On the electronic structures of the 1,3-diboracyclobutane-1,3-diyls and their valence isomers with a B2E2 skeleton (E = N, P, AS)

The concept of through-space versus through-bond interactions on the stabilization of biradical structures with a singlet or triplet ground state is evaluated for the 1,3-diboracyclobutane-1,3-diyls and related congeners. Singlet biradicals are favored when the intermediate units E feature singlet character (PH2+, AsH2+), while E fragments with triplet character (NH2+) induce small energy separations between the lowest singlet and triplet states. These considerations are supported by quantum chemical calculations with energy optimization at 1) MCSCF level plus MR-MP2 correction, 2) MRMP2 level, and 3) two different types of density functional levels for the planar (D-2h) geometries. The singlet-triplet energy separations in the planar compounds increase with increasing singlet stability of the corresponding E fragments. In addition to this newly developed principal features for singlet stabilization, which primarily occurs in bonded structures with higher main-group elements, the corresponding valence isomers with bicyclobutane, cyclobutene and cis-butadiene structures are investigated.