Structural and spectroscopic studies of 16-electron, unsaturated derivatives of low-valent, group 6 carbonyl complexes containing pi-donor ligands

Several eighteen- and sixteen-electron derivatives of tungsten(0), molybden um(0), and chromium(0) carbonyl complexes, including [PPN](2)[Cr(CO)(3)(O,S -C6H4)] (2c), [PPN](2)[W(CO)(3)(NH,S-C6H4)] (5c), [PPN](2)[W(CO)(3)(O,S-C6H 4)] (6c), [PPN](2)[W(CO)(4)(S,S-C6H4)] (7b) have been synthesized from the reaction of photochemically generated M(CO)(5)THF with a series of doubly d eprotonated 1,2-disubstituted benzene rings with the appropriate oxygen, ni trogen, and sulfur donor atoms. These complexes have been characterized in the solid state by X-ray crystallography and in solution by IR and C-13 NMR spectroscopies. The crystal of 2c (C84H71N3O5P4SCr) is triclinic P (1) ove r bar, a = 13.869(3) Angstrom, b = 23.128(5) Angstrom, c = 12.056(2) Angstr om, alpha = 104.84(3)degrees, beta = 106.91(3)degrees, gamma = 95.29(3)degr ees, Z = 2; that of 5c (C89H77N7O3P4SW) is monoclinic P2(1), a = 11.054(2) Angstrom, b = 28.140(6) Angstrom, c = 12.556(2) Angstrom, beta = 90.58(1)de grees, Z = 2; that of 6c (C85H70N4O4P4SW) is triclinic P (1) over bar, 12.2 36(2) Angstrom, b = 14.419(2) Angstrom, c = 22.748(4)Angstrom, alpha = 76.4 4(1)degrees; beta = 75.98(2)degrees, gamma = 70.98(1)degrees, Z = 2; that o f 7b (C82H64N2O4P4S2W) is triclinic P (1) over bar, a = 12.650(1) Angstrom, b = 14.810(1) Angstrom, c = 21.053(2) Angstrom, alpha = 77.182(7)degrees b eta = 78.334(7)degrees, gamma = 66.579(7), Z = 2; and that of 8 (C10H8O4P2W ) is monoclinic P2(1)/c, a = 11.582(1) Angstrom, b = 10.791(1) Angstrom, c = 10.449(1) Angstrom, beta = 100.867(7)degrees, Z = 2. The average nu(CO) f requencies for each tricarbonyl species reported are compared to those rela ted dianions previously reported in order to gauge the pi-donor character o f the different ligands. The C-13 NMR spectrum for each tricarbonyl derivat ive consists of a single sharp peak for the three inequivalent carbonyls as a result of a low-energy, fast intramolecular exchange process. Both inter - and intramolecular CO-exchange processes have been probed via variable te mperature C-13 NMR. In the case of the 16-electron species the geometry of the metal dianion is that of a distorted trigonal bipyramid consisting of t hree carbonyl ligands and a five-membered chelate ring bound through the pi -donor atoms at an equatorial and an axial position, with the stronger pi-d onor atom in the equatorial site. The equatorial site for the most effectiv e pi-donor is preferred over the axial position because the unoccupied d(xy ) orbital lies in the equatorial plane, and may be stabilized via a pi-dono r ligand in the equatorial position. The axial position exhibits a filled/f illed repulsion as both orbitals available for pi-bonding are filled.