SQUARE FE2AU2 AND TRIANGULAR FE2AU CLUSTERS - A REVERSIBLE TRANSFORMATION - X-RAY CRYSTAL-STRUCTURE OF [FE2AU2(CO)8(MU-DPPM)] (DPPM = BIS(DIPHENYLPHOSPHINO)METHANE)

The salt (NEt4)2[Fe2(CO)8] reacts with (ClAu)2(mu-L) (L = bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp)) in tetrahydrofuran to yield [Fe2Au2(CO)8(mu-L)] (L = dppm, 2a; L = dppe, 2b; L = dppp, 2c). The structure of 2a has been determined by X-ray diffraction methods. Crystals are monoclinic, space group P2(1)/n, with Z = 4 in a unit cell of dimensions a = 16.405 (5) angstrom, b = 17.031 (8) angstrom, c = 12.410 (4) angstrom and beta = 95.64 (2)-degrees. The structure has been solved from diffractometer data by direct and Fourier methods and refined by full-matrix least squares on the basis of 3640 observed reflections to R and R(w) values of 0.0291 and 0.0318, respectively. The metal core consists of an unprecedented, nearly square tetrametallic Fe2Au2 framework with Au-Au = 2.915 (1) angstrom, Au-Fe = 2.527 (2) and 2.534 (2) angstrom, and Fe-Fe = 2.900 (2) angstrom distances. 2a does not react with an excess of (NEt4)2[Fe2(CO)8], but 2b,c add another [Fe2(CO)8]2- fragment to give the hexametallic complexes (NEt4)2]{Fe2Au(CO)6(mu-CO)2}2(mu-L)] (L = dppe, 3b; L = dppp, 3c), which are postulated to be formed by two Fe2Au units joined by an L ligand. Furthermore, 3b,c react reversibly with 1 equiv of (ClAu)2(mu-L) to again afford 2b,c, respectively. An extended Huckel molecular orbital study of the bonding capabilities of two forms of [Fe2(CO)8]2- has been carried out. Formation of terminal Fe-Au bonds is found to be only compatible with the unbridged form of [Fe2(CO)8]2-, whereas the formation of a triangular Fe2Au cluster is most favorable for the bridged form of the iron-carbonyl fragment. Several alternative semibridging interactions between carbonyls and Au atoms in [Fe2Au2(CO)8(mu-dppm)] are analyzed, and the structure with only one cis-equatorial carbonyl bent toward each Au atom is found to be the most stable one.