Activation of C-C and C-H bonds by the Rh(I) and Ir(I) complexes (PCP) MCl (M) Rh, Ir; PCP=C6H3(CH3)(CH2PH2)2) has been studied by density functional methodology. C-H activation from either of the three-coordinate intermediates 1a and 1b has a high barrier (>25 kcal/mol). Direct C-C activation does not occur from either 1a or 1b because the C-C bond is sterically inaccessible. Plausible C-C and C-H activation mechanisms under mild conditions are related to four-coordinate