(POCOP)­IrI(CO) [POCOP = κ3-C6H3-2,6-(OPR2)2 for R = tBu, iPr] and (PCP)­IrI(CO) [PCP = κ3-C6H3-2,6-(CH2PR2)2 for R = tBu and iPr] complexes can add hydrogen via two distinct pathways. When R = tBu, (POCOP)­Ir­(CO) and (PCP)­Ir­(CO) complexes only add hydrogen via a proton-catalyzed pathway due to steric effects, yielding trans-dihydride complexes. For R = iPr, both systems oxidatively add hydrogen to give cis-dihydride complexes which thermally isomerize to more thermodynamically favorable trans-dihydride species, consistent with previous reports. Proton-catalyzed hydrogen addition pathways are also observed for both iPr-substituted (pincer)­Ir­(CO) complexes. (PCP)­Ir­(CO) complexes add hydrogen under milder conditions than the analogous POCOP species. Intermediate hydrido-pyridine Ir­(III) carbonyl complexes from the proton-catalyzed pathway have been synthesized and characterized. This is the first report of a series of complexes shown to add hydrogen via concerted oxidative addition or a proton-catalyzed pathway to the same iridium center.

Pincer ligand metalation is presumed to proceed via initial coordination to the phosphorus atoms followed by C− H oxidative addition. Few isolated intermediates in this process are known. A rhodium phosphinite complex has been isolated and structurally characterized that exhibits a strong agostic interaction with a C−H bond in the ligand backbone. The [( tBu POCOP)-Rh(CO)H] + system exhibits greater acidity and reactivity than the analogous iridium species ( tBu POCOP = κ 3 -C 6 H 3 -1,3-[OP-(tBu) 2 ] 2 ).