Ab initio and density functional study of structure and bonding of cage compounds containing boron, phosphorus and group 14 atoms

Abstract

Ab initio and density functional calculations for five vertex cage compounds of the type P₂(YB)₂EH₂, (E = C, Si, Ge, Sn, Y = H, H₂N) using 6-311G++(d,p) and 3-21G++(d,p) basis sets, showed that they adopt a trigonal bipyramidal structure of C_2v symmetry with the two phosphorus atoms occupying the apical position, the HB or NB groups residing in the trigonal basal plane. The theoretical results agree well with the experimental data for E = Si, Ge and Sn. For the carbon cage P₂(HB)₂CH₂ this structure is a transition state and the minimum is a cage of C_s symmetry with a distorted B–B–C ring in the equatorial plane. Key structural parameters within the cages show a linear dependence on the covalent radii and the diffuseness of the orbitals of the heteroatom E. The results of the natural population analysis (NPA) and the values of nucleus independent chemical shifts (NICS) calculated reveal a bonding description very close to the idealized localized Lewis structure, in contrast to the parent P₂(HB)₃ cage. This is more pronounced in the case of the NH₂ substituted cages.