Covalency in metal complexes of 1,4-diazabutadiene (dab). A density functional investigation of the electronic structures of [M(dab)2](M = Li, Ga or Co) and [Th(NH3)(NH2)3(dab)]

Abstract

Density functional methods have been used to investigate the valence electronic structures of four complexes containing 1,4-diazabutadiene (dab) ligands. One compound was chosen from each of the four ‘blocks’ of the Periodic Table, viz.[Li(dab)₂], [Ga(dab)₂], [Co(dab)₂] and [Th(dab)(NH₃)(NH₂)₃]. The well documented structural differences between the two ligand rings of [Ga(dbdab)₂](dbdab = Bu^tNCHCHNBu^t) are interpreted in terms of a Jahn–Teller distortion arising from the partial occupation of the 7e highest occupied molecular orbital of the (hypothetical) molecule in which both rings are equivalent. This distortion brings about unequal occupancy of the dab π₃-localised molecular orbitals, with one ring becoming singly reduced and the other doubly reduced. The molecular orbital containing the single π₃ electron of the uninegative ring has a more negative eigenvalue than that containing the two electrons of the dianionic ring. While this runs contrary to the Aufbau principle, it provides a simple explanation of the 2 : 1 intensity ratio of the first two bands in the photoelectron spectrum of [Ga(dbdab)₂]. A similar effect is observed for [Li(dab)₂]. An analysis of the metal–dab bonding in all four compounds suggests predominantly ionic interactions in [Li(dab)₂] and [Th(dab)(NH₃)(NH₂)₃], with much greater covalency in [Co(dab)₂]. The conclusion for [Ga(dab)₂] is less clear cut, being dependent on the method used to gauge covalency/ionicity.