Structure and bonding in stannadiphospholes and their dianions SnC<SUB>2</SUB>P<SUB>2</SUB>R<SUP>m</SUP><SUB>2</SUB> (R=H, tBu m=0, -2): A comparative study with C<SUB>5</SUB>H<SUB>5</SUB><SUP>+</SUP> and C<SUB>5</SUB>H<SUB>5</SUB><SUP>-</SUP> analogues

Abstract

The potential energy surfaces of both neutral and dianionic SnC2P2R2 (R=H, tBu) ring systems have been explored at the B3PW91/LANL2DZ (Sn) and 6-311+G* (other atoms) level. In the neutral isomers the global minimum is a nido structure in which a 1,2-diphosphocyclobutadiene ring (1,2-DPCB) is capped by the Sn. Interestingly, the structure established by X-ray diffraction analysis, for R=tBu, is a 1,3-DPCB ring capped by Sn and it is 2.4 kcal mol-1 higher in energy than the 1,2-DPCB ring isomer. This is possibly related to the kinetic stability of the 1,3-DPCB ring, which might originate from the synthetic precursor ZrCp2tBu2C2P2. In the case of the dianionic isomers we observe only a 6π-electron aromatic structure as the global minimum, similarly to the cases of our previously reported results with other types of heterodiphospholes.1,4,19 The existence of large numbers of cluster-type isomers in neutral and 6π-planar structures in the dianions SnC2P2R22- (R=H, tBu) is due to 3D aromaticity in neutral clusters and to 2D π aromaticity of the dianionic rings. Relative energies of positional isomers mainly depend on: 1) the valency and coordination number of the Sn centre, 2) individual bond strengths, and 3) the steric effect of tBu groups. A comparison of neutral stannadiphospholes with other structurally related C5H5+ analogues indicates that Sn might be a better isolobal analogue to P+ than to BH or CH+. The variation in global minima in these C5H5+ analogues is due to characteristic features such as 1) the different valencies of C, B, P and Sn, 2) the electron deficiency of B, 3) weaker pπ-pπ bonding by P and Sn atoms, and 4) the tendency of electropositive elements to donate electrons to nido clusters. Unlike the C5H5+ systems, all C5H5- analogues have 6π-planar aromatic structures as global minima. The differences in the relative ordering of the positional isomers and ligating properties are significant and depend on 1) the nature of the π orbitals involved, and 2) effective overlap of orbitals