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

Abstract

Structure and stability of monocyclic (CH) 4−n (BL) 2− n dianions (L = CO, N 2 , CS) with 6π-electrons, which are isolobal with cyclobutadiene dianion (CH) 2− 4 , have been investigated at the B3LYP and CCSD(T) levels of theory. (CH) 3 (BL) 2− have non-planar singlet ground states. 1,2-(CH) 2 (BL) 2− 2 have planar singlet ground states, while 1,3-(CH) 2 (BL) 2− 2 isomers have folded four-membered central rings. Both (CH)(BCO) 2− 3 and (CH)(BN 2 ) 2− 3 have planar ground states, the ground state of (CH)(BCS) 2− 3 has a six-membered ring with two bridging and one terminal CS. Both (BCO) 2− 4 and (BN 2 ) 2− 4 have reduced aromaticity compared to (CH) 2− 4 as indicated by the less negative nucleus independent chemical shifts (NICS) values. The mono-, di-and trisubstituted (CH) 3 (BL) 2− , 1,2-(CH) 2 (BL) 2− 2 , and (CH)(BL) 2− 3 also have reduced aromaticity, while the 1,3-substituted (CH) 2 (BL) 2− 2 have positive NICS values due to the localization of the negative charge at the ring carbon centers. In addition, the electrostatic stabilization of Li + in favor of the singlet or triplet states depends on the nature of their structures.