STEREOCHEMISTRY OF THE COMPOUNDS [FE3(CO)9XY] - CRYSTAL AND MOLECULAR-STRUCTURES OF THE MIXED TRIIRON CARBONYL-COMPOUNDS [FE3(CO)9XY] (X, Y = S, SE, OR TE)

X-ray analyses of [Fe3(CO)9SSe], [Fe3(CO)9STe] and of [Fe3(CO)9SeTe] was performed. The three compounds are isomorphous and belong to the triclinic space group P1BAR with a = 6.835(1), 6.996(1), 6.964(1) angstrom, b = 9.186(2), 9.367(1), 9.360(2) angstrom, c = 13.121(3), 13.058(2), 13.189(2) angstrom, alpha = 93.79(3), 93.23(1), 94.09(2)-degrees, beta = 94.21(3), 94.03(1), 94.54(2)-degrees, gamma = 110.89(3), 111.14(1), 110.67(2)-degrees for the SSe, STe and SeTe complexes, respectively. The structures were refined anisotropically to R = 0.026, 0.042, 0.030 for 3127, 3465, 2564 observed reflections for the SSe, STe and SeTe complexes, respectively. They belong to the well known series of [Fe3(CO)9XY] complexes formed by an open triangle of iron atoms each linked to three terminal CO groups, with two faces capped by a chalcogen atom. The bond distances and angles of these mixed complexes arc well within the ranges imposed due to the different dimensions of the chalcogen atoms. Intermolecular contacts occur when at least one chalcogen atom is tellurium. Examining some relevant geometrical features of the complexes with X,Y = S, Se, Te, NR, PR, AsR..., a regular trend of the Fe-Fe distances is found, depending on the dimensions of the capping atoms; an asymmetry of the Fe-Fe distances is attributed to steric factors.