Aza‐ versus Phospha‐Substitution in Cyclobutadiene: A Theoretical Evaluation of Bonding Properties

Completely different effects on the stability of cyclobutadiene are found upon replacement of C with N or P. Whereas in the phospha derivatives the cyclobutadiene structure is less stable than the tetrahedrane, and the barrier to valence isomerization is reduced, the exchange of N for C increases this barrier, and the tetrahedranes are prone to fragmentation. According to the results of the ab initio calculations described here, not only the monoaza‐ and monophosphacyclobutadienes but also their analogues with two heteroatoms should, in principle, be experimentally accessible.