Reactions of H₂XXH₂ and H₂XO Double Bonds (X = Si, Ge, Sn, Pb):  Are 1,3-Dioxa-2,4-dimetaletanes Unusual Molecules?

Abstract

1,3-Dioxa-2,4-dimetaletanes (X = Si, Ge, Sn, Pb) are possible products from the reaction of the H 2 X XH 2 double bonds with dioxygen as well as from H 2 X O dimerization. The energies of these reactions were investigated with DFT pseudopotential calculations (using the Becke3LYP functional and DZ+P basis sets) in order to assess the relative stabilities of X X and X O double bonds. While ethene and formaldehyde are exceptionally stable in this context, the reactions of most of the heavier group 14 element compounds are very exothermic. The H 2 X O dimerization energy trend follows the electronegativity of X. However, the H 2 X XH 2 oxygenation energy decreases down group 14 from silicon to lead, and the dioxygen reaction of the trans-bent Pb 2 H 4 minimum is even less favorable than that of of H 2 C CH 2 . Nevertheless, Pb Pb π bonds are not very stable. Diplumbene can dissociate easily; reaction of the resulting divalent PbH 2 subunits to tetravalent lead−oxygen compounds is not very favorable. The 1,3-dioxa-2,4-dimetaletanes have short through-ring X···X distances, but no evidence for an X···X bond was found by natural bond orbital analysis. Instead, the intramolecular distances are due to the short and inflexible X−O bond lengths and to the electrostatic O···O repulsion. In (H 2 XO) 2, the hybridization not only of the group 14 elements down the group but also of the oxygen lone pairs and bond orbitals changes significantly.