Synthesis and chemical behavior of [MFe4(CO)16]n− (M=Au, Zn, Cd, Hg) clusters: X ray structure of [NMe3CH2Ph]2[Au{Fe2(CO)8}2]Cl and [PPh4]2[Cd{Fe2(CO)6(μ-CO)2}2]2CH3CN

The oxidation reaction of [Fe(CO)(4)](2-) with [AuBr2](-) or Au(SEt2)Cl complexes in its early stages parallels the corresponding oxidation with silver(I) derivatives by leading in sequence to [Au{Fe(CO)(4)}(2)](3-), [Au-4{Fe(CO)(4)}(4)](4-) and the unstable [Au-5{Fe(CO)(4)}(4)](3-) clusters. Further addition of gold(I) salts gives rise to formation of [Au{Fe-2(CO)(8)}(2)](-) and gold metal, rather than the expected gold congener of [Ag-13{Fe(CO)(4)}(8)](n-). The green [Au{Fe-2(CO)(8)}(2)](-) anion is also obtained by reaction of gold(III) halides with the [Fe(CO)(4)](2-), [Fe-2(CO)(8)](2-) or [Fe-3(CO)(11)](2-) dianions, and a has been isolated as tetrasubstituded ammonium or phosphonium salts. The X ray structural study of its trimelhylbenzylammonium salt disclosed a bow-tie metal framework centered by the unique gold atom, so that the compound may formally be considered a gold(III) complex of the [Fe-2(CO)(8)](2-) bidentate ligand. In keeping with the above description, the [Au{Fe-2(CO)(8)}(2)](-) anion can be reversibly reduced to the corresponding [Au{Fe-2(CO)(8)}(2)](2-) and [Au{Fe-2(CO)(8)}(2)](3-) anions. Owing to their ready re-oxidation by moisture or air to the parent compound both anions could not be isolated. The probable change of gold coordination in these formally Au(II) and Au(I) di- and tri-anions is suggested by their red color, whereas the change in the carbonyl stereochemistry is pointed out by the presence of edge bridging carbonyl ligands in their IR spectra. In the attempt to simulate the above structural changes, we tried the synthesis of related a(9) Cu(II) and d(10) Cu(I), Ag(I), Zn(II), Cd(II) and Hg(II) congener complexes of the [Fe-2(CO)(8)](2-) and. The new red-orange [M{Fe-2(CO)(8)}(2)](2-) (M=Zn, Cd, Hg) dianions have been obtained in almost quantitative yields by reacting [Fe-2(CO)(8)](2-) with 0.5 equivalents of MCl2 and display infrared spectra identical to that of [Au{Fe-2(CO)(8)}(2)](2-). The structure of the [Cd{Fe-2(CO)(8)}(2)](2-) dianion in its PPh4+ salt confirmed both the presence of isomerized [Fe-2(CO)(6)(mu-CO)(2)](2-) ligands and their twist toward a tetrahedral coordination around the central cadmium atom.