An SCCC xnolecular orbital c;12culation was performed for low spin square-pyramid& Co(CN)$-. The results show that the ordering of the d fevels xy<xz, yzC za & xz--y2, which is in unexpected from the point of view of crystal field theory, is due to the fact that rhe metal lies above the equatorIj1 plane offhe ligands.
The energy of the 3d metal orbit& in squarepyranidat complexes with rr acceptor iigands is genera& assumed to be bz<e<a,<b,.
From the point of view of crystal field theory, the energies of the b, and e levels are inverted. Two interpretations for this fact have been proposed. Firstly fl] , assuming that back- donation is of importance, the b2 orbital has ?arger overlap with the ?r* (CN-) functions than the e orbitais. Thus, stabilization due to back-donation would favour the bz level. Secondly 123, taking into account that the metal ion is expected to lie above tile plane of the equatorial Iigands, the e orbit& should have non-zero overlap with the ligands o functions 2~"(a, -03) and 2-~(02-(Tq) (see fig. I). This fact should raise tile energy of the e teveis above that of b?, which oniy has a weak rr interaction, IR ord& to gain further insight into the electronic energy levels of square-pyramidal complexes, a molecular orbital calculation was performed for the Cog-ion (point group C,,+,). The SCCC MO method was employed, as described in refs. f3--51. Cobalt 3d, 4s and 4p functions were taken from [6] ; Co(H) functions were used and ~on~guration d7p was chosen for the 4p orbital. &fofecular orbit& for CN-, which are based on Ciementi' s doubfe !: atomic functions for carbon and nitrogen [7] , were taken from {4J. interatomic distances were assumed to be 1.89 ,& (Co-C) and I .I5 A (C-N), as in the hexacyanide complex ion [4] _ L&and-i&and overlap corrections were included. Metal VOiPs were taken from f8] .
J_ig&nd orbital energy levels were given the values: