Complexes of pincer-ligated iridium fragments (tBuPCP)Ir and (tBuPOCOP)Ir are effective catalysts for the transfer-dehydrogenation of alkanes, a reaction with great potential value. Precursors of both species are effective co-catalysts as components in alkane metathesis systems based on tandem alkane-dehydrogenation/olefin-metathesis. However, of this pair, only (tBuPCP)Ir has been reported to effect metathesis of Cn alkanes with the desirable selectivity for the C2n-2 product. Although these ligands might seem quite similar, especially in terms of steric parameters, we find profound differences in their reactivity. For example, (tBuPOCOP)Ir(C2H4) is approximately 6 times faster than (tBuPCP)IrH2 for cyclooctane/t-butylethylene transfer dehydrogenation. But for n-octane/1-hexene transfer dehydrogenation, the order is markedly reversed: (tBuPCP)IrH2 is > ca. 100 times faster than (tBuPOCOP)Ir(C2H4). Preliminary results indicate that the rate-determining step in (tBuPCP)Ir-co-catalyzed n-alkane metathesis is hydrogenation of the internal olefin intermediate, while n-alkane dehydrogenation is rate-determining with (tBuPOCOP)Ir. DFT calculations are in accord with these results, and with the surprising conclusion that the differences are largely due to steric effects. The smaller O linkage in the POCOP ligand (vs. CH2 in PCP) "holds back" the PR2 groups to significantly "open up" the ligand, allowing easier access for alkanes (especially non-linear) but also much stronger binding of olefins which inhibits reactivity. The selectivity between branched and n-alkanes has also been studied. (tBuPCP)Ir-based catalysts show remarkably high selectivity (>1000:1) for the dehydrogenation of a terminal alkyl group vs. dehydrogenation of a primary-tertiary C-C linkage.