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Ab initio and extended Hueckel calculations have been used to discuss the bonding scheme in X{sub 2}Y{sub 2} neutral and ionic main group clusters. A qualitative analysis suggests that two different electron counts, 20 and 22, are possible for the butterfly structures of these systems. This results from two orbital crossings in the correlation diagram for the tetrahedral (T{sub d}) {yields} butterfly (C{sub 2{nu}}) {yields} square-planar (D{sub 2h}) transformation. Detailed ab initio computations substantiate this analysis and show that the 20-electron butterfly structure becomes increasingly favored over the tetrahedral one in X{sub 2}Y{sub 2} clusters when the 2 atoms have increasing electronegativity difference. These results are in agreement with the known structures for the Pb{sub 2}Sb{sub 2}{sup 2{minus}} and Sb{sub 2}Bi{sub 2}{sup 2{minus}} clusters (tetrahedral-like) and the Tl{sub 2}Te{sub 2}{sup 2{minus}} one (butterfly-like).

A theoretical study of models for X2Y2 Zintl ions