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)MCl ₃(NNR ₂) (dme = 1,2-dimethoxyethane; M = Nb, Ta; R = alkyl, aryl) or (MeCN)WCl ₄(NNR ₂) complexes have been synthesized and characterized. Perturbing the electronic environment of the β (NR ₂) nitrogen affects the energy of the lowest-energy charge-transfer (CT) transition in these complexes. For group 5 complexes, increasing the energy of the N _β lone pair decreases the ligand-to-metal CT (LMCT) energy, except for electron-rich niobium dialkylhydrazides, which pyramidalize N _β in order to reduce the overlap between the Nb=N _α π bond and the N _β lone pair. For W complexes, increasing the energy of N _β eventually leads to reduction from formally [W ^VI≡N-NR ₂] with a hydrazido(2-) ligand to [W ^IV=N=NR ₂] 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.