The relative stabilities of trans-[Ir(XMe)(CO)(PR3)2] species (X = O, CH2) toward ß-H elimination have been studied via combination of density functional and hybrid density functional/Hartree-Fock calculations. For both small (R = H) and full (R = Ph) model systems ß-H elimination from the methoxide species is found to be disfavored both kinetically and thermodynamically compared to that from the analogous ethyl complexes. This is consistent with the greater stability of alkoxide species seen experimentally (R = Ph). In all cases the major contribution to the activation barrier is phosphine dissociation, and for the alkyl systems this leads directly to an agostically stabilized intermediate from which ß-H transfer readily occurs. In contrast, with the trans-[Ir(OMe)(CO)(PR 3)2] species a p-stabilized intermediate is formed and a further isomerization barrier must be overcome before ß-H transfer can be accessed. Further calculations were performed on the acetophenone complex [Ir(H)(?2-O=C(Me)Ph)-(CO)(PPh3)], and a low-energy pathway for face exchange of the metal-bound ketone has been characterized. This involves an ?1-intermediate and provides a mechanism for facile racemization of the precursor alkoxide. Selected calculations using alternative hybrid calculations showed the sensitivity of PPh3 binding energies to the methodology employed. This is especially the case for the final step in the ß-H elimination reaction, the formation of [Ir(H)(CO)(PPh 3)3] from tIr(H)(CO)(PPh3)2] and free PPh3, where the use of the UFF approach appears to be particularly unreliable. © 2007 American Chemical Society.