Bonding analysis of transition metal NNR end-on complexes and comparison with isoelectronic NNR₂ species

DFT calculations have been carried out on various NNR and NNR₂ transition metal complexes. The theoretical results have been analyzed together with a collection of structural data obtained through a Cambridge Data Base search covering ca. 140 compounds. When linearly coordinated in mono-substituted complexes and assuming the respective formal charges of -3 and -2, both hydrazide(-3) (NNR^3-) and hydrazide(-2) (NNR₂^2-) ligands act as 6-electron donors through their 3 occupied FMOs (one σ-type FMO and two non-equivalent π-type FMOs). Hydrazide(-3) is found to be a weaker π-donor ligand, in agreement with most of the reported X-ray structures. Calculations of the FMO occupations suggest that the ligand formal oxidation state of an NNR ligand is close to -1 and that of NNR₂ is intermediate between 0 and -1. In the case of the cis-di-substituted complexes, and still assuming the NNR^3- and NNR₂^2- formal ligand charges, either the d⁰ or the d^-2 metal configuration is always found. In the former case, both cis ligands act together as a 10-electron system leading to the 18-electron count. In the latter case, the d^-2 value is meaningless. Calculations show that the 2-electron oxidation of the d⁰ species involves a ligand-based MO that is metal-ligand non-bonding, leaving the metal oxidation state unchanged, as well as the 18-electron count of the complex. Such an oxidation is associated with a 90° rotation of the cis ligands. A similar situation is computed for the 2-electron oxidation of d² 18-MVE trans-dihydrazide(-2) models, in which the NNR₂ formal charge varies from -2 to -1, whereas the hydrazide system acts as an 8-electron donor in both the reduced and oxidized states. The trans-di(NNR^3-) compounds behave somewhat differently since their oxidized form is better described as a d^o hydrazide(-3) 16-MVE system.