ORIGIN OF THE UNEXPECTED SP VS SP(2) HYBRIDIZATION OF CARBON IN THE MB(2)C (M=TH, U) SOLID-STATE PHASES - AN AB-INITIO STUDY OF THE [H2BCBH2](4-) MODEL AND ISOELECTRONIC SPECIES

In the solid-state actinoid-boron-carbon MB(2)C compounds, the nonmetal atoms form two-dimensional planar anionic nets in which the carbon is sp-hybridized, whereas the sp(2) hybridization is expected. This paper describes an ab initio study of the [H2BCBH2](4-) anion and several other isoelectronic systems chosen to modelize the [B2C](4-) repeat unit present in the MB(2)C phases. It is shown that the three-orbital/four-electron interaction pattern usually invoked to explain the bending of triatomic units is still valid. However, the electronegativity difference between B and C forces the system to work in the other way, i.e, favors the linearity or quasi-linearity. Calculations indicate that the potential energy surfaces associated with the bending are extremely flat over a large angle range around linearity. It is likely that in the real MB(2)C compounds crystal forces and the presence of B-B bonds tend to reinforce the preference for BCB linearity. This trend of opposition to bending, which is caused by a large electronegativity difference effect, should be quite general in chemistry.