In order to elucidate the electronic nature of our recently reported first hexacoordinated carbon (12-C-6), density functional theory (DFT) calculations of sulfide precursor, sulfone derivative, and S+-F derivative were carried out and compared with those of the reported S+-Me hexacoordinated carbon. Computations on the hexacoordinated carbon, indicating that four attractive C-O interactions with the central hexacoordinate carbon atom exist, also revealed that the interactions consist of two different types of three-center four-electron bonds, which can be regarded as electron donation by the lone pairs of the oxygen atoms to the empty low-lying π"-orbitals of the allene. The optimized structures of the sulfide, sulfone, S+-F, and the original S+-Me suggested that the introduction of electron-withdrawing groups at the sulfur atoms would make the C-O attractive interactions stronger by a larger contribution of the carbon dication resonance structure. Thus, allene compounds (sulfide, sulfone, sulfonium) with two different thioxanthene ligands (one with 1,8-dimethoxy groups as in the S+-Me compound and the other with 1,8-diphenoxy groups) were synthesized to confirm the predicted substituent effects on the C(central)-O interactions. Electron-withdrawing substituents at the sulfur atoms were found to give rise to strong C(central)-O attractive interactions; the average values of the four C-O distances were smaller as the electron-withdrawing ability of the sulfur atoms rose. Additionally, C(central)···OMe distances were shorter than the corresponding C(central) ···OPh distances, reflecting the higher electron-donating ability of the oxygen atoms at these 1,8-positions of the thioxanthene skeleton. [ABSTRACT FROM AUTHOR]