Structural and Spectroscopic Studies of 16-Electron, Unsaturated Derivatives of Low-Valent, Group 6 Carbonyl Complexes Containing π-Donor Ligands

Several eighteen- and sixteen-electron derivatives of tungsten(0), molybdenum(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-C6H4)] (6c), [PPN]2[W(CO)4(S,S-C6H4)] (7b) have been synthesized from the reaction of photochemically generated
M(CO)5THF with a series of doubly deprotonated 1,2-disubstituted benzene rings with the appropriate oxygen,
nitrogen, and sulfur donor atoms. These complexes have been characterized in the solid state by X-ray
crystallography and in solution by IR and 13C NMR spectroscopies. The crystal of 2c (C84H71N3O5P4SCr) is
triclinic P1̄, a =13.869(3) Å, b = 23.128(5) Å, c = 12.056(2) Å, α = 104.84(3)°, β = 106.91(3)°, γ = 95.29(3)°,
Z = 2; that of 5c (C89H77N7O3P4SW) is monoclinic P21, a = 11.054(2) Å, b = 28.140(6) Å, c = 12.566(2) Å,
β = 90.58(1)°, Z = 2; that of 6c (C85H70N4O4P4SW) is triclinic P1̄, 12.236(2) Å, b = 14.419(2) Å, c = 22.748(4)
Å, α = 76.44(1)°, β = 75.98(2)°, γ = 70.98(1)°, Z = 2; that of 7b (C82H64N2O4P4S2W) is triclinic P1̄, a =
12.650(1) Å, b = 14.810(1) Å, c = 21.053(2) Å, α = 77.182(7)°, β = 78.334(7)°, γ = 66.579(7), Z = 2; and that
of 8 (C10H8O4P2W) is monoclinic P21/c, a = 11.582(1) Å, b = 10.791(1) Å, c = 10.449(1) Å, β = 100.867(7)°,
Z = 2. The average ν(CO) frequencies for each tricarbonyl species reported are compared to those related dianions
previously reported in order to gauge the π-donor character of the different ligands. The 13C NMR spectrum for
each tricarbonyl derivative 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 temperature 13C NMR. In the case of the 16-electron species the geometry of the metal
dianion is that of a distorted trigonal bipyramid consisting of three carbonyl ligands and a five-membered chelate
ring bound through the π-donor atoms at an equatorial and an axial position, with the stronger π-donor atom in
the equatorial site. The equatorial site for the most effective π-donor is preferred over the axial position because
the unoccupied dxy orbital lies in the equatorial plane, and may be stabilized via a π-donor ligand in the equatorial
position. The axial position exhibits a filled/filled repulsion as both orbitals available for π-bonding are filled.