Molecular-orbital calculations cyclopropenes

The s character of the vinylic C-H bond of cyclopropene is midway between that of sp and sp hybridized bonds and some similarity in behaviour to the acetylenic C—H bond is noted, particularly with regard to the acidity of the proton. Molecular orbital calculations at the ab initio level support an enhanced acidity of C(l)-H over C(3)-H in cyclopropene and correctly predict the preference for a 1-lithio derivative. A bis-lithio derivative is expected to have both lithium atoms bridging the C(l)-C(2) a bond. 

The reactivity of carbene-metal complexes, amongst others the reactivity with respect to alkenes and alkynes, has been reviewed by Dotz Just like free carbenes the coordinated carbenes add to triple bonds to give cyclopropene derivatives. Other reaction products, however, are also possible. For instance, the carbene ligand of chromium complex 23 reacts with diphenylacetylene to a mixture of products, including naphthalene derivative 24 and furan derivative 25 (equation 18). A carbonyl ligand has participated. Molecular orbital calculations by Hofmann and Hammerle " on this system reveal that the reaction would pass through an y-vinylcarbene type of complex (26) instead of through a planar chromacyclobutene 27. The subsequent steps to yield either phenol or furan could involve vinylketene 28, but this still is a matter of debate. Similar, but more selective, furan syntheses have been observed for carbene complexes based on iron and cobalt. ... 

MINDO/3 calculations relative to cyclopropane and cyclopropene show that the two highest molecular orbitals for silacyclopropane and silacyclopropene are destabilized. As a result, silacyclopropane and silacyclopropene are predicted to be more reactive toward electrophiles than their carbon analogues <79JCS(P1)443>. 

---