Ab initio and extended Hückel calculations have been used to discuss the bonding scheme in X₂Y₂ 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_d) -> butterfly (C_2v) -> square-planar (D_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 X₂Y₂ clusters when the 2 atoms have increasing electronegativity difference. These results are in agreement with the known structures for the Pb₂Sb₂²̄ and Sb₂Bi₂²̄ clusters (tetrahedral-like) and the Tl₂Te₂²̄ one (butterfly-like).
© 1989 American Chemical Society
Cave, R.J.; Davidson, E.R.; Sautet, P.; Canadell, E.; Eisenstein, E. “A Theoretical Study of Models for X2Y2 Zintl Ions,” J. Am. Chem. Soc. 1989, 111, 8105. DOI: 10.1021/ja00203a006