# Metal sulfur dioxide complex

In organometallic chemistry, metal sulfur dioxide complexes are complexes that contain sulfur dioxide, SO$_2$, 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.

## Bonding modes

Sulfur dioxide forms complexes with many transition metals. Most numerous are complexes with metals in oxidation state 0 or +1.
In most cases SO$_2$ 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:
* η$^1$-SO$_2$, planar (meaning that the MSO$_2$ subunit forms a plane). In such complexes, SO$_2$ is classified as a 2e donor complemented by pi-back bonding into the empty p$_z$ orbital localized on sulfur.
* η$^1$-SO$_2$, pyramidal (meaning that the MSO$_2$ subunit is pyramidal at sulfur). In such complexes, SO$_2$ is classified as a pure Lewis acid. The structure is similar to that for conventional Lewis base adducts of SO$_2$.
* η$^2$-SO$_2$. Both S and one O centre are attached to the metal. The MSO$_2$ subunit is pyramidal at sulfur. This bonding mode is more common for early metals, which are typically strongly pi-donating.
* η$^1$-SO$_2$, O-bonded. In such cases, SO$_2$ attaches to a metal via one of its two oxygen centres. Such complexes are prevalent for hard metal cations such as Na$^+$ and Al$^{3+}$. In these compounds the M-O interaction is usually weak.
More exotic bonding modes are known for clusters.
    Illustrative SO$_2$ complexes of soft metal centers. From the left, Fe(CO)$_2$[P(OPh)$_3$]$_2$(η$^1$-SO$_2$), IrCl(CO)(PPh$_3$)$_2$(η$^1$-SO$_2$), Mo(CO)$_2$(PMe$_3$)$_3$(η$^2$-SO$_2$), and the A-frame complex Rh$_2$(bis(diphenylphosphino)methane)$_2$Cl(CO)$_2$(μ-SO$_2$).

## Preparation

Complexes of the transition metals are usually generated simply by treating the appropriate metal complex with SO$_2$. The adducts are often weak. In some cases, SO$_2$ displaces other ligands.
A large number of labile O-bonded SO$_2$ complexes arise from the oxidation of a suspension of the metals in liquid SO$_2$, an excellent solvent.

## Reactions

The main reaction of sulfur dioxide promoted by transition metals is its reduction by hydrogen sulfide. Known as the Claus process, this reaction is conducted on a large scale as a way to remove hydrogen sulfide that arises in hydrotreating processes in refineries.

### Insertion of SO$_2$ into metal-ligand bonds

Of academic interest, SO$_2$ acts like a Lewis acid towards the alkyl ligand. The pathway for the insertion of SO$_2$ into metal alkyl bond begins with attack of the alkyl nucleophile on the sulfur centre in SO$_2$. 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 SO$_2$ inserts into a square planar alkyl complexes involves the formation of an adduct. Thereafter, the alkyl ligand migrates to the SO$_2$.

## Related complexes
### Dithionite complexes

Dithionite, the reductively coupled derivative of sulfur dioxide is observed as a ligand when some reduced metals are treated with sulfur dithioxide. One example is [(C$_5$(CH$_3$)$_5$)$_2$Sm]$_2$(S$_2$O$_4$).

### S$_2$O complexes

Several complexes of disulfur monoxide are known. Most are formed by oxidation peroxide oxidation of a disulfur ligand. In these complexes, the S$_2$O ligand is invariably bound in an η$^2$-S,S manner. Selected examples: [Ir(dppe)$_2$S$_2$O]$^+$, OsCl(NO)(PPh$_3$)$_2$S$_2$O, NbCl(η-C$_5$H$_5$)$_2$S$_2$O, Mn(CO)$_2$(η-C$_5$Me$_5$)S$_2$O, Re(CO)$_2$(η-C$_5$Me$_5$)S$_2$O, Re(CO)$_2$(η-C$_5$H$_5$)S$_2$O.
Mo$_2$(S$_2$O)$_2$(S$_2$CNEt$_2$)$_4$ arises when the dithiocarbamate complex Mo(CO)$_2$(S$_2$CNEt$_2$)$_2$ is oxidized with elemental sulfur in air. Another way to form these complexes is to combine OSNSO$_2$·R complexes with hydrogen sulfide. Complexes formed in this way are: IrCl(CO)(PPh$_3$)$_2$S$_2$O; Mn(CO)$_2$(η-C$_5$H$_5$)S$_2$O. With hydrosulfide and a base followed by oxygen, OsCl(NO)(PPh$_3$)$_2$S$_2$O can be made.