CLUSTER-BOUND KETENYLIDENES AS PRECURSORS TO DICARBIDE LIGANDS - SYNTHESIS AND CHARACTERIZATION OF [PPN][FE3CO3(C2)(CO)18]

The triiron acetylide cluster compound [PPN][Fe3(CO)9CCOAc] (1) [PPN = bis(triphenylphosphine) nitrogen(1+), (Ph3P)2N+] reacts with [Fe(CO)4]2- to produce the metalated acetylide cluster [PPN]2[Fe3(CO)9CCFe(CO)4] (2). Further metalation of 2 with excess Co2(CO)8 produces a hexametallic dicarbido cluster [PPN][Fe3Co3(C2)(CO)18] (5). The reaction proceeds by formation of two intermediate compounds, the dicarbide-containing cluster compound [PPN]2[Fe4Co2(C2)(CO)18] (3) and the acetylide [PPN][Fe2-Co(CO)9CCFe(CO)4] (4). Compound 4 also forms in moderate yield directly from 1, and the reaction of 5 with [Fe2(CO)8 ] 2- regenerates 3. [PPN] [Fe3Co3(C2) (CO) 18] (5) was the subject of a single-crystal X-ray structure determination. The compound crystallizes in the space group P2(1)/c (No. 14), with a = 15.878 (2) angstrom, b = 29.774 (4) angstrom, c = 11.930 (1) angstrom, beta = 90.32 (1)-degrees, V = 5640 (2) angstrom3, and Z = 4. Refinement of 568 variables on 5217 observed [I > 3-sigma(I)] reflections converged to R = 4.4% and R(W) = 5.6%. The structure of the cluster anion 5 consists of two metal triangles at either end of the dicarbide bond, with the metal triangles in a nonparallel configuration. Extended Huckel MO calculations indicate that the nonparallel configuration of the metal triangles in solution gives rise to the observed diamagnetic ground state.