Fig. shows relevant MOs in the acetylene-derived diradical, C 2 H 2 O. The electronic structure of diradicals derived from triple-bond compounds is different from that of C 2 H 4 O. In the lowest triplet state, there are two planar isomers, Z and E. The symmetry plane separates the orbitals into the π(a ) and σ(a ) manifolds. The π-system and the orbital occupations are remarkably similar to the π-system of the allyl radical (see Fig. in SI), giving rise to the CC and CO bond orders of ∼1.5 and and an increased barrier to rotation. The σ system comprises lone pairs on oxygen and carbon. While these orbitals do not contribute towards bonding in the triplet state, they are important for the analysis of excited states and transition properties because they are involved in the A transitions.