The four-membered Cyclic C4H2, resulting from electrocyclization of butadiyne has been found 75.2 kcal/mol above the latter species. In its ground state, it is a singlet non-planar biscarbene of C-2 symmetry. The electronic structures of the singlet and triplet have been studied using ELF analysis. The opening into butadiyne needs ca. 20 kcal/mol activation and follows a pathway which does not preserve the C-2 symmetry. Breaking the carbene-carbene bond results in a bicyclo-biscarbene, a degenerate isomerization which offers a reaction path for the scrambling Of C2C3 atoms in butadiyne. Breaking the HC-C bond results in a sigmatropic shift yielding cyclopropylidene. (C) 2004 Elsevier B.V. All rights reserved.