Structural and electronic features of Group 8 metal complexes containing one α-diiminobenzene chelate ligand

Known complexes of general formula LnM(diim), where diim=C6H4(NH)2-o, α-diiminobenzene, n=3, 4 and M is a metal of Group 8 (Ru(II), in particular) are analyzed in terms of their electronic structure and associated energetics. The newly synthesized compound [(CO)2(PPh3)BrRu(C6H4(NH)2-o)]Br, 1, is a new member of the category. This is confirmed by the X-ray structure of 1 that is presented together with electrochemical measurements and other spectroscopic characterization. At variance with the isoelectronic L4M(o-phenylenediamido) compounds (M=Group 6 metal) that are affected by structural deformations, 1 and similar complexes have a regular pseudo-octahedral structure. Density functional theory calculations and qualitative MO arguments provide an explanation for the different electronic trends as well as for the diimino/diamido dichotomy of the non-innocent chelate ligand. The latter is affected, not only by the nature of the metal (energy of d orbitals), but also by the surrounding environment (three or four coligands). Moreover, the calculations allow to outline the stability trends for the loss of one coligand from L4M(diim) systems when the Ru(II) complexes are chemically or electrochemically reduced.