www.elsevier.nl/locate/ica Inorganica Chimica Acta 300–302 (2000) 319–327 The effect of divergent-bite ligands on metal – metal bond distances in some paddlewheel complexes F. Albert Cotton a, * 1 , Lee M. Daniels a , Carlos A. Murillo a,b, * 2 , Hong-Cai Zhou a a The Laboratory for Molecular Structure and Bonding, Department of Chemistry, PO Box 30012, Texas A&M Uniersity, College Station, TX 77842 -3012, USA b Escuela de Quı ´mica, Uniersidad de Costa Rica, Ciudad Uniersitaria, Costa Rica Received 9 September 1999; accepted 22 October 1999 Abstract In a paddlewheel type bimetal unit, the most frequent arrangement of the bridging ligands has a conformation such that the two donor orbitals are directed along approximately parallel lines. However, when a ligand has one of the two donor atoms derived from a six-membered ring and the other from a fused five-membered ring, the two donor orbitals will be directed divergently. Such divergent-bite ligands might be expected to elongate the MM bonds. A CrCr core supported by a set of four divergent-bite ligands, Cr 2 (CHIP) 4 , where CHIP is the anion of 1,3-dihydrospiro[cyclohexane-1,2-[2H]imidazo[4,5-b ]pyridine], has been synthesized and crystallographically characterized. It has a CrCr distance of 2.016(1) A  , which is much longer than those in paddlewheel complexes with non-divergent dinitrogen ligands. This is the first example of a dichromium complex supported by a set of four divergent-bite ligands and no axial coordination. An analogous complex, namely Mo 2 (azin) 4 , where azin represents the anion of 7-azaindole, has also been characterized. Together with W 2 (azin) 4 , which was previously reported, the first series of quadruple bonds wrapped in a sheath of four divergent-bite ligands has been obtained. A comparison of the MM bond lengths with those in a homologous series having a more flexible ligand, DTolF, which is the anion of di-p -tolylformamidine, reveals that the CrCr quadruple bonds are much more sensitive to the effect of the bridging ligand geometry than are MoMo and WW quadruple bonds; this is very similar to the trend in susceptibility of quadruple bonds towards axial coordination. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Crystal structures; Chromium complexes; Metal – metal bonds 1. Introduction The term paddlewheel complex [1,2] is applied to a complex of the type I (see Scheme 1), where only the essential features are shown. A central pair of metal atoms, bonded, sometimes strongly, to each other, is surrounded by four bridging bidentate ligands of the type minimally represented by IIa (see Scheme 2), where X and Y are donor atoms. Most often, but not necessarily, there is a negative charge present. Common and representative examples of IIa are IIb and IIc,a carboxylate ion and an amidate ion, respectively. In some cases a paddlewheel complex may have a helical twist about the MM axis. Strictly speaking, the paddlewheel analogy is then compromised. Imagine trying to steer a Mississippi steamboat propelled by a helical paddlewheel! Nevertheless, we also call the twisted ones paddlewheel complexes. Scheme 1. 1 *Corresponding author. E-mail: cotton@tamu.edu 2 *Corresponding author. E-mail: murillo@tamu.edu 0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 5 3 7 - X