Metal sulfur dioxide complex
{{short description|Chemical compound containing SO2 bound to a transition metal}}
In organometallic chemistry, metal sulfur dioxide complexes are complexes that contain sulfur dioxide, {{chem2|SO2}}, bonded to a transition metal.{{Greenwood&Earnshaw2nd}} 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
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Sulfur dioxide forms complexes with many transition metals. Most numerous are complexes with metals 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:
- 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 pz 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.{{cite journal|author1 = Mews, R.|author2 = Lork, E.|author3 = Watson, P. G.|author4 = Görtler, B.|title = Coordination Chemistry in and of Sulfur Dioxide|journal = Coord. Chem. Rev.|year = 2000|volume = 197|issue = 1|pages = 277–320|doi = 10.1016/S0010-8545(99)00191-5}}
More exotic bonding modes are known for clusters.
:File:SO2CmpxesUpgrade.png
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.{{cite journal|author = Schenk, W. A.|title = Sulfur Oxides as Ligands in Coordination Compounds|journal = Angew. Chem. Int. Ed.|year = 1987|volume = 26|issue = 2|pages = 98–109|doi = 10.1002/anie.198700981}}
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 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<sub>2</sub> into metal-ligand bonds=
Of academic interest, SO2 acts like a Lewis acid towards the alkyl ligand.{{cite book|author1 = Douglas, B. E.|author2 = McDaniel, D. H.|author3 = Alexander, J. J.|title = Concepts and Models of Inorganic Chemistry|edition = 3rd|year = 1994|location = New York|publisher = John Wiley & Sons, Inc.|isbn = 978-0-471-62978-8}} 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.{{cite book|chapter=Insertion Reactions of Transition Metal–Carbon σ-Bonded Compounds II. Sulfur Dioxide and Other Molecules|year = 1974|author=Wojcicki, A.|volume = 12|title = Advances in Organometallic Chemistry|editor1-last = Stone|editor1-first = F. G. A.|editor1-link = F. Gordon A. Stone|editor2-last = West|editor2-first = R.|editor2-link = Robert West (chemist)|pages = 31–81|doi = 10.1016/S0065-3055(08)60450-5|isbn = 9780120311125}} 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.{{cite book|title = The Chemistry of the Metal-Carbon Bond, Vol. 2: The Nature and Cleavage of Metal-Carbon Bonds|year = 1985|publisher = John Wiley & Sons|location = New York|author = Alexander, J. J.|editor1-last = Hartley|editor1-first = F. R.|editor2-last = Patai|editor2-first = S.}}
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.{{cite journal|author1 = Puddephatt, R. J.|author2 = Stalteri, M. A.|title = Competition between Insertion of Sulfur Dioxide into the Methyl– or Phenyl–Transition Metal Bond|journal = J. Organomet. Chem.|year = 1980|volume = 193|issue = 1|pages = C27–C29|doi = 10.1016/S0022-328X(00)86091-X}}
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 {{chem2|[(C5(CH3)5)2Sm]2(S2O4)}}.{{cite journal |doi=10.1039/C5NJ00318K |title=Dithionite and Sulfinate Complexes from the Reaction of SO2with Decamethylsamarocene |date=2015 |last1=Klementyeva |first1=Svetlana V. |last2=Arleth |first2=Nicholas |last3=Meyer |first3=Karsten |last4=Konchenko |first4=Sergey N. |last5=Roesky |first5=Peter W. |journal=New Journal of Chemistry |volume=39 |issue=10 |pages=7589–7594 }}{{cite journal |doi=10.1016/S0022-328X(00)89402-4 |title=Synthesis of a Transition Metal-Dithionite Complex, (η5-C5H5)(CO)2Fe-S(O)2S(O)2-Fe(CO)2(η5-C5H5) |date=1975 |last1=Tennent |first1=Norman H. |last2=Su |first2=Sophia R. |last3=Poffenberger |first3=Craig A. |last4=Wojcicki |first4=Andrew |journal=Journal of Organometallic Chemistry |volume=102 |issue=4 |pages=C46–C48 }}
=S<sub>2</sub>O complexes=
Several complexes of disulfur monoxide are known. Most are formed by oxidation peroxide oxidation of a disulfur ligand. In these complexes, the {{chem2|S2O}} ligand is invariably bound in an {{chem2|\h{2}S,S}} manner. Selected examples: {{chem2|[Ir(dppe)2S2O]+}}, {{chem2|OsCl(NO)(PPh3)2S2O}}, {{chem2|NbCl(η\-C5H5)2S2O}}, {{chem2|Mn(CO)2(η\-C5Me5)S2O}}, {{chem2|Re(CO)2(η\-C5Me5)S2O}}, {{chem2|Re(CO)2(η\-C5H5)S2O}}.{{cite book|doi=10.1016/S0065-3055(08)60391-3|title=Organotransition Metallic Chemistry of Sulfur Dioxide Analogs|series=Advances in Organometallic Chemistry|year=1994|last1=Hill|first1=Anthony F.|volume=36|pages=159–227|isbn=9780120311361}}
{{chem2|Mo2(S2O)2(S2CNEt2)4}} arises when the dithiocarbamate complex {{chem2|Mo(CO)2(S2CNEt2)2}} is oxidized with elemental sulfur in air. Another way to form these complexes is to combine {{chem2|OSNSO2*R}} complexes with hydrogen sulfide. Complexes formed in this way are: {{chem2|IrCl(CO)(PPh3)2S2O}}; {{chem2|Mn(CO)2(η\-C5H5)S2O}}. With hydrosulfide and a base followed by oxygen, {{chem2|OsCl(NO)(PPh3)2S2O}} can be made.