Triplet ground state, dioxygen

Figure 11.2. Molecular orbital diagrams for O2 and O2. The ground state normal O2 (dioxygen) = triplet ground state) has two unpaired ir-electrons (it is a biradical) and is paramagnetic. Two excited forms of singlet oxygen O2 ( A and E ) can be generated (often by light irradiation in the presence of a sensitizer). The first product in a one-electron reduction of oxygen is the superoxide ) anion. (Adapted from Sawyer, 1991.)...

As mentioned above, NO can also be reduced to the nitroxyl anion (NO-) (Eq. (5)), which is isoelectronic with 02, and like dioxygen has a triplet ground state (15). 

Dioxygen could overcome the kinetic barrier of its unpaired electrons and triplet ground state by excitation to its first excited state (xAg), in which all electrons are paired. Unfortunately, this species, referred to as singlet oxygen, is generally too reactive and too short-lived for most situations (lb, lc). However, dioxygen complexation to a transition metal can also result in activation and create stable complexes that can be studied, modified, and used in further reactions in a controlled manner (2). This latter type of activation is the subject of this chapter. 

As noted earlier, there is a kinetic restriction on the reactivity of dioxygen, as the one-electron reduction to superoxide is unfavoured. This is demonstrated in the redox potentials shown in Figure 51. The triplet ground state of the molecule may also impose spin restrictions on its reactions. 

In the ground state of O2, the outermost two electrons occupy a doubly degenerate set of antibonding tt orbitals with parallel spins. Dioxygen is thus a paramagnetic molecule with a triplet ground state (3 ), and its formal double bond has a length of 120.752 pm. 

Triplet states are generally molecules in which one electron is promoted to a higher electronic orbital (e.g. by the absorption of light) under spin reversal, i.e. it is no longer paired with its partner in the orbital it has left and the two electrons have a combined magnetic moment what is twice that of a radical. Some molecules, e.g. dioxygen, have a triplet ground state. 

Dioxygen is a colourless gas, but condenses to a pale blue liquid or solid. Its bonding was described in Sections 2.2 and 2.3. In all phases, it is paramagnetic with a triplet ground state, i.e. the two unpaired electrons have the same spin, with the valence electron configuration being ... 

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