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

The oxidation reaction of [Fe(CO)(4)](2-) with [AuBr2](-) or Au(SEt2)Cl com plexes in its early stages parallels the corresponding oxidation with silve r(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. Furthe r 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 r eaction 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 a mmonium or phosphonium salts. The X ray structural study of its trimelhylbe nzylammonium salt disclosed a bow-tie metal framework centered by the uniqu e gold atom, so that the compound may formally be considered a gold(III) co mplex 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-) an ions. Owing to their ready re-oxidation by moisture or air to the parent co mpound both anions could not be isolated. The probable change of gold coord ination 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 p ointed out by the presence of edge bridging carbonyl ligands in their IR sp ectra. In the attempt to simulate the above structural changes, we tried th e synthesis of related a(9) Cu(II) and d(10) Cu(I), Ag(I), Zn(II), Cd(II) a nd Hg(II) congener complexes of the [Fe-2(CO)(8)](2-) and. The new red-oran ge [M{Fe-2(CO)(8)}(2)](2-) (M=Zn, Cd, Hg) dianions have been obtained in al most 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 cent ral cadmium atom.