X-ray and neutron diffraction studies of the polyhydrido complex FeH6Mg4Br3.5Cl0.5(C4H8O)8: discussion of binary transition-metal hydride anions of the type [MHx]n-

Abstract

The molecular structure of the polyhydrido complex FeH6Mg4X4(THF)8 (X4 = Br/sub 3.5/Cl/sub 0.5/) has been determined by single-crystal X-ray and neutron diffraction methods. The X-ray structure determination was carried out at room temperature, revealing an octahedral (FeH6)U core with (MgBr(THF)2) units situated over four of the eight faces of the octahedron. The (FeH6)U core exhibits crystallographic twofold symmetry, with an average Fe-H distance (as determined in the X-ray study) of 1.69 (9) A. Neutron diffraction analysis of the title compound (carried out at 80 K at the Brookhaven High-Flux Beam Reactor on a crystal of volume 12 mmT) fully confirmed this basic molecular geometry, yielding much more precise H positions. The Fe-H (1.609 (2) A) and Mg-H (2.045 (18) A) distances obtained from the neutron study support the idea that the Fe-H bond is largely covalent in nature while the Mg-H interaction is largely ionic. A detailed discussion of available structural information for binary transition-metal hydride anions, (MH/sub x/)/sub n-/, is given in the text. Crystallographic details for the X-ray analysis of the title compound C32H70Br/sub 3.5/Cl/sub 0.5/FeMg4O8 is included. Details for the neutron analysis at 80 Z are also reported. 52 references, 2 figures, 4 tables.