Dynamics of dioxygen binding to vacant cobalt(II) sites in lacunar cyclidene complexes: barrier-free oxygenation

The kinetics of O2 binding to a demonstrably vacant coordination site on a cobalt(II) ion are determined, revealing a radical-like character for the reaction, with a very low activation barrier (ca. 1–2 kcal mol–1) and large entropically controlled rate constants, whose values approach those for myoglobin and haemoglobin (up to 108 dm3 mol–1 s–1).