Coupled-cluster calculations were performed for cyclobutane-1,3-diylidene dicarbenes 2 at the CCSD(T)//CCSD/ cc-pVDZ level of theory, in which the ground-state spin multiplicity and the structures of unique molecules were investigated in detail. The closed-shell singlet state 2(S σπ ) with a bicyclo-[1.1.0]but-1(3)-ene (BBE) structure found to be the groundstate was much lower in energy than the corresponding singlet dicarbene structure 2(S ** ), the quintet state 2(Q), and the triplet state 2(T), suggesting that the hitherto experimentally unknown BBE structure can be synthesized by the intramolecular dimerization of two carbene units. The energy gap between the BBE structures 2(S σπ ) and corresponding quintet states 2(Q) with electron-withdrawing substituents (X = F) at the C2 and C4 positions was found to be larger than that with electrondonating substituents (X = SiH 3 ), i.e., ca. 100 kcal mol ¹1 for 2b (X = F) > ca. 85 kcal mol ¹1 for 2a (X = H) > ca. 70 kcal mol ¹1 for 2c (X = SiH 3 ). Two unique structures, 2(T σ ) with a C1σC3 bond and 2(T π ) with a C1πC3 bond, were found to be the equilibrium structures for the triplet state of cyclobutane-1,3-diylidene dicarbenes 2.

Coupled-cluster calculations were performed for 1,3-disilacyclobutane-1,3-diylidene disilylenes at the CCSD(T)//CCSD/cc-pVDZ level of theory to investigate the ground-state spin multiplicity and structures of these unique molecules in detail. The cis-bent disilene with an R 2 Si═SiR 2 double-bonding structure was found to be the most energetically stable structure for the 2,2,4,4-tetrafluoro-substituted compound. In contrast, the singlet disilylene structures (R 2 Si:) with a 4-membered ring geometry were calculated to be more energetically stable than the disilene structures for 1,3-disilacyclobutane-1,3-diylidene disilylenes with electron-donating substituents such as H and SiH 3 . Thus, the perfluoro-substituted disilylene was found to be a candidate for the synthesis of cis-bent disilenes.