Broken-symmetry DFT spin densities of iron nitrosyls, including Roussin's red and black salts: striking differences between pure and hybrid functionals.

Pure and hybrid exchange-correlation functionals, as typified by OLYP and B3LYP, respectively, yield dramatically different spin density profiles for a variety of paramagnetic iron nitrosyls. Not unexpectedly, based on the strongly noninnocent nature of the NO ligand, the strength of metal-NO spin coupling appears to vary widely as a function of the functional chosen. For the diamagnetic Roussin's red salt, red salt esters, and black salt, OLYP and B3LYP yield very different Fe-Fe spin coupling strengths, with OLYP exhibiting a greater preference for a more spin-coupled description.