O-Phenylenebis(Dimethylarsine)Decacarbonyltriiron. Preparation and Structure of a Compound with Two Semibridging Carbonyl Ligands

The reaction of Fe₂(CO)₉ with o-phenylenebis(dimethylarsine) (diars) under a dynamic atmosphere of argon (to sweep out CO) allows the isolation of the title compound, (diars) Fe₃(CO)₁₀. The structure of this dark brown crystalline compound has been investigated by X-ray crystallography. The crystals are monoclinic, and a unit cell in space group P2₁/n has dimensions of a = 9.538 (2) Å, b = 11.101 (2) Å, c = 21.070 (3) Å, β = 99.81 (1)°, V= 2580 (1) ų, and Z = 4. The molecules consist of triangular Fe3 clusters with the diars chelated to Fe(l). There are no symmetrically bridging CO groups, but one CO group from Fe(2) and one from Fe(3) lean toward Fe(l) and may be described as semibridging CO groups with Fe(l)-C distances of 2.23 ± 0.01 Å. These two semibridging CO groups may be assigned the role of accepting electron density from the electron-rich Fe(l) atom. The two semibridging CO groups are on opposite sides of the Fe₃ plane, and there is a concerted twist of all other ligand atoms so that the only symmetry element retained by the molecule is a twofold axis passing through Fe(l) and bisecting the Fe(2)-Fe(3) bond. The Fe-Fe bonds with the semibridging CO groups are appreciably shorter, 2.602 (1) and 2.597 (1) Å, than the third one, 2.662 (1) Å, between Fe(2) and Fe(3).