Density functional calculations have been applied to models of both transfer and acceptorless alkane dehydrogenation for the iridium(III) pincer complex, ((PCP)-P-R)IrH2 ((PCP)-P-R=,eta(3) -C6H3(CH2PR2)(2)-1,3). The iridium pincer complex is the first efficient homogeneous catalyst that does not require photons or sacrificial hydrogen acceptors to drive the reaction, although with the latter the transfer reaction can accomplished under mild conditions. There are four essential steps for the catalytic cycle in both transfer and acceptorless reactions: oxidative addition of alkane, reductive elimination, beta-hydrogen (beta-H) transfer, and loss of olefin. The transfer reaction can utilize a hydrogen acceptor to produce a 14-electron intermediate, ((PCP)-P-R)Ir(I), and the acceptorless reaction can produce an 18-electron intermediate, ((PCP)-P-R)Ir(V)(H)(3)(alkyl) species. Because the critical barriers are well-balanced in the lr(V) catalytic cycle for acceptorless alkane dehydrogenation, no transition state (TS) is higher in energy than the products and no intermediate is lower in energy than the reactant. This balance could explain why pincer complexes are more efficient catalysts for alkane dehydrogenation than the corresponding Cp-Ir(III) complex. Entropy contributions, which play a larger role at high temperature, open new pathways for the acceptorless reaction. Free energies of activation suggest that the dynamics of the acceptorless reaction might be sampling the entire range of pathways and Ir oxidation states from Ir(l) to Ir(V). (C) 2002 Elsevier Science B.V. All rights reserved.