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 group P2₁/n, Z=2, with (S compound)a=10.42(2)Å, b=6.794(1)Å, c=12.421(2)Å, β=97.27(1) and (Se compound)a=10.110(2)Å, b=6.747(2)Å, c=12.592(4)Å, β=96.37(2); final R(S)=0.029, R_w=0.032 for 1414 reflections and final R (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 the v(CO) infrared spectrum of polycrystalline Co₄(CO)₁₀S₂. In this molecule, intramolecular vibrational coupling in the crystal leads to extensive modifications of the infrared-active v(CO) molecular coordinates.