Comparing the Ligand Behavior of N-Heterocyclic Phosphenium and Nitrosyl Units in Iron and Chromium Complexes

N-Heterocyclic phosphenium (NHP) and nitrosonium (NO+) ligands are often viewed as isolobal analogues that share the capability to switch between different charge states and thus display redox “noninnocent” behavior. We report here on mixed complexes [(NHP)M(CO)n(NO)] (M = Fe, Cr; n = 2, 3), which permit evaluating the donor/acceptor properties of both types of ligands and their interplay in a single complex. The crystalline target compounds were obtained from reactions of N-heterocyclic phosphenium triflates with PPN[Fe(CO)3(NO)] or PPN[Cr(CO)4(NO)], respectively, and fully characterized (PPN = nitride-bistriphenylphosphonium cation). The structural and spectroscopic (IR, UV–vis) data support the presence of carbene-analogue NHP ligands with an overall positive charge state and π-acceptor character. Even if the structural features of the M-NO unit were in all but one product blurred by crystallographic CO/NO disorder, spectroscopic studies and the structural data of the remaining compound suggest that th...