Experiments have indicated that bulky ligands are required for efficient H-2 activation by Pt-Sn complexes. Herein, we unravel the mechanisms for a Pt-Sn complex, Pt((SnBu3)-Bu-t)(2)((CNBu)-Bu-t)(2) (1a), catalyzed reversible H-2 activation. Among a number of Pt-Sn catalysts used to model H-2 activation and H-2/D-2 exchange reactions, only 1a with large strain was found to be suitable because the addition of H-2 to 1a requires lowest distortion energy, minimal structural changes, and smallest entropy of activation. The activity of this Pt-Sn complex was compared vis-a-vis its Pt-Ge and Pt-Si analogues, and we predicted that strained Pt-Ge complex can efficiently activate H-2 reversibly. Direct dynamics calculations for the rate of reductive elimination of H-2, HD, and D-2 from Pt((SnBu3)-Bu-t)((CNBu)-Bu-t)(2)H-3 (4a) and Pt((SnBu3)-Bu-t)((CNBu)-Bu-t)(2)HD2 (4a([2D])) shows that H/D atom tunneling contributes significantly, which leads to an enhanced kinetic isotope effect. Strain control is suggested as a design concept in H-2 activation.