ELECTRONIC-STRUCTURE OF LIGAND-BRIDGED COMPLEXES CONTAINING THE [RH2[3- ESR SPECTROSCOPY, MO CALCULATIONS, AND X-RAY STRUCTURES OF THE 3 REDOX PAIRS [RH2(CO)2LL'(MU-PHNC(ME)NPH)2]Z [Z=0,1 L=L'=PPH3, P(OPH)3 - L=PPH3, L'=P (OPH)3]( CORE )

The X-my crystal structure analyses of the three redoX pairs [Rh2(CO)2 LL'{mu-PhNC(Me)NPh}2]z [z = 0, 1; L = L' = PPh3 (1, 1+); L - L' = P(OP h)3 (2, 2+); L = PPh3, L' - P(OPh)3 (3, 3+)] are reported and the mole cular structures compared with those Of [Rh2(CO)2(PPh3)2(mu-RNNNR)2]z [z - 0, 1; R = p-tolyl (4,4+)]. These have locally planar rhodium coor dination with the planes splayed apart between 20 and 40-degrees. The, non Rh...Rh distance of ca. 2.9 angstrom in the neutral complexes is reduced by between 0.14(2) and 0.262(4) angstrom on oxidation. These a nd other changes in the molecular structure arc consistent with the HO MO of the neutral complexes being delocalized, over the two metals, an d having rhodium-rhodium sigma character. ESR spectroscopic analysis is reported on 1+-4+ and on the related ligand-bridged [Rh2]3+ species [Rh2(CO)2(mu-dPPM)(mu-RNNR)2]+ [R = p-tolyl (5+)], [Rh2(mu-diMen)2(mu -dppM)2]3+ (6+, dimen = 1,8-diisocyanomenthane), [Rh2(mu-TM4)2(mu-dppM )2]3+ [TM4 = 2,5-diisocyano-2,5-dimethylhexane (7+)], [Rh2(mu-CNCH2CH2 CH2NC)2(mu-dppm)2]3+ (8+), [Rh2(mu-HTP5)2(mu-dppm)2]3+ (9+,HTP5 - 1,5- diisocyano-1,1,5-triphenylpentane), [Rh2(CO)2(PMe2Ph)2(mu-chp)2]+(10+) , [Rh2(mu-mhp)2(COD)2]+ (11+). [Rh2(mu-ChP)2(NBD)2]+ (12+), and [Rh2(m u-mhp)2(NBD)2]+ (13+). The spectroscoPiC parameters are shown to be de pendent on the geometry of the cations and especially on the splay ang le between the rhodium coordination planes. Quantitative interpretatio n of the ESR parameters suggests an orbital composition for the SOMO w hich is in good agreement with extended Huckel MO calculations on the model compounds [{Rh(H)2(CO)2}2]2- (14) and [Rh2(CO)4(mu-HNNNH)2] (15) . The theoretical and ESR analyses are in full accord with the X-ray r esults and together provide a coherent picture of the electronic and g eometric structures of ligand-bridged [Rh2]3+ complexes.