Comparison of pure and hybrid DFT functionals for geometry optimization and calculation of redox potentials for iron nitrosyl complexes with μ-SCN bridging ligands

The geometry and electronic structure of neutral molecules and mono-anions of iron nitrosyl complexes with mu-SCN bridging ligands [(mu-SC7H4SN)Fe(NO)2](2) (I) and [(mu-SC2H3N4)Fe(NO)(2)](2) (II) were studied using the density functional theory: pure functionals BP86, OLYP, OPBE, TPSS, PW91 and hybrid functionals B3LYP, B1B95, B3PW91 along with 6-311++G**//6-31G*. For geometry optimization, the pure functionals are more appropriate than the hybrid functionals. For mono-anions, pure functionals favor the doublet over the quartet state by 8.0-13.9 kcal/mol, whereas hybrid functionals yield quartet spin state of these anions to be more stable. The redox potentials of complexes I and II have been computed by DFT and compared with experimental results obtained from cyclic voltammetry method. The calculated redox potentials for I and II with pure functionals (especially BP86) are close to the experimental values, whereas those obtained by using hybrid functionals significantly deviate from the experiment.