Vaska's complex

Vaska's complex is the trivial name for trans-chlorocarbonylbis(triphenylphosphine)iridium(I) with the formula IrCl(CO)[P(C6H5)3]2. This square planar diamagnetic organometallic complex consists of a central iridium atom bound to two mutually trans triphenylphosphine ligands as well as carbon monoxide, and chloride. The complex was reported by Di Luzio and Vaska in 1961. Vaska's complex can undergo oxidative additions and is notable for its ability to bind to O2 reversibly. It is a bright yellow crystalline solid.

Preparation
The synthesis involves heating virtually any iridium chloride salt with P(C6H5)3 with a CO source. The most popular method uses dimethylformamide (DMF) as a solvent. Sometimes aniline is added to accelerate the reaction. Another popular solvent is 2-methoxyethanol. The reaction typically is conducted under nitrogen. In the synthesis, triphenylphosphine serves as both a ligand and a reductant, and the carbonyl ligand is derived by decomposition of dimethylformamide probably via a deinsertion of an intermediate Ir-C(O)H species. The following is a possible balanced equation for this complicated reaction.


 * H2IrCl6 + 3.5P(C6H5)3 + HCON(CH3)2 + 4C6H5NH2 + 1.5H2O → IrCl(CO)[P(C6H5)3]2 + (CH3)2NH2+Cl- + 1.5 OP(C6H5)3

Typical sources of iridium used in this preparation are IrCl3.xH2O and H2IrCl6.

Reactions
Studies on Vaska's complex provided a conceptual framework for homogeneous catalysis. Vaska's complex, with 16 valence electrons, is considered "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)[P(C6H5)3]2 + O2 ↔ IrCl(CO)[P(C6H5)3]2O2

The dioxygen ligand is bonded to Ir(I) via both oxygen atoms, so-called side-on bonding. In myoglobin and hemoglobin, O2 binds "end-on," attaching to the metal via only one of the two oxygen atoms. The oxygenation reaction is carried out simply by bubbling O2 through a solution of Vaska's complex in toluene, 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.

Spectroscopy
Infrared spectroscopy can be used to analyse the products of oxidative addition to Vaska's complex because the reactions induce characteristic shifts of the stretching frequency of the coordinated carbon monoxide. These shifts are dependent on the amount of π-back bonding allowed from the newly associated ligands. The CO stretching frequencies for Vaska's complex and oxidatively added ligands have been documented in literature.
 * Vaska's Complex: 1967 cm-1
 * Vaska's + O2: 2015 cm-1
 * Vaska's + MeI: 2047 cm-1
 * Vaska's + I2: 2067 cm-1

Oxidative addition to give Ir(III) products reduces the π-bonding from Ir to C, which causes the increase in the frequency of the carbonyl stretching band. The stretching frequency changes depending on the ligands that have been added, but is always greater than 2000 cm-1 for an Ir(III) complex.