The geometry of small rings—IV: Molecular geometry of cyclopropene and its derivatives

Numeric structural data for 34 derivatives of cyclopropene and cyclopropenium ion have been retrieved from the Cambridge Crystallographic Database and analysed in conjunction with available gas-phase results. Geometric data indicate that the vinylic C atoms in cyclopropene use sp^1.19 hybrids in bond formation to substituents and contribute sp^2.68 hybridges to the ring σ-framework. The D_3h-symmetric cyclopropenium ion has a bond length of 1.373(3)Å, which can be related to distances in unstrained systems. Comparison of data for cyclopropenylidenes and 3,3-difluorocyclopropene with analogous cyclopropanes shows that π-donor effects (distal bond lengthening, vicinal bond shortening) are apparent in cyclopropene. Rehybridization and π-donation are largely responsible for cyclopropenylidene geometry, rather than significant contributions from dipolar and pseudo-aromatic resonance forms. Insufficient data exist to quantify the effect of π-acceptor substituents on cyclopropene, but some lengthening of vicinal bonds is apparent. Three major bonding patterns are exhibited by organomental derivatives of cyclopropenium and cyclopropene.