Metal-Metal Bond Cleavage Reactions. The Crystallization and Solid State Structural Characterization of Cadmium Tetracarbonyliron, CdFe(CO)4

The classical metal-metal bonded oligomer CdFe(CO)4 (but not the isomorphous HgFe(CO)4) can be completely cleaved by certain Lewis bases (B) to yield species of the formulation BnCdFe(CO)4, n = 2 or 3. Reversal of the cleavage allows, for the first time, crystallization of CdFe(CO)4 and determination of its molecular structure by single-crystal x-ray diffraction. The compound crystallizes from acetone-water in the monoclinic space group C2h5-P21/n, with two tetrameric [CdFe(CO)4]4 units and four molecules of acetone in a unit cell of dimensions a = 6.292 (4), b = 10.566 (6), c = 26.024 (11) Å, β = 97.80 (3)°. Full-matrix least-squares refinement gave a final value of the conventional R index (on F) of 0.032 for 1602 reflections having Fo2 > 3σ(Fo2). The molecular structure consists of a nearly planar, centrosymmetric, eight-membered ring of alternating Cd and cis-Fe(CO)4 units. The geometry deviates from D4h, square symmetry principally in that two opposite Fe-Cd-Fe angles are 170.25 (5)° and two are 189.85 (5)°. This reflects weak coordination of two cadmium atoms to acetone oxygen atoms (Cd-O = 2.688 (9) Å). The Cd-Fe distances are within experimental error all equal at 2.562 (3) A. The major distortion from octahedral symmetry about the Fe(CO)4 groups is a bending of the axial carbonyl ligands toward the ring centroid and away from the vector perpendicular to the plane containing the metal atoms. The result is an average C-Fe-C angle of 154.7 (4)° for these groups. The acetone can be removed from the structure with minimal changes in the metal-metal bonding geometry. The base-induced cleavage reaction for this group II system is similar to the previously reported cleavage of [R2MFe(CO)4]2 molecules where M = Ge, Sn, Pb.