New Complexes of Methyldiphenylphosphonium Cyclopentadienylide, Representative of a Class of Ligands Heretofore Much Neglected

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. The new group 6 coordination complexes (η5-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 ν(CO) values of these complexes with those of the isoelectronic (η6-C6H6)M(CO)3 and
[(η5-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 (η5-C5H4PMePh2)M(CO)3 (M = Cr, Mo) by equimolar amounts of PMe3 and PPh3
were not observed, but the ylide is displaced photochemically from (η5-C5H4PMePh2)Mo(CO)3 by excess
PMe3 to form fac-Mo(CO)3(PMe3)3, while (η5-C5H4PMePh2)Mo(CO)3 reacts with I2 to form [(η5-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 also is oriented
toward it in an edge-on fashion. The intramolecular edge−face orientations involve interactions of an
aromatic hydrogen atom with the C5H4 aromatic π 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 (η5-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 π involvement of an orbital on the phosphorus atom, consistent with
the low P−C bond order implicit in the zwitterionic structure IIb. The primary ylide−metal interactions
in (η5-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 (η6-C6H6)Cr(CO)3.