We investigated the electronic structure of 1-methylene-3,5-didehydrobenzene triradical (MDB, C7H5) containing a sigma(1)sigma(1) biradical benzene ring coupled with an exocyclic eta(1) CH2. MS-CASPT2(9,9)/ANO-L calculations reveal that MDB exhibits a low-spill doublet B-2(1) ((II)-I-2) ground state, followed by an open-shell doublet (2)A(2) state with a (2)A(2)-B-2(1) gap of 17 +/- 3 kcal mol(-1). The energy\ ordering of MDB is (kcal mol(-1)): (XB1)-B-2 (0) < A(2)A(2) (17) < a(4)A(2) (20) < (BB2)-B-2 (39) < b(4)A(1) (59) < C(2)A(1) (61) < (CB1)-B-4 (75) < d(4)B(2) (79). The doublet states are lower in energy than the quartets, violating Hund's rule, due to a coupling of the two unpaired sigma-electrons transforming the triradical into a radical. (C) 2005 Elsevier B.V. All rights reserved.