Crystal structure of Co4(CO)10S2 and Co4(CO)10Se2; v(CO) vibrations of a model cluster molecule: Co4(CO)10S2

The crystal structure of Co₄(CO)₁₀S₂ has been redetermined and that of Co₄(CO)₁₀Se₂ obtained by X-ray analysis. The compounds are monoclinic and isomorphous, space groupP2₁/n,Z=2, with (S compound)a=10.42(2)Å,b=6.794(1)Å,c=12.421(2)Å,=97.27(1) and (Se compound)a=10.110(2)Å,b=6.747(2)Å,c=12.592(4)Å,=96.37(2); finalR(S)=0.029,R _w =0.032 for 1414 reflections and finalR (Se)=0.052,R _w =0.054 for 1264 reflections. The molecules, which lie on a crystallographic center of symmetry, consist of a rectangle with Co atoms at the corners, each face of the rectangle being capped by a chalcogen atom. Each Co atom is bonded to two terminal CO groups; there are two bridging CO groups, one on each member of a pair of opposite sides of the rectangle. The vibrational spectra of the sulfur compound are considered in detail. A study of the crystal structure, and recognition of an approximate spectroscopic space group, are shown to be essential for an understanding of thev(CO) infrared spectrum of polycrystalline Co₄(CO)₁₀S₂. In this molecule, intramolecular vibrational coupling in the crystal leads to extensive modifications of the infrared-activev(CO) molecular coordinates.