Transition Metal Complexes with CO, N2, NO and O2 Ligands

The diatomic molecules and ions, CO, N2, CN, NO and O2, can bond to transition metals. Isoelectronic CO, N2 and CIST, with ten valence-shell electrons, have triple bonds and no unpaired electrons in their Lewis octet structures (l)-(3). 

Each of these structures can use a lone-pair of electrons to coordinate with a transition metal, as in structures (4)-(6), thereby functioning initially as Lewis bases. The ground-states of NO and O2, with 11 and 12 valence-shell electrons respectively, have one and two unpaired electrons. Their valence-bond structures (7) and (8) involve one or two Pauling 3-electron bonds as well as lone-pairs of electrons. The possibility exists that these molecules may react either as Lewis bases or as free radicals. For NO, examples of both Lewis base and free radical behaviour are known, but ground-state O2 seems to behave exclusively as a free radical. We shall now describe valence-bond structures for a few transition metal complexes that involve some of these ligands. 

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Chapter 18 Transition Metal Complexes with CO, N2, NO and O2 Ligands... 

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