SYNTHESIS AND CHARACTERIZATION OF THE PARAMAGNETIC [AG13FE8(CO)32]4- TETRAANION - A CUBOCTAHEDRAL AG13 CLUSTER STABILIZED BY FE(CO)4 GROUPS BEHAVING AS 4-ELECTRON DONORS

The new [Ag13Fe8(CO)32]4- paramagnetic cluster anion has been obtained in a mixture with other yet uncharacterized products from the reaction of Na2[Fe(CO)4].xTHF with ca. 1.5 equiv of AgBF4 or AgNO3 in tetrahydrofuran and acetonitrile solution, respectively. It has been separated from the mixture via differential solubility of the tetrasubstituted ammonium or phosphonium salts and isolated in a crystalline state as [NMe3CH2Ph]+ and [N(PPh3)2]+ salts. The structure has been determined by X-ray diffraction studies on a crystal of the latter salt: monoclinic, space group P2(1)/n, a = 18.321 (2) angstrom, b = 22.429 (3) angstrom, c = 22.150 (1) angstrom, beta = 92.24 (2)-degrees, Z = 2, R = 0.0410. The molecular structure of the anion consists of an Ag13 centered cuboctahedron capped on all the triangular faces by Fe(CO)4 groups displaying C3-upsilon, symmetry; the eight iron atoms describe a distorted cube. The idealized molecular symmetry is O(h). The unpaired electron present in [Ag13Fe8(CO)32]4- is strongly coupled with the unique interstitial silver atom and loosely coupled with the 12 peripheral silvers. The electron count of the cluster, having extra nine electrons with respect to the number predicted by most theories, as well as the hyperfine structure of the ESR spectrum, have both been interpreted on the basis of extended Huckel molecular orbital calculations.