Several nitrosyl complexes of Fe and Co have been prepared using the sterically hindered Ar-nacnac ligand (Ar-nacnac = anion of [(2,6- diisopropylphenyl)NC(Me)]2CH). The dinitrosyliron complexes [Fe(NO)2(Ar-nacnac)] (1) and (Bu4N)[Fe(NO) 2(Ar-nacnac)] (2) react with [FeIII(TPP)Cl] (TPP = tetraphenylporphine dianion) to generate [FeII(NO)(TPP)] and the corresponding mononitrosyliron complexes. The factors governing NO transfer with dinitrosyliron complexes (DNICs) 1 and 2 are evaluated, together with the chemistry of the related mononitrosyliron complex, [Fe(NO)Br(Ar-nacnac)] (4). The synthesis and properties of the related cobalt dinitrosyl [Co(NO) 2(Ar-nacnac)] (3) is also discussed for comparison to DNICs 1 and 2. The solid-state structures of several of these compounds as determined by X-ray crystallography are reported. © 2011 American Chemical Society.