Structural and Spectroscopic Studies of 16-Electron, Unsaturated Derivatives of Low-Valent, Group 6 Carbonyl...

Several eighteen- and sixteen-electron derivatives of tungsten(0), molybdenum(0), and chromium(0) carbonyl complexes, including [PPN](2)[Cr(CO)(3)(O,S-C(6)H(4))] (2c), [PPN](2)[W(CO)(3)(NH,S-C(6)H(4))] (5c), [PPN](2)[W(CO)(3)(O,S-C(6)H(4))] (6c), [PPN](2)[W(CO)(4)(S,S-C(6)H(4))] (7b) have been synthesized from the reaction of photochemically generated M(CO)(5)THF 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 (13)C NMR spectroscopies. The crystal of 2c (C(84)H(71)N(3)O(5)P(4)SCr) is triclinic P&onemacr;, a =13.869(3) Å, b = 23.128(5) Å, c = 12.056(2) Å, alpha = 104.84(3) degrees, beta = 106.91(3) degrees, gamma = 95.29(3) degrees, Z = 2; that of 5c (C(89)H(77)N(7)O(3)P(4)SW) is monoclinic P2(1), a = 11.054(2) Å, b = 28.140(6) Å, c = 12.566(2) Å, beta = 90.58(1) degrees, Z = 2; that of 6c (C(85)H(70)N(4)O(4)P(4)SW) is triclinic P&onemacr;, 12.236(2) Å, b = 14.419(2) Å, c = 22.748(4) Å, alpha = 76.44(1) degrees, beta = 75.98(2) degrees, gamma = 70.98(1) degrees, Z = 2; that of 7b (C(82)H(64)N(2)O(4)P(4)S(2)W) is triclinic P&onemacr;, a = 12.650(1) Å, b = 14.810(1) Å, c = 21.053(2) Å, alpha = 77.182(7) degrees, beta = 78.334(7) degrees, gamma = 66.579(7), Z = 2; and that of 8 (C(10)H(8)O(4)P(2)W) is monoclinic P2(1)/c, a = 11.582(1) Å, b = 10.791(1) Å, c = 10.449(1) Å, beta = 100.867(7) degrees, Z = 2. The average nu(CO) frequencies for each tricarbonyl species reported are compared to those related dianions previously reported in order to gauge the pi-donor character of the different ligands. The (13)C 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 (13)C 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 pi-donor atoms at an equatorial and an axial position, with the stronger pi-donor atom in the equatorial site. The equatorial site for the most effective 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-donor ligand in the equatorial position. The axial position exhibits a filled/filled repulsion as both orbitals available for pi-bonding are filled.