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

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) pi-acceptor and sigma-electron ligands by the o-electron rich ammonia ligands permits a significant improvement in coupling/pi-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, (C) 2000 Elsevier Science S.A. All rights reserved.