Metal sulfur dioxide complex

Metal sulfur dioxide complexes are complexes that contain sulfur dioxide, SO₂, bonded to a transition metal.^[1] 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.

Bonding modes

Sulfur dioxide forms complexes with many transition metals. For the most stable complex the metal is in oxidation state 0 or +1.^[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:

• η¹-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 p_z orbital localized on sulfur.
• η¹-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.
• η²-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.
• η¹-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.^[2]

More exotic bonding modes are known for clusters.

Illustrative SO₂ complexes. From the left, Fe(CO)₂[P(OPh)₃]₂(η¹-SO₂), IrCl(CO)(PPh₃)₂(η¹-SO₂), Mo(CO)₂(PMe₃)₃(η²-SO₂), and the A-frame complex Rh₂(bis(diphenylphosphino)methane)₂Cl(CO)₂(μ-SO₂).

Preparation

Complexes of the transition metals are usually generated simply by treating the appropriate metal complex with SO₂. The adducts are often weak. In some cases, SO₂ displaces other ligands.^[3]

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.^[2]

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 SO₂ into metal-ligand bonds

Of academic interest, SO₂ acts like a Lewis acid towards the alkyl ligand.^[4] The pathway for the insertion 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.^[5] 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.^[6] 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.^[7]

Pathway for insertion of SO₂ into Au-methyl bonds of a square planar Au(III)) complex.

References

1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
2. Mews, R.; Lork, E.; Watson, P. G.; Görtler, B. (2000). "Coordination Chemistry in and of Sulfur Dioxide". Coord. Chem. Rev. 197 (1): 277–320. doi:10.1016/S0010-8545(99)00191-5.
3. Schenk, W. A. (1987). "Sulfur Oxides as Ligands in Coordination Compounds". Angew. Chem. Int. Ed. 26 (2): 98–109. doi:10.1002/anie.198700981.
4. Douglas, B. E.; McDaniel, D. H.; Alexander, J. J. (1994). Concepts and Models of Inorganic Chemistry (3rd ed.). New York: John Wiley & Sons, Inc. ISBN 978-0-471-62978-8.
5. Wojcicki, A. (1974). "Insertion Reactions of Transition Metal–Carbon σ-Bonded Compounds II. Sulfur Dioxide and Other Molecules". In Stone, F. G. A.; West, R. Advances in Organometallic Chemistry 12. pp. 31–81. doi:10.1016/S0065-3055(08)60450-5.
6. Alexander, J. J. (1985). Hartley, F. R.; Patai, S., eds. The Chemistry of the Metal-Carbon Bond, Vol. 2: The Nature and Cleavage of Metal-Carbon Bonds. New York: John Wiley & Sons.
7. Puddephatt, R. J.; Stalteri, M. A. (1980). "Competition between Insertion of Sulfur Dioxide into the Methyl– or Phenyl–Transition Metal Bond". J. Organomet. Chem. 193 (1): C27–C29. doi:10.1016/S0022-328X(00)86091-X.