Groups 5 and 6 Terminal Hydrazido(2−) Complexes: N_β Substituent Effects on Ligand-to-Metal Charge-Transfer Energies and Oxidation States

Brightly colored terminal hydrazido(2−) (dme)­MCl3(NNR2) (dme = 1,2-dimethoxyethane; M = Nb, Ta; R = alkyl, aryl) or (MeCN)­WCl4(NNR2) complexes have been synthesized and characterized. Perturbing the electronic environment of the β (NR2) nitrogen affects the energy of the lowest-energy charge-transfer (CT) transition in these complexes. For group 5 complexes, increasing the energy of the Nβ lone pair decreases the ligand-to-metal CT (LMCT) energy, except for electron-rich niobium dialkylhydrazides, which pyramidalize Nβ in order to reduce the overlap between the NbNα π bond and the Nβ lone pair. For W complexes, increasing the energy of Nβ eventually leads to reduction from formally [WVIN–NR2] with a hydrazido(2−) ligand to [WIVNNR2] with a neutral 1,1-diazene ligand. The photophysical properties of these complexes highlight the potential redox noninnocence of hydrazido ligands, which could lead to ligand- and/or metal-based redox chemistry in early transition metal derivatives.