We report measurements of effective bimolecular rate constants for reaction of the gas phase, neutral, and ground state Ir(5d(7)6s(2),4F(9/2)) and Pt(5d(9)6s,D-3(3)) atoms with small linear alkanes at 300 K in 0.5-1.1 Torr of He buffer gas. Iridium shows no reaction with methane, ethane, propane, or n-butane within our detection limits. Ln contrast, platinum reacts with all four alkanes. The reaction efficiency increases with the size of the alkane from 0.01 for methane to 0.5 for n-butane. Thus far, ground state Pt is unique among the neutral transition metal atoms in its ability to react with methane at 300 K. To understand the experiments, we carried out quantum chemical calculations on the PtCH4 system using the parametrized configuration interaction method called PCl-80 with approximate corrections for spin-orbit interactions derived from experimental atomic data. The calculations explore both the lowest triple and singlet potential energy surfaces along paths leading from Pt + CH4 to the CH bond insertion intermediate and onward to elimination of H-2. The calculations make a clear prediction that the product of the observed reaction is the collisionally stabilized, long-lived H-Pt-CH3 insertion complex rather than H-2 elimination. A large potential barrier separates H-Pt-CH3 from H-2 elimination products, which are in any event substantially endothermic from ground state Pt + CH4. The hint of a mild pressure dependence of the observed rate constant and statistical estimates of the H-Pt-CH3 lifetime are consistent with the inferred termolecular mechanism. The key feature that enables tripler ground state reactants to access the singlet insertion well is the low energy of the intersection between triplet and singlet surfaces, which the calculations find only 1 kcal/mol above ground state reactants. The Pt reactions with alkanes larger than methane could involve H-2 elimination or a saturated termolecular mechanism like Pt + CH4. In contrast to Pt, the calculations find a large potential barrier between ground state Ir + CK4 and the insertion intermediate H-Ir-CH3 due to the large promotion energy from the chemically inert quartet 5d(7)6s(2) ground state to 5d(8)6s(1) excited states. The chemical behavior of the naked Pt and Ir atoms in the gas phase thus differs significantly from that of saturated Pt and Lr complexes in solution phase.