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J . A m . Chem. SOC.1988, 110, 5764-5767
species) again causes a sizeable increase in the M-M distance. Our results and conclusions are in excellent accord with the theoretical and photoelectron spectroscopic studiesI0 recently reported for Rh,[(t~l)NCHN(tol)]~.This molecule has two more electrons than our Ru2[(tol)NNN(tol)],, and the calculation, supported by the measured UV-PES, assigns them to an orbital of bl, symmetry that is primarily a 6' (Rh-Rh) orbital. This orbital (the HOMO) lies about 8000 cm-' above the a* orbital. 17.3 16.7 Thus, if we were to deduce the electron configuration of Ru22.622 (3)d 2.563 ( Pd2,+ [(tol)NNN(tol)], from that established for Rh2[(tol)NCHN(tol)], 15 15 by removing two electrons (which seems an eminently reasonable "This work. bLe, J. C.; Chavan, M. Y.; Chau, L. K.; Bear, J. L.; configuration, which thing to do), we should arrive at the 2a4627r*4 Kadish, K . M. J . A m . Chem. SOC.1985, 107, 7195. No esd given. is exactly the one indicated directly by experimental data for 'Piraino, P.; Bruno, G.; Lo Schiavo, S.; Laschi, F.; Zanello, P. fnorg. Chem. 1987, 26, 2205. dCotton, F. A,; Matusz, M.; Poli, R.; Feng, X. Ru2[(tol)NNN(tol)],. The similarity of the Ru-Ru and Rh-Rh J . A m . Chem. SOC.1988, 110, 1144. eCorbett, M.; Hoskins, B. F.; distances in these two compounds shows that the 6* orbital is not McLeod, N. J.; O'Day, B. P. Aust. J . Chem. 1975, 28, 2377. a major factor in determining M-M bond strength. The major result of losing the two 6* electrons is the reestablishment of a this conclusion in two ways. The eclipsed configuration indicates net 6 bond, and this imposes an eclipsed configuration in Ru2that a net 6 bond is present. In all comparable structures (see [(toi)NNN(tol)], instead of the twisted one (16.7') found in Table IV) where the 6* orbital must contain two electrons, thus Rh2[(tol)NCHN(tol)]4. abolishing the 6 bond, there are torsion angles of ca. 16'. Another important result given by the calculations on the The Ru-Ru bond length is also in excellent accord with the rhodium compoundsI0 is that the large (ca. 1 eV) separation of u27r462a*4configuration. For R U ~ ( O ~ C Rspecies ) ~ + with three the 6* and K* orbitals in the Rh,(RNCHNR), type compound ~*6*)~, unpaired electrons, the configuration must be ~ ~ a ~ 6 ~ ( where is in sharp contrast to the small (
