Molybdenum(II) and tungsten(II) M(CO)(RC2R′)L2X2 alkyne complexes

Article abstract of DOI:10.1021/om00139a032

A series of molybdenum and tungsten alkyne complexes, M(CO)(RC₂R′)L₂X₂ (various combinations of M = Mo, W; L = PPh₃, PEt₃, py; L₂ = dppe (Ph₂PCH₂CH₂PPh₂); X = Cl, Br), has been prepared by treating M(CO)_nL₂X₂ (n = 2 or 3) reagents with free alkyne. Nuclear magnetic resonance, infrared, and electronic absorption spectroscopies have been used to probe the metal-alkyne bonding interaction. Both ¹H and ¹³C NMR chemical shift values for the alkyne ligands are compatible with a four-electron donor description for the alkyne ligands in these complexes. Dynamic NMR studies of complexes containing 2-butyne revealed metal-alkyne rotational barriers ranging from 9 to 13 kcal/mol. The structure of Mo(CO)(PhC₂H)(PEt₃)₂Br₂ has been determined and refined to R = 0.048 and R_w = 0.035 using 1660 reflections [monoclinic, P2₁/n; a = 7.995 (3) ?, b = 14.559 (4) ?, c = 22.209 (10) ?; β = 98.04 (3)°; Z = 4]. The molecule adopts a distorted octahedral geometry with the alkyne cis and parallel to the carbon monoxide ligand. The phosphine ligands are mutually trans with the bromides cis. Strong metal-alkyne bonding is evident in the short M-C distances (1.99-? average). The molecule possesses effective C_s symmetry with Mo, Br1, Br2, CO, and the entire alkyne, including the phenyl substituent, defining the mirror plane.