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

Methyldiphenylphosphonium cyclopentadienylide, C 5 H 4 PMePh 2 ( 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 (η 5 -C 5 H 4 PMePh 2 )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 (η 6 -C 6 H 6 )M(CO) 3 and [(η 5 -C 5 H 5 )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 (η 5 -C 5 H 4 PMePh 2 )M(CO) 3 (M = Cr, Mo) by equimolar amounts of PMe 3 and PPh 3 were not observed, but the ylide is displaced photochemically from (η 5 -C 5 H 4 PMePh 2 )Mo(CO) 3 by excess PMe 3 to form fac -Mo(CO) 3 (PMe 3 ) 3, while (η 5 -C 5 H 4 PMePh 2 )Mo(CO) 3 reacts with I 2 to form [(η 5 -C 5 H 4 PMePh 2 )Mo(CO) 3 I]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 C 5 H 4 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 C 5 H 4 aromatic π systems and are rationalized in terms of electrostatic attractive forces between the slightly positive phenyl hydrogen atom and the negatively charged C 5 H 4 ring. The electronic structures of II and of (η 5 -C 5 H 4 PMePh 2 )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 (E 1 symmetry) and that a lower energy, almost fully symmetric orbital corresponds to the fully symmetric bonding A 1 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 (η 5 -C 5 H 4 PMePh 2 )Cr(CO) 3 involve donation of the HOMO and HOMO-1 orbitals into the d xz and d yz 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 (η 6 -C 6 H 6 )Cr(CO) 3 .