Why does the yellow isomer of [Sn{PPh2Se)(2)-Se,Se ')}(2)] present a square planar four-co-ordinated tin(II)? A molecular orbital approach

Ab initio (RHF/3-21G*) calculations on Sn[(SePR2)(2)N](2) (R=H) suggest tha t tin coordinated by four selenium atoms does not have a strong preference either for square planar nor for pyramidal geometry. According to the semie mpirical PM3 calculations performed at the experimental geometries of Sn[(S ePPh2)(2)N](2) it is most likely that the isomer containing the planar Sn(I I) is stabilized by pi-pi interactions between the interlocked phenyl rings .