Biliverdine-Based Metalloradicals:  Sterically Enhanced Noninnocence WasbottenIngar GhoshAbhik 2006 This is a first density functional theory survey of transition-metal biliverdines (Blv), where we have chosen to focus on key Mn, Fe, Co, and Cu complexes. According to the calculations, the complexes are invariably noninnocent, featuring Blv<b><sup>•</sup></b><sup>2-</sup> ligand radicals. In this, biliverdine complexes resemble metallocorroles, but the parallels are only approximate. Briefly, metallobiliverdines exhibit a much greater tendency to adopt noninnocent electronic structures than analogous metallocorroles. The O···O nonbonded contacts in biliverdines apparently preclude the formation of short metal−N bonds that, in turn, could stabilize high-valent metal ions. Thus, while most copper corroles (Cor) exhibit diamagnetic Cu<sup>III</sup> ground states, copper biliverdines are clearly Cu<sup>II</sup>Blv<sup>•2-</sup> species. In the same spirit, while chloroiron corroles are best described as Fe<sup>III</sup>(<i>S</i> = <sup>3</sup>/<sub>2</sub>)Cor<sup>•2-</sup>, the analogous biliverdine derivative seems best described as Fe<sup>III</sup>(<i>S</i> = <sup>5</sup>/<sub>2</sub>)Blv<sup>•2-</sup>, i.e., featuring a high-spin Fe<sup>III</sup> center with long (>2.0 Å) Fe−N bond distances. Overall, the results highlight the important role that steric effects may play in modulating the electronic structures and the potentially noninnocent nature of transition-metal complexes.