Sigma-only octahedral molecular orbital model

A model in which six donor orbitals of an octahedral complex form $a_{1g}+e_g+t_{1u}$ symmetry-adapted combinations. metal s, p and two d orbitals supply matching symmetry partners; the other three d orbitals remain nonbonding $t_{2g}$ orbitals. The model neglects ligand pi donation, backbonding and departures from octahedral symmetry.