Triplet ground state of 2,5-dimethylene five-member heterocycles

2,5-Dimethylene five-member heterocycles have been thought to possess exclu sively singlet ground states. On the basis of the analysis of charge redist ribution upon switching the state multiplicity of pi-CASSCF optimized geome tries, we calculate that the 2,5-dimethyleneborole dianion should exist in a triplet ground state. Additionally, the 2,5-dimethyleneborole dianion and the analogous 2,5-dimethylenealanacyclopentadiene dianion, 2,5 -dimethylen eberyllacyclopentadiene trianion, and 2,5-dimethylenemagnesacyclopentadiene trianion have triplet ground states at the MCQPDT2 level in the CASSCF geo metry. Furthermore, we find the upper limit of the S-T gap, \Delta ES-T\ to be 50 kcal/mol, which correlates with the S-T gap calculated for the paren t hexatriene hydrocarbon. The Delta ES-T trend approximately follows the gr oup electronegativity scale of Boyd and Boyd derived from the bond critical point model for the heteroatom and protons attached to it. The 2,5-dimethy leneborole dianion and its analogues are members of a new class of ferromag netic coupling units (FCs) for use in the construction of molecular organic ferromagnets.