Are the C4v complexes of cyclobutadiene with CO, NO+, and CS minima?

While the C 4v pyramidal structures of the cyclobutadiene-CO and cyclobutadiene-NO + complexes are second-order saddle points rather than minima, the pyramidal C 4v cyclobutadiene-CS complex is a minimum due to stronger interactions of the e π * orbitals of the CS fragment with the cyclobutadiene e g π-orbitals. However, the isomerization barrier of this complex into bicyclo[2.1.0]pententhione, only 0.5 kcal/mol at MP2/6–31G * //MP2/6–31G * +ZPE(HF/6–31G * ), disappears at higher levels. This C 4v C 4 H 4 CS complex is unlikely to be a viable candidate for experimental observation as an isolated entity.