STRUCTURE AND STABILITY OF MONOCYCLIC $({\rm CH})_{4-n} ({\rm BL})_{n}^{2-}$(L = CO, N2, CS) DIANIONS AND THEIR DILITHIUM COMPLEXES

Structure and stability of monocyclic $({\rm CH})_{4-n}({\rm BL})_{n}^{2-}$ dianions (L = CO, N2, CS) with 6π-electrons, which are isolobal with cyclobutadiene dianion $({\rm CH})_{4}^{2-}$, have been investigated at the B3LYP and CCSD(T) levels of theory. (CH)3(BL)2- have non-planar singlet ground states. $1,2-({\rm CH})_{2}({\rm BL})_{2}^{2-}$ have planar singlet ground states, while $1,3-({\rm CH})_{2}({\rm BL})_{2}^{2-}$ isomers have folded four-membered central rings. Both $({\rm CH})({\rm BCO})_{3}^{2-}$ and $({\rm CH})({\rm BN}_{2})_{3}^{2-}$ have planar ground states, the ground state of $({\rm CH})({\rm BCS})_{3}^{2-}$ has a six-membered ring with two bridging and one terminal CS. Both $({\rm BCO})_{4}^{2-}$ and $({\rm BN}_{2})_{4}^{2-}$ have reduced aromaticity compared to $({\rm CH})_{4}^{2-}$ as indicated by the less negative nucleus independent chemical shifts (NICS) values. The mono-, di- and trisubstituted (CH)3(BL)2-, $1,2-({\rm CH})_{2}({\rm BL})_{2}^{2-}$, and $({\rm CH})({\rm BL})_{3}^{...