Topographical analyses of homonuclear multiple bonds between main group elements

Recent experiments have resulted in the completion of the series of Group 14 and Group 15 element double-bond systems, R_nE=ER_n (E = C-Pb, n = 2; E = N-Bi, n = 1). Furthermore, new families of multiple-bonded species have been discovered, such as the radical anion [RSnSnR]^-, the close ion pairs [RE(μ₂Na)₂-ER] (E = Ge,Sn), and a digallyne [RGa(μ₂Na)₂GaR] for which a Ga=Ga triple bond was formulated. Some of these compounds show classical multiple bond features (i.e. the dipnictogens RE=ER, E = N - Bi) in the sense that planar structures with short E-E distances are observed. However, many (i.e. R₂E=ER₂, E = Si - Pb) do not behave as expected for compounds with multiple bonds. They have trans bent structures, show enormous variation in their E-E distances, and some dissociate easily under E-E bond cleavage in solution. These properties raised doubts as to whether these compounds can be formulated as multiple-bonded systems. Using the electron localization function (ELF) it is possible to clearly show the topographical similarities between classical and nonclassical multiple bonds; ELF divides these systems into unslipped (classical) and slipped (nonclassical) systems. ELF can also be employed to confirm the nonexistence of multiple bonds. Therefore, topographical analyses using ELF are useful to categorize a bonding system. In particular, the bonds in the heavier Group 14 double systems and the Ga-Ga bond in digallyne are clearly shown by this method as slipped double and triple bonds, respectively.