The NMR chemical shift has been the most versatile marker of chemical structures, by reflecting global and local electronic structures, and is very sensitive to any change within the chemical species. In this work, Ru(II) complexes with the same five ligands and a variable sixth ligand L (none, H2O, H2S, CH3SH, H-2, N-2, N2O, NO+, C=CHPh, and CO) are studied by using as the NMR reporter the phosphorus P-A of a coordinated bidentate P-A-N ligand (P-A-N = o-diphenylphosphino-N,N'-dimethylaniline). The chemical shift of P-A in RuCl2(P-A-N)(PR3)(L) (R = phenyl, p-tolyl, or p-FC6H4) was shown to increase as the Ru-P-A bond distance decreases, an observation that was not rationalized. This work, using density functional theory (DFT) calculations, reproduces reasonably well the observed P-31 chemical shifts for these complexes and the correlation between the shifts and the Ru-P-A bond distance as L varies. An interpretation of this correlation is proposed by using a natural chemical shift (NCS) analysis based on the natural bonding orbital (NBO) method. This analysis of the principal components of the chemical shift tensors shows how the sigma-donating properties of L have a particularly high influence on the phosphine chemical shifts.