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     John H. Brownie, Michael C. Baird, Hartmut Schmider
     New Complexes of Methyldiphenylphosphonium cyclopentadienylide; Representative of a Class of Ligands Heretofore Much Neglected
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     Methyldiphenylphosphonium cyclopentadienylide, C5H4PMePh2 (II), has been synthesized and characterized spectroscopically and crystallographically, and has been found to exhibit properties consistent with the zwitterionic structure IIb. New group 6 coordination complexes (?5-C5H4PMePh2)M(CO)3 have been synthesized via substitution reactions of II with M(CO)6 (M = Cr, Mo) or W(CO)3(MeCN)3. Comparisons of ?(CO) of these complexes with those of the isoelectronic (?6-C6H6)M(CO)3 and [(?5-C5H5)M(CO)3]- suggest that the electron donating ability of the ylide is less than that of the cyclopentadienyl anion but much greater than that of benzene. Thermal and photochemical substitutions of the CO ligands of (?5-C5H4PMePh2)M(CO)3 (M = Cr, Mo) by equimolar amounts of PMe3 and PPh3 were not observed, but the ylide is displaced photochemically from (?5-C5H4PMePh2)Mo(CO)3 by excess PMe3 to form fac-Mo(CO)3(PMe3)3 while (?5-C5H4PMePh2)Mo(CO)3 reacts I2 to form [(?5-C5H4PMePh2)Mo(CO)3I]I. The crystal structures of the free ylide and all four of its coordination complexes show that in each case one of the ylide phenyl rings not only eclipses the C5H4 ring but is oriented towards it in an edge on fashion. The intramolecular edge-face orientations involve interactions of an aromatic hydrogen atom with the C5H4 aromatic ? systems, and are rationalized in terms of electrostatic attractive forces between the slightly positive phenyl hydrogen atom and the negatively charged C5H4 ring. The electronic structures of II and of (?5-C5H4PMePh2)Cr(CO)3 have been investigated using ab initio methodologies; it is found that the near degenerate HOMO and (HOMO-1) orbitals of the free ylide exhibit symmetries very similar to those of the corresponding, doubly degenerate HOMO of the cyclopentadienyl anion (E1 symmetry), and that a lower energy, almost fully symmetric orbital corresponds to the fully symmetric bonding A1 MO of the cyclopentadienyl anion. In none of these three orbitals does there appear to be significant ? involvement of an orbital on the phosphorus atom, consistent with the low P-C bond order implicit in zwitterionic structure IIb. The primary ylide-metal interactions in (?5-C5H4PMePh2)Cr(CO)3 involve donation of the HOMO and (HOMO-1) orbitals into the dxz and dyz orbitals of the chromium, while the calculated ylide-Cr(CO)3 bond dissociation energy is about 30% higher than the analogous ring-metal bond dissociation energy of (?6-C6H6)Cr(CO)3.
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