Why the mechanisms of digermyne and distannyne reactions with H2 differ so greatly.
Despite their formal relationship to alkynes, ArGeGeAr, ArSnSnAr, and Ar*SnSnAr* [Ar = 2,6-(2,6-iPr(2)C(6)H(3))(2)C(6)H(3); 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. ArGeGeAr can react with 1-3 equiv of H(2) to give mixtures of ArHGeGeHAr, ArH(2)GeGeH(2)Ar, and ArGeH(3). In contrast, ArSnSnAr and Ar*SnSnAr* react with only 1 equiv of H(2) but give different types of products, ArSn(?-H)(2)SnAr and Ar*SnSnH(2)Ar*, 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. ArGeGeAr reacts with H(2) via three sequential steps: H(2) addition to ArGeGeAr to give singly H-bridged ArGe(?-H)GeHAr; isomerization of the latter to the more reactive Ge(II) hydride ArGeGeH(2)Ar; and finally, addition of another H(2) to the hydride, either at a single Ge site, giving ArH(2)GeGeH(2)Ar, or at a Ge-Ge joint site, affording ArGeH(3) + ArHGe:. Alternatively, ArGe(?-H)GeHAr also can isomerize into the kinetically stable ArHGeGeHAr, which cannot react with H(2) directly but can be transformed to the reactive ArGeGeH(2)Ar. The activation of H(2) by ArSnSnAr is similar to that by ArGeGeAr. The resulting singly H-bridged ArSn(?-H)SnHAr then isomerizes into ArHSnSnHAr. The subsequent facile dissociation of the latter gives two ArHSn: species, which then reassemble into the experimental product ArSn(?-H)(2)SnAr. The reaction of Ar*SnSnAr* with H(2) forms in the kinetically and thermodynamically more stable Ar*SnSnH(2)Ar* product rather than Ar*Sn(?-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 ArEEAr (E = Ge, Sn) involve both E atoms, and the products with H(2) are the singly H-bridged ArE(?-H)EHAr species rather than ArHEEHAr or ArEEH(2)Ar. (II) The heavier alkene congeners ArHEEHAr (E = Ge, Sn) cannot activate H(2) directly. Instead, ArHGeGeHAr must first isomerize into the more reactive ArGeGeH(2)Ar. Interestingly, the subsequent H(2) activation by ArGeGeH(2)Ar can take place on either a single Ge site or a joint Ge-Ge site, but ArSnSnH(2)Ar is not reactive toward H(2). The higher reactivity of ArGeGeH(2)Ar in comparison with ArSnSnH(2)Ar 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 ArEEAr (E = Ge, Sn) with H(2). Similarly, the carbene-like ArHGe: is more reactive toward H(2) than is ArHSn:. (III) The doubly H-bridged ArE(?-H)(2)EAr (E = Ge, Sn) species are not reactive toward H(2).