Back-Donation in High-Valent d⁰ Metal Complexes: Does It Exist? The Case of Nb^V

In the last years, some N-heterocyclic carbene (NHC) complexes of high-valent d⁰ transition-metal halides have been structurally characterized, showing a significant short distance between the carbene carbon and the cis-halide ligands (Cl_ax). Some authors attributed this arrangement to a halide → C_carbene unusual "back-donation", whereas, according to others, the M-carbene bond is purely σ. More, in general, the ability of d⁰ metal centers to provide back-donation to suitable ligands is still debated, and detailed bond analyses for this class of systems are missing in the literature. In this contribution, we analyze in detail the Nb^V-L bond within neutral, cationic, and anionic derivatives of NbCl₅, with L = NHC, CO, CNH, and CN^-. In [Nb^VCl_6-x(NHC)_x]^x-1 complexes, with NHC being either a model carbene (1,3-dimethylimidazol-2-ylidene, IMe) or a realistic one [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IPr], we demonstrate that the metal center is really capable of back-donation to the carbene ligand by a charge flux that involves the chloride in the trans position and, directly, the metal. In this case, a direct interaction between Cl_ax and C_carbene can be excluded, while if different π-acceptor ligands, such as CO or CNH, are used (instead of NHC), the direct Cl_ax → L interligand interaction becomes predominant.