Comparison of o-benzoquinonediimine with bipyridine and bipyrazine in electronic coupling to ruthenium(II), as a function of spectator ligand

Abstract When comparing [Ru(LL) 3 ] 2+ with [Ru(NH 3 ) 4 (LL)] 2+ , the question is raised whether the replacement of the relatively poor LL (LL=2,2′-bipyridine, 2,2′-bipyrazine or o -benzoquinonediimine) π-acceptor and σ-electron ligands by the σ-electron rich ammonia ligands permits a significant improvement in coupling/π-back donation to the remaining LL. Using a zindo analysis of the electronic structures of these species, it is shown that 2,2′-bipyridine is unable to accept the extra electron density from the ruthenium center, while o -benzoquinonediimine can readily do so. The ligand 2,2′-bipyrazine is slightly more able to accept the electron density than 2,2′-bipyridine.