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 d_xy 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.