Polyhedron zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Vol. 6, No. 4, pp. 705-714, 1987 Printed in Great Britain 0277-5387/87 S3.00+.00 Pcrgamon zyxwvutsrqpon Journals Ltd zyxwvutsr S + 6* REVISITED: WHAT THE ENERGIES AND INTENSITIES MEANT MICHAEL D. HOPKINS and HARRY B. GRAY Arthur Amos Noyes Laboratory, California Institute of Technology, Pasadena, CA 91125, U.S.A. and VINCENT M. MISKOWSKI Applied Sciences and Microgravity Experiments Section, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, U.S.A. zyxwvutsrqponmlkjihgfe (Received 19 November 1986) Abstract-Attempts to extract estimates of the &bond strength of quadruply metal-metal- bonded molecules from their 6 + 6* electronic transitions have been hindered in the past by limited understanding of the origins of the energies and intensities of these transitions. We show that the energies of the 6 + 6* transitions of a wide variety of these molecules are adequately interpreted in terms of a simple zero-differential-overlap model that yields one-electron 6-6* splittings of 5000-10,000 cm-’ and two-electron exchange terms [K@, S*)] of 5000-8OOOcm-‘. Because of the magnitude of K, singlet-triplet 6 --, 6* splittings are very large, and configuration interaction is important for the correct description of the ground state. The intrinsic intensity of ‘(6 + 6*) is estimated to be quite low. The considerable intensities observed in many cases do not correlate with &bond strength, but instead reflect intensity stealing from charge-transfer excited states as a result of 6,6*-orbital mixing with ligand orbitals. The intrinsic &bond stabilization is estimated to be on the order of 10 kcalmol- ‘. In 1964 Cotton proposed’ the existence of a metal- metal S-bond for Re,Cli- in order to account for its diamagnetism and eclipsed (D.$h) geometry. The d4- d4 electron count yields, within this molecular orbital framework, a [a2~462] electronic configura- tion and a lot+lying empty 6* antibonding orbital, with the &HOMO and S*-LUMO being derived from metal d,-orbitals. The &bond completes a metal-metal quadruple bond. A central question in the years following this discovery has concerned the strength of the &bond. A primary experimental probe in this regard has been the determination of the spectroscopic energies of the electronic transitions between the 6- and 6*- orbitals. The earliest attempts along these lines t Contribution No. 7482 from the Arthur Amos Noyes Laboratory. were adversely affected by the primitive calculation methods that were then available,2 and extremely inflated estimates of b-bond strength, as well as incorrect spectroscopic assignments, resulted. Work on three different fronts ushered in a new era in this field during 1973-75. On the theoretical side, calculations employing the then-novel SCF- Xc+SW method were published by Norman and Kolari,3 and Mortola et ~1.~ The postulated molecu- lar-orbital ordering was confirmed, although the splitting of the one-electron 6- and 6*-levels was much smaller than had previously been envision- ed. Meanwhile, Cowman and Gray’ showed that the lowest-energy visible absorption in the elec- tronic spectrum of Re,Cli- (v,,, = 14,7OOcm-‘, E = 2500) and related quadruply bonded com- pounds could be assigned to the ‘(6 + 6*) transition on the basis of its molecular z-polarization and pronounced vibronic structure in the metal-metal 705