Metal sulfur dioxide complex
Encyclopedia
Metal sulfur dioxide complexes are complexes
that contain sulfur dioxide
, SO2, bonded to a transition metal
. Such compounds are common but are mainly of theoretical interest. Historically, the study of these compounds has provided insights into the mechanisms of migratory insertion
reactions in organometallic chemistry
.
forms complexes with many transition metals. For the most stable complex the metal is in oxidation state
0 or +1.
In most cases SO2 binds in monodentate fashion, attaching to the metal through sulfur. Such complexes are further subdivided according to the planarity or pyramidalization at sulfur. The various bonding modes are:
More exotic bonding modes are known for clusters.
s are often weak. In some cases, SO2 displaces other ligands.
A large number of labile O-bonded SO2 complexes arise from the oxidation of a suspension of the metals in liquid SO2, an excellent solvent.
, this reaction is conducted on a large scale as a way to remove hydrogen sulfide that arises in hydrotreating processes in refineries.
of SO2 into metal alkyl bond begins with attack of the alkyl nucleophile on the sulfur centre in SO2. The "insertion" proceed the sulfur dioxide between the metal and the alkyl ligand leads to the O, O'-sulphinate. Alternatively an O-sulphinate can arise. Both of these intermediates commonly convert to an S-sulphinate. S-sulphinate has sulfur–oxygen stretching frequencies from 1250–1000 cm−1 and 1100–1000 cm−1. The O, O'-sulphinate and O-sulphinate are difficult to distinguish as they have stretching frequencies from 1085–1050 cm−1 and 1000–820 cm−1 or lower. The pathway involving the O, O' sulphinate can generally be ruled out if the original metal complex fulfilled the 18-electron rule
because the two metal–oxygen bonds would exceed the 18 electron rule.
The pathway by which SO2 inserts into a square planar alkyl complexes involves the formation of an adduct
. Thereafter, the alkyl ligand migrates to the SO2.
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
that contain sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
, SO2, bonded to a transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
. Such compounds are common but are mainly of theoretical interest. Historically, the study of these compounds has provided insights into the mechanisms of migratory insertion
Migratory insertion
A migratory insertion is a type of reaction in organometallic chemistry wherein two ligands on a metal complex combine. It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanism that leads to the resulting stereochemistry of the...
reactions in organometallic chemistry
Organometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
.
Bonding modes
Sulfur dioxideSulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
forms complexes with many transition metals. For the most stable complex the metal is in oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
0 or +1.
In most cases SO2 binds in monodentate fashion, attaching to the metal through sulfur. Such complexes are further subdivided according to the planarity or pyramidalization at sulfur. The various bonding modes are:
- ηHapticityThe term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...
1-SO2, planar (meaning that the MSO2 subunit forms a plane). In such complexes, SO2 is classified as a 2e donor complemented by pi-back bonding into the empty pz orbital localized on sulfur. - ηHapticityThe term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...
1-SO2, pyramidal (meaning that the MSO2 subunit is pyramidal at sulfur). In such complexes, SO2 is classified as a pure Lewis acid. The structure is similar to that for conventional Lewis base adducts of SO2. - ηHapticityThe term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...
2-SO2. Both S and one O centre are attached to the metal. The MSO2 subunit is pyramidal at sulfur. This bonding mode is more common for early metals, which are typically strongly pi-donating. - ηHapticityThe term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...
1-SO2, O-bonded. In such cases, SO2 attaches to a metal via one of its two oxygen centres. Such complexes are prevalent for hard metal cations such as Na+ and Al3+. In these compounds the M-O interaction is usually weak.
More exotic bonding modes are known for clusters.
Preparation
Complexes of the transition metals are usually generated simply by treating the appropriate metal complex with SO2. The adductAdduct
An adduct is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is considered a distinct molecular species...
s are often weak. In some cases, SO2 displaces other ligands.
A large number of labile O-bonded SO2 complexes arise from the oxidation of a suspension of the metals in liquid SO2, an excellent solvent.
Reactions
The main reaction of sulfur dioxide promoted by transition metals is its reduction by hydrogen sulfide. Known as the Claus processClaus process
The Claus process is the most significant gas desulfurizing process, recovering elemental sulfur from gaseous hydrogen sulfide. First patented in 1883 by the scientist Carl Friedrich Claus, the Claus process has become the industry standard....
, this reaction is conducted on a large scale as a way to remove hydrogen sulfide that arises in hydrotreating processes in refineries.
Insertion of SO2 into metal-ligand bonds
Of academic interest, SO2 acts like a Lewis acid towards the alkyl ligand. The pathway for the insertionMigratory insertion
A migratory insertion is a type of reaction in organometallic chemistry wherein two ligands on a metal complex combine. It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanism that leads to the resulting stereochemistry of the...
of SO2 into metal alkyl bond begins with attack of the alkyl nucleophile on the sulfur centre in SO2. The "insertion" proceed the sulfur dioxide between the metal and the alkyl ligand leads to the O, O'-sulphinate. Alternatively an O-sulphinate can arise. Both of these intermediates commonly convert to an S-sulphinate. S-sulphinate has sulfur–oxygen stretching frequencies from 1250–1000 cm−1 and 1100–1000 cm−1. The O, O'-sulphinate and O-sulphinate are difficult to distinguish as they have stretching frequencies from 1085–1050 cm−1 and 1000–820 cm−1 or lower. The pathway involving the O, O' sulphinate can generally be ruled out if the original metal complex fulfilled the 18-electron rule
18-Electron rule
The 18-electron rule is a rule of thumb used primarily for predicting formulas for stable metal complexes. The rule rests on the fact that valence shells of a transition metal consists of nine valence orbitals, which collectively can accommodate 18 electrons either as nonbinding electron pairs or...
because the two metal–oxygen bonds would exceed the 18 electron rule.
The pathway by which SO2 inserts into a square planar alkyl complexes involves the formation of an adduct
Adduct
An adduct is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is considered a distinct molecular species...
. Thereafter, the alkyl ligand migrates to the SO2.