CONFORMATIONAL ENERGY CALCULATIONS ON 5-MEMBERED RINGS IN PENTACOORDINATE PHOSPHORUS (REPRINTED FROM J AM CHEM SOC, VOL 97, PG 5379, 1975)

shown in agreement with X-ray data that the square pyramid (SP) confor mation of five-membered ring containing pentacoordinate phosphorus is stabilized relative to the corresponding trigonal bypyramid (TP) when the ring systems are unsaturated and contain highly electronegative he teroatoms directly attached to phosphorus. The primary cause of the st abilization is attributed to ring strain produced by differences in bo nd lengths which are inherent in the TP form for equatorial vs. apical bonds. For dioxaphospholene compounds examined, ring strain is estima ted to be about 3-4 kcal/mol greater in the TP. This enhances the over all stability of the SP conformation relative to the TP form. As the e lectronegativity of the heteroring atoms is reduced, ring strain effec ts become less important and the TP appears as the more stable structu re in line with structural evidence. The presence of mixed heteroring atoms directly attached to phosphorus and the presence of saturated ri ngs are expected to reduce ring strain differences, thus favoring the formation of the TP, while the presence of four-membered rings will in tensify the formation of the SP conformation.