The effect of trans thiolate ligation on the coordinated nitric oxide in ferric heme nitrosyl complexes as a function of the thiolate donor strength, induced by variation of NH-S(thiolate) hydrogen bonds, is explored. Density functional theory (DFT) calculations (BP86/TZVP) are used to define the electronic structures of corresponding six-coordinate ferric [Fe(P)(SR)(NO)] complexes. In contrast to N-donor-coordinated ferric heme nitrosyls, an additional Fe-N(O) sigma interaction that is mediated by the d(z)(2)/d(xz) orbital of Fe and a sigma*-type orbital of NO is observed in the corresponding complexes with S-donor ligands. Experimentally, this is reflected by lower nu(N-O) and nu(Fe-N) stretching frequencies and a bent Fe-N-O moiety in the thiolate-bound case.