Vaska's Complex - Reactions

Reactions

Studies on Vaska's complex helped provide the conceptual framework for homogeneous catalysis. Vaska's complex, with 16 valence electrons, is considered "coordinatively unsaturated" and can thus bind to one two-electron or two one-electron ligands to become electronically saturated with 18 valence electrons. The addition of two one-electron ligands is called oxidative addition. Upon oxidative addition, the oxidation state of the iridium increases from Ir(I) to Ir(III). The four-coordinated square planar arrangement in the starting complex converts to an octahedral, six-coordinate product. Vaska's complex undergoes oxidative addition with conventional oxidants such as halogens, strong acids such as HCl, and other molecules known to react as electrophiles, such as iodomethane (CH₃I).

An interesting characteristic of Vaska's complex is that it binds O₂ reversibly.

IrCl(CO)₂ + O₂ ⇌ IrCl(CO)₂O₂

The dioxygen ligand is bonded to Ir via both oxygen atoms, so-called side-on bonding. (In myoglobin and hemoglobin, by contrast, O₂ binds "end-on," attaching to the metal via only one of the two oxygen atoms.) The oxygenation reaction is carried out simply by purging a solution of Vaska's complex in toluene with O₂, which results in a colour change from yellow to orange. The resulting dioxgen adduct reverts to the parent complex upon heating in boiling benzene solution, or by flushing the solution with an inert gas.