Through-Ring Bonding in Edge Sharing Dimers of Octahedral Complexes Ana A. Palacios, Gabriel Aullo ´ n, Pere Alemany, and Santiago Alvarez* Departament de Quı ´mica Inorga `nica, Departament de Quı ´mica Fı ´sica and Centre de Recerca en Quı ´mica Teo `rica (CeRQT), Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain ReceiVed January 5, 2000 A study of the preferred structures for the M 2 X 2 rings in the binuclear complexes of types [M 2 (µ-XR 2 ) 2 L 8 ] and [M 2 (µ-XR 3 ) 2 L 8 ] is presented, based on qualitative orbital arguments supported by extended Hu ¨ckel calculations on Cr compounds. The main conclusions are confirmed by DFT calculations on key compounds of Cr and Mn and agree well with the results of a structural database analysis. With the simplified electron counting scheme deduced, complexes with six or four electrons available for bonding of the M 2 X 2 framework are predicted to have two possible minimum energy structures, with either a short M-M or X-X distance, whereas compounds with eight framework electrons are expected to present no short through-ring distance. Such a behavior is consistent with the framework electron rules reported earlier for compounds with different coordination spheres and provides a general description of the structure and bonding in a variety of compounds with M 2 X 2 diamonds. Metal-metal bonding across the ring can be equally predicted taking into account only the bonding characteristics of the t 2g -like orbitals for the XR 2 - but not for the XR 3 -bridged complexes. In addition, the framework electron counting scheme has the advantage of being independent of the formal oxidation state assigned to the metal atom. The edge sharing bis-octahedral structure of general formula [M 2 (µ-XR n ) 2 L 8 ] is a very common pattern in the chemistry of coordination and organometallic compounds. In such a structure, two ML 4 fragments are joined by two bridging ligands that complete an octahedral coordination sphere around each metal atom (1). We include in this family those complexes with metal fragments of the type MCpL, since the η 5 -cyclopentadienide ligand can be considered as electronically tridentate. 1 Not only can one find an assortment of bridging ligands of general type XR n but also a variety of metal atoms with different oxidation states, providing different electron counts which may or may not give rise to metal-metal bonding across the ring, resulting in three possible alternative structures of the M 2 X 2 framework (2a-c). In many complexes with XR 2 bridges having a d n electron configuration of the metal atom (where n e 5), the partially occupied t 2g orbitals of each metal atom can account for the presence of a short through-ring metal-metal distance. The structures of complexes with XR 3 or isolobal bridging ligands (e.g., hydride) are not so easy to explain, since these bridging ligands cannot be considered as two-electron donors toward each metal atom. For analogous edge-sharing binuclear compounds of transition metal d 10 ions with tetrahedral, or d 8 ions with square planar coordination spheres, we have shown 2,3 that a delocalized MO description results in simple electron counting rules for the M 2 X 2 framework. 4 In brief, if the number of electrons available for the σ bonding of that framework is eight (framework electron count, FEC ) 8), one should expect a regular ring, with no short distance across the ring, whereas for smaller FECs (6 or 4), a metal-metal (or a ligand-ligand) bond across the ring should be expected. Since such short through-ring distances in those cases cannot be directly associ- ated with metal-metal bonds involving the metal d orbitals, we wish to explore the orbital analogies and differences between the bis-octahedral complexes and the previously studied bi- nuclear structures. Our final goal is to establish simple rules to describe the bonding and structure in a wide variety of compounds with M 2 X 2 cores. A detailed study of the molecular orbital diagrams for edge- sharing bis-octahedral complexes with X or XR 2 bridges was reported early by Hoffmann and co-workers, 5 but the case of XR 3 bridges, the possibility of an alternative structure with a short X-X distance, and the changes in orbital localization that accompany the distortion of the M 2 X 2 ring were not analyzed. (1) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 711. (2) Alemany, P.; Alvarez, S. Inorg. Chem. 1992, 31, 4266. (3) Aullo ´ n, G.; Alemany, P.; Alvarez, S. J. Organomet. Chem. 1994, 478, 75. (4) Alvarez, S.; Palacios, A. A.; Aullo ´n, G. Coord. Chem. ReV. 1999, 185-186, 431. (5) Shaik, S.; Hoffmann, R.; Fiesel, C. R.; Summerville, R. H. J. Am. Chem. Soc. 1980, 102, 4555. (6) Mealli, C.; Orlandini, A. Metal Clusters in Chemistry In Braunstein, P., Oro, L. A., Raithby, P. R., Ed.; Wiley-VCH: New York, 1999; Vol. 1, p 143 (7) Rohmer, M.-M.; Be ´nard, M. Organometallics 1991, 10, 157. (8) DeKock, R. L.; Peterson, M. A.; Reynolds, L. E. L.; Chen, L.-H.; Baerends, E. J.; Vernooijs, P. Organometallics 1993, 12, 2794. (9) Janiak, C.; Silvestre, J.; Theopold, K. H. Chem. Ber. 1993, 126, 631. (10) Cotton, F. A. Polyhedron 1987, 6, 667. 3166 Inorg. Chem. 2000, 39, 3166-3175 10.1021/ic000017i CCC: $19.00 © 2000 American Chemical Society Published on Web 06/28/2000