Transition metal oxides: CrO, MoO, NiO, PdO, AgO

The transition metal oxides are quite ionic, Mulliken population analyses for several oxides give a negative charge of ∼0.7 electrons for oxygen. When the transition metal d shell is only partially filled, the d orbitals are involved in covalent bonds with O; both two electron (bonding2–antibonding0) and one electron (bonding2–antibonding1) bonds are formed. These covalent bonds occur in addition to the ionic bonding. There is dσ-O2pσ repulsion and this repulsion is reduced when the dσ electron is promoted into an orbital which has dominantly 4spσ character and is polarized away from O. This effect is quite large, greater than half and electron, for some states of CrO and MoO.