Dynamic properties and solution structures of the clusters [Fe3(CO)9X2](X = S, Se, Te, or NMe) and their derivatives
Variable-temperature 13C n.m.r. spectra of the title species have shown that two distinguishable steps occur in the exchange of carbonyl groups: in the first step the carbonyls at the apical iron atom become equivalent and in the second those at the basal Fe. The replacement of one carbonyl group with a Group 5 ligand yields one isomer, whereas in the case of the disubstituted derivatives two positional isomers have been obtained. The substitution with Group 5 ligands occurs more easily at the basal iron atoms: at the axial position in the monosubstituted derivative and at the axial and an equatorial position at each basal Fe in the most abundant disubstituted isomer. The structure of the adduct [Fe3(CO)9(PBun3)Te2], an intermediate in the formation of the monosubstituted derivatives, is discussed. The dynamic properties of the phosphorus derivatives have also been determined.