Despite their formal relationship to alkynes, Ar'GeGeAr', Ar'SnSnAr', and Ar*SnSnAr* [Ar' = 2,6-(2,6-iPr(2)C(6)H(3))(2)C6H3; Ar* = 2,6-(2,4,6-iPr(3)C(6)H(2))(2)-3,5-iPr(2)C(6)H] exhibit high reactivity toward H-2, quite unlike acetylenes. Remarkably, the products are totally different. Ar'GeGeAr' can react with 1-3 equiv of H-2 to give mixtures of Ar'HGeGeHAr', Ar'H2GeGeH2Ar', and Ar'GeH3. In contrast, Ar'SnSnAr' and Ar*SnSnAr* react with only 1 equiv of H-2 but give different types of products, Ar'Sn(mu-H)(2)SnAr' and Ar*SnSnH2Ar*, respectively. In this work, this disparate behavior toward H-2 has been elucidated by TPSSTPSS DFT computations of the detailed reaction mechanisms, which provide insight into the different pathways involved. Ar'GeGeAr' reacts with H-2 via three sequential steps: H-2 addition to Ar'GeGeAr' to give singly H-bridged Ar'Ge(mu-H)GeHAr'; isomerization of the latter to the more reactive Ge(II) hydride Ar'GeGeH2Ar'; and finally, addition of another H-2 to the hydride, either at a single Ge site, giving Ar'H2GeGeH2Ar', or at a Ge-Ge joint site, affording Ar'GeH3 + Ar'HGe:. Alternatively, Ar'Ge(mu-H)GeHAr' also can isomerize into the kinetically stable Ar'HGeGeHAr', which cannot react with H-2 directly but can be transformed to the reactive Ar'GeGeH2Ar'. The activation of H-2 by Ar'SnSnAr' is similar to that by Ar'GeGeAr'. The resulting singly H-bridged Ar'Sn(mu-H)SnHAr' then isomerizes into Ar'HSnSnHAr'. The subsequent facile dissociation of the latter gives two Ar'HSn: species, which then reassemble into the experimental product Ar'Sn(mu-H)(2)SnAr'. The reaction of Ar*SnSnAr* with H-2 forms in the kinetically and thermodynamically more stable Ar*SnSnH2Ar* product rather than Ar*Sn(mu-H)(2)SnAr*. The computed mechanisms successfully rationalize all of the known experimental differences among these reactions and yield the following insights into the behavior of the Ge and Sn species: (I) The active sites of Ar'EEAr' (E = Ge, Sn) involve both E atoms, and the products with H-2 are the singly H-bridged Ar'E(mu-H)EHAr' species rather than Ar'HEEHAr' or Ar'EEH2Ar'. (II) The heavier alkene congeners Ar'HEEHAr' (E = Ge, Sn) cannot activate H-2 directly. Instead, Ar'HGeGeHAr' must first isomerize into the more reactive Ar'GeGeH2Ar'. Interestingly, the subsequent H-2 activation by Ar'GeGeH2Ar' can take place on either a single Ge site or a joint Ge-Ge site, but Ar'SnSnH2Ar' is not reactive toward H-2. The higher reactivity of Ar'GeGeH2Ar' in comparison with Ar'SnSnH2Ar' is due to the tendency of group 14 elements lower in the periodic table to have more stable lone pairs (i.e., the inert pair effect) and is responsible for the differences between the reactions of Ar'EEAr' (E = Ge, Sn) with H-2. Similarly, the carbene-like Ar'HGe: is more reactive toward H-2 than is Ar'HSn:. (III) The doubly H-bridged Ar'E(mu-H)(2)EAr' (E = Ge, Sn) species are not reactive toward H-2.