Stabilization of octacoordinate carbon center in metal-containing derivatives of orthocarbonic acid

Abstract

The electronic and spatial structures of alkali metal compounds CO 3 M 2 , CO 3 M 3 + , and CO 4 M 4 (M = Li, Na, K) were investigated by the ab initio (MP2(full)/6 311+G**) and density functional (B3LYP/6 311+G**) methods. The calculated energies of formation decrease in the order E Li > E Na > E K for all structural types, being determined by steric and orbital interactions. Stable structures with octacoordinate carbon are formed in the case of CO 4 M 4 salts.Key words: octacoordinate carbon, carbon oxoanions, orthocarbonic acid, alkali metals, quantum chemical calculations.Recently, search for and studies of novel types of non classical valence systems containing hypercoordinate atoms of main group elements have been the subject of increasing interest. 1-9 An important problem in chemis try of hypercoordinate compounds, together with the problems of the maximum possible coordination number of an atom of a certain element and corresponding stereo chemical conditions for stabilization of such systems, 1,2,8 is the design of novel systems with unusual stereochemis try or coordination. In this respect, the best developed field is hypercarbon chemistry for which not only a vari ety of non classical hypercarbon compounds have been found but also efficient methods for the design and stabi lization of such systems were developed. 1-9One of the most convenient and promising approaches to the design of systems with hypercoordinate centers is based on the "induced aromaticity" effect. 10 This meth odology involves the incorporation of counterions (usu ally, lithium cations) into originally unstable anionic non classical systems. 1,10-13 This has been successfully imple mented taking various non classical systems including pla nar (1), 11 pyramidal (2), 11 and sandwich (3) 13 structures with hypercoordinate carbon as examples.Recent studies 2,11,13 showed that the role of the lithium counterions is not reduced to the electrostatic stabiliza tion; they are also involved in the charge transfer pro cesses leading to formation of a closed electron shell of * Dedicated to Academicians A. L. Buchachenko and N. S. Zefirov on the occasions of their 70th birthdays.

D3h CN3Be3+ and CO3Li3+: viable planar hexacoordinate carbon prototypes

“…The extreme case, CO 3 Li 3 + , has a D 3h 3a global minimum, which is 32.3 kcal mol À1 more stable than the second lowest isomer (3b). We note that a study in 2005 analysing CO 3 M 3 + had included CO 3 Li 3 + , 21 and a recent study of superalkali cations also explored the PES of CO 3 Li 3 + . The later work only located 3a and 3b, but reported much higher third and fourth lowest isomers, rather than 3c and 3d.…”