO)₂]⁻ (X1, X2 = Cl, Br, I)

DFT calculations within the two-component quasirelativistic approach and further analysis of ¹⁰³Rh NMR shielding (including the natural bond orbitals method) in a series of flat-square complex anions cis-[X1X2Rh(CO)₂]⁻ (X1, X2 = Cl, Br, I) are used to determine the role of relativistic spin-orbit effects in ¹⁰³Rh NMR shielding in these systems. Electron delocalization observed in these complexes provides the possibility of relativistic spin-orbit effects but complicates the interpretation of ¹⁰³Rh NMR shielding in terms of chemically meaningful fragments of electronic system (lone pairs, chemical bonds).