0 = CO, C 2 H 4 ; L = PhCN, L 0 = C 2 H 4 ) are highly fluxional, with ligand site interconversion free energy barriers determined by VT NMR of 9.7 kcal mol -1 (L = L 0 = CO), 12.2 kcal mol -1 (L = L 0 = C 2 H 4 ), and 16.1 kcal mol -1 (L = C 2 H 4 , L 0 = PhCN). A dissociative site exchange mechanism is proposed. ( CF 3 PCP)Ir(L) complexes readily undergo oxidative addition reactions. Addition of H 2 to ( CF 3 PCP)-Ir(CO) reversibly forms trans-( CF 3 PCP)Ir(CO)(H) 2 at ambient temperatures. In contrast, addition of H 2 to ( CF 3 PCP)Ir(dfmp) affords fac,cis-( CF 3 PCP)Ir(dfmp)(H) 2 as the major product, with an unusual facially coordinated pincer group. VT NMR monitoring of the reaction of ( CF 3 PCP)Ir(CO) with H 2 established the initial formation of fac,cis-( CF 3 PCP)Ir(CO)(H) 2 followed by conversion to mer, cis-( CF 3 PCP)Ir(CO)(H) 2 prior to isomerization to mer,trans-( CF 3 PCP)Ir(CO)(H) 2 . The unusual stability of ( CF 3 PCP)Ir(L) 2 and fac,cis-( CF 3 PCP)Ir(L)(H) 2 complexes is attributable to the increased stability of nonplanar (PCP)M moieties possessing strongly π-accepting phosphorus groups. (1) (a) Albrecht, M.; van Koten, G. Zhu, K.; Kissin, Y. V.; Cherian, A. E.; Coates, G. W.; Goldman, A. S. Chem. Commun. 2005, 3388-3390. (i) Kuklin, S. A.; Sheloumov, A. M.; Dolgushin, F. M.; Ezernitskaya, M. G.; Peregudov, A. S.; Petrovskii, P. V.; Koridze, A. A. Organometallics 2006, 25, 5466-5476. (3) (a) Goldman, A. S.; Roy, A. H.; Huang, Z.; Ahuja, R.; Schinski, W.; Brookhart, M. Science (Washington, DC, U.S.) 2006, 312, 257-261. (b) Ahuja, R.; Kundu, S.; Goldman, A. S.; Brookhart, M.; Vicente, B. C.; Scott, S. L. Chem. Commun. 2008, 253-255. (c) Bailey, B. C.; Schrock, R. R.; Kundu, S.; Goldman, A. S.; Huang, Z.; Brookhart, M. Rolfe, E.; Carson, E. C.; Brookhart, M.; Goldman, A. S.; El-Khalafy, S. H.; MacArthur, A. H. R. Adv. Synth. Catal. 2010, 352, 125-135. (5) Kundu, S.; Choliy, Y.; Zhuo, G.; Ahuja, R.; Emge, T. J.; Warmuth, R.; Brookhart, M.; Krogh-Jespersen, K.; Goldman, A. S.
