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Atransition metal sulfoxide complex is acoordination complex containing one or moresulfoxide ligands. The inventory is large.[2][3]
The most common sulfoxide ligand isdimethyl sulfoxide (dmso). Many sulfoxides are known because an enormous range of organic substituents are possible. When the two substituents differ, the ligand is chiral. Chiral sulfoxides are configurationally stable. One example ismethyl phenyl sulfoxide.
ruthenium(II)-from-xtal-2008-3D-balls.png) | ruthenium(II)-from-xtal-1990-3D-balls.png) |
| cis-dichlorotetrakis(dimethylsulfoxide)ruthenium(II) | trans-dichlorotetrakis(dimethylsulfoxide)ruthenium(II) |
Sulfoxides can bind to metals by the oxygen atom or by sulfur. This dichotomy is calledlinkage isomerism. O-bonded sulfoxide ligands are far more common, especially for 1st row metals. S-bonded sulfoxides are only found forsoft metal centers, such as Ru(II). Complexes with both O- and S-bonded sulfoxide ligands are known.[4] In some cases, sulfoxides arebridging ligands, with S bonded to one metal and O bonded to the other.
Being a polar solvent with a highdielectric constants, dmso dissolves many metal salts to give the corresponding complexes. Other ligand-solvent combinations includeacetonitrile andwater, which respectively formmetal-acetonitrile complexes andmetal aquo complexes. Treatment ofthioether complexes with peroxide reagents gives sulfoxide complex. In rare cases, sulfoxide complexes are prepared by S-alkylation of sulfenito complexes.[5]
Metal thioether complexes are susceptible to sulfoxidation withdimethyldioxirane.[6][7]
Being weakly basic, sulfoxide ligands are generally labile, i.e. they are rapidly displaced by other more basic ligands.
O-bonded sulfoxide ligands are susceptible to oxidation at sulfur. In this way, the weakly bonded ligand is converted into a leaving group, such asdimethylsulfone. Since dmso is susceptible to deprotonation by strong base, cationic dmso complexes might be expected to undergo H-D exchange under basic conditions. Such behavior is not observed even for the trication[Co(NH3)5(dmso)]3+.[8]
Several metal sulfoxide complexes have been investigated as catalysts.[9] ThemolybdoenzymeDMSO reductase catalyzes the reduction of dmso todimethyl sulfide.
Several homoleptic octahedral complexes of sulfoxides have been characterized byX-ray crystallography. These include the[M(dmso)6]2+ complexes for M = Cr(III), Mn(II), Fe(II), Fe(III), Co(II), Co(III), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II). All such derivatives feature O-bonded sulfoxides. The tricationic complex in[Rh(dmso)6](O3SCF3)3 features one S-bonded and five O-bonded sulfoxide ligands.[10] The complex[Cu(Ph2SO)4]2+ is square planar, in contrast to the derivative with dmso ligands. The square planar d8 complex[Rh(dmso)4]+ features a pairs of S- and O-bonded sulfoxide ligands.[11]