The unsaturated PCP-type complexes Rh(L){2,6-(CH2PtBu2)2C6H3}X (L = Et, nPr; X = Cl, I) complexes convert upon heating to the corresponding Rh(III)−hydride complexes Rh(H){2,6-(CH2PtBu2)2C6H3}X (X = Cl, I) and ethylene or propylene, products indicative of a β-H elimination process. The iPr analogue is observed upon reaction of Rh(η 1-N2){2,6-(CH2PtBu2)2C6H3} with iPrI at −10 °C and decomposes readily at room temperature to give Rh(H){2,6-(CH2PtBu2)2C6H3}I and propylene. Analogous alkyl complexes  lacking β-hydrogens  are stable under the applied reaction conditions. The mechanism of this process has been studied by NMR, using 13C and deuterium labeling of the alkyl ligand (L = Et-d 5, 13CH2CH3). 13C labeling shows that the β-H elimination is irreversible. A deuterium isotope effect of k Et/k Et - d 5 = 1.4 and a rate order of Et < nPr ≪ iPr were observed. The overall process follows first-order kinetics in the Rh(III)−alkyl complexes. The activation parameters for the thermolysis of Rh(Et)(2,6-(CH2PtBu2)2C6H3)I in toluene were determined:  ΔH ⧧ = 21.2 kcal/mol, ΔS ⧧ = −21.1 eu, and ΔG ⧧ 298   K = 27.5 kcal/mol.