carbon uses strong 2s/2p-derived covalent bonding, whereas retention or involvement of the low-energy Pb 6s pair distinguishes the two lead oxides. In CO, carbon has an approximately triple-bonded description with one carbon lone pair; the latter has substantial 2s character, while the sigma and two pi bonding combinations involve the valence shell, particularly carbon 2p orbitals for the pi bonds. In CO, carbon is formally in oxidation state +4 and its four valence electrons participate in two strongly covalent C=O double-bond frameworks. A localized account uses sp hybrids for sigma bonding and two p orbitals for the pi systems. oxidation states are bookkeeping devices here: neither molecule contains a bare C or C ion.
In PbO, the Pb(II) description removes or employs mainly the two 6p electrons, retaining a relatively low-energy 6s pair. In PbO, the Pb(IV) description additionally involves those two 6s electrons. The stabilization and poor availability of that pair underlie the inert-pair effect and the relative preference for Pb(II) chemistry. “Inert” does not mean that the 6s electrons have zero influence on bonding or shape: mixing with oxygen states and empty Pb states can make the Pb(II) lone pair stereochemically active. The more ionic formal electron counts in the lead oxides and the covalent molecular-orbital description of the carbon oxides are complementary models, not literal assignments of individual electrons to immutable atomic orbitals.[ref]