William C. Trogler Northwestern University Evanston, IL 60201
Orbital Concepts and the Metal-Metal Bond
I n the last decade the synthesis and characterization of compounds containing metal-metal bonds have been vigorously pursued (I). Two important types of complexes exhibit strong metal-metal interactions. Metal-halide cluster complexes and their derivatives are known for the elements Nb, Ta, Mo, W, Tc, and Re in I-IV oxidation states ( 2 ) .Metal carhonyl cluster complexes, and related compounds which possess "r-acceptor" ligands, generally contain transition metals which lie to the right of group V B (3).I n these complexes, the metals usually exist in some low (-I, 0 , I) oxidation state. Table 1lists some examples of binuclear metal-metal honded snecies from both classes. One Duroose . . of this article is tocons~dcrthe elerrronicnipects d m e t a l cluster iystems. t~tdizineorbital WnCrDtS (41. 1)isrussion will be restrirted t ~ , the simplest of binuclear metal clusters in order to illustrate fundamental features of the metal-metal interactions. ~
Table 1. Some Blnuclear Metal Cluster Complexes Metal-Halide Dimers Re2C1.2MO~CI~~+ pReCi,* W2ci93NbCI,=
Metal Carbanyl Dimers MndC0)to Ren(C0)w CodCOh FedCOk IMo(?-CsHs)(CO)&
a T ~ solid C phase el these mrnpawds contain chains of binmiear cluster units linked by bridginghalide ions.
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The Metal-Metal Bond and Molecular Orbital Theory A qualitative description of the electronic structure associated with a binuclear metal unit in transition metal complexes logically begins with the valence d orbital splitting pattern expected for an isolated Mp (M = a transition metal) fragment. Although metals a n d p wavefunctions are among the low-lvine orbitals in a neutral and isolated Mz fragment ( 5 ) ,met2-lGand interactions and charge effects in cbordination complexes greatly destabilize these orbitals and minimize their hmdini conr;it,utitm. One must thercforervnsidrr rhc types UI cwerlap between \ , a h r e d urbitals in dinucleur .;y
