Vaska's complex
Encyclopedia
Vaska's complex is the trivial name
for the chemical compound
trans-chlorocarbonylbis(triphenylphosphine)iridium(I), which has 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
ligand
s, carbon monoxide
, and a chloride
ion. The complex was first reported by J. W. DiLuzio and Lauri Vaska
in 1961.
Vaska's complex can undergo oxidative addition
and is notable for its ability to bind to O2
reversibly. It is a bright yellow crystalline solid.
and a carbon monoxide
source. The most popular method uses dimethylformamide
(DMF) as a solvent, and sometimes aniline
is added to accelerate the reaction. Another popular solvent is 2-methoxyethanol. The reaction is typically 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.
Typical sources of iridium used in this preparation are IrCl3.xH2O
and H2IrCl6.
. 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 electrophile
s, such as iodomethane
(CH3I).
An interesting characteristic of Vaska's complex is that it binds O2 reversibly.
The dioxygen ligand is bonded to Ir 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 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.
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 by the newly associated ligands. The CO stretching frequencies for Vaska's complex and oxidatively added ligands have been documented in the literature.
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 change depends upon the ligands that have been added, but the frequency is always greater than 2000 cm-1 for an Ir(III) complex.
Trivial name
In chemistry, a trivial name is a common name or vernacular name; it is a non-systematic name or non-scientific name. That is, the name is not recognised according to the rules of any formal system of nomenclature...
for the chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
trans-chlorocarbonylbis(triphenylphosphine)iridium(I), which has the formula IrCl(CO)[P(C6H5)3]2. This square planar diamagnetic organometallic complex consists of a central iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
atom bound to two mutually trans triphenylphosphine
Triphenylphosphine
Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature...
ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s, carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
, and a chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
ion. The complex was first reported by J. W. DiLuzio and Lauri Vaska
Lauri Vaska
Lauri Vaska is an Estonian-born chemist who has made noteworthy contributions to organometallic chemistry....
in 1961.
Vaska's complex can undergo oxidative addition
Oxidative addition
Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre...
and is notable for its ability to bind to O2
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
reversibly. It is a bright yellow crystalline solid.
Preparation
The synthesis involves heating virtually any iridium chloride salt with triphenylphosphineTriphenylphosphine
Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature...
and a carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
source. The most popular method uses dimethylformamide
Dimethylformamide
Dimethylformamide is an organic compound with the formula 2NCH. Commonly abbreviated as DMF , this colourless liquid is miscible with water and the majority of organic liquids. DMF is a common solvent for chemical reactions...
(DMF) as a solvent, and sometimes aniline
Aniline
Aniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...
is added to accelerate the reaction. Another popular solvent is 2-methoxyethanol. The reaction is typically 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.
- IrCl3(H2O)3 + 3 P(C6H5)3 + HCON(CH3)2 + C6H5NH2 → IrCl(CO)[P(C6H5)3]2 + [(CH3)2NH2]Cl + OP(C6H5)3 + [C6H5NH3]Cl + 2 H2O
Typical sources of iridium used in this preparation are IrCl3.xH2O
Iridium(III) chloride
Iridium chloride is the inorganic compound with the formula IrCl3. This material is relatively rare, but the related hydrate is useful for preparing other iridium compounds. The anhydrous salt is a dark green crystalline solid...
and H2IrCl6.
Reactions
Studies on Vaska's complex helped provide the conceptual framework for homogeneous catalysisHomogeneous catalysis
In chemistry, homogeneous catalysis is a sequence of reactions that involve a catalyst in the same phase as the reactants. Most commonly, a homogeneous catalyst is codissolved in a solvent with the reactants.-Acid 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
Oxidative addition
Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre...
. 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
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
, 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 electrophile
Electrophile
In general electrophiles are positively charged species that are attracted to an electron rich centre. In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile...
s, such as iodomethane
Iodomethane
Methyl iodide, also called iodomethane, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one hydrogen atom by an atom of iodine. It is naturally...
(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 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 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.
Spectroscopy
Infrared spectroscopyInfrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...
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
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
.
These shifts are dependent on the amount of π-back bonding allowed by the newly associated ligands. The CO stretching frequencies for Vaska's complex and oxidatively added ligands have been documented in the 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 change depends upon the ligands that have been added, but the frequency is always greater than 2000 cm-1 for an Ir(III) complex.