A THEORETICAL-STUDY OF THE DIMERIZATION OF PHOSPHAETHYNE (HCP) - HEAD-TO-TAIL VERSUS HEAD-TO-HEAD CYCLOADDITION

Ab initio molecular orbital calculations (at the MP4/6-31G(d,p) level using MP2/6-31G(d,p) geometries and ZPE corrections) on the cycloaddition of the model HC=P + HC=P system suggest that the head-to-head dimerization of phosphaalkyne giving 1,4-diphosphacyclobutadiene is inherently favored over the head-to-tail approach yielding 1,3-diphosphacyclobutadiene (by 37 kJ/mol); thus the experimental observation of the latter can be understood by a steric effect of the bulky substituents employed or a catalytic effect of metal fragments. Both dimers undergo, however, further reactions giving the most stable structure, diphosphabicyclo[1.1.0]butane. The latter molecule, also trapped by metal fragments, is likely to be an intermediate of the thermal tetramerization of HCP, leading to tetraphosphacubane.