Treatment of either the dichloro or diiodo nitrosyl precursors Cp'M(NO)X2 (Cp' = Cp (eta-5-C5H5) or Cp* (eta-5-C5Me5); M = Mo or W; X = Cl or I) in THF/Et2O with 2 equiv of a benzyl Grignard reagent, Ph'CH2MgCl (Ph' = C6H5 or 2,4,6-Me3C6H2), at 0-degrees-C affords the corresponding bis(benzyl) nitrosyl complexes, Cp'M(NO)(CH2Ph')2, isolated yields typically being 50-65%. All the bis(benzyl) nitrosyl complexes are relatively high melting (143-163-degrees-C), orange-red, diamagnetic solids that are soluble in most common organic solvents. They are all stable to air in the solid state for at least 2 months, and only solutions of the tungsten species react with O2 to produce moderate yields of the corresponding dioxo benzyl complexes, Cp'W(O)2(CH2Ph'). The spectroscopic properties of the bis(benzyl) complexes are consistent with their possessing 18-electron, monomeric Cp'M(NO)(eta-1-CH2Ph')(eta-2-CH2Ph') molecular structures. Variable-temperature H-1 and C-13{H-1} NMR spectra of all the compounds in CD2Cl2 establish that they are stereochemically nonrigid in solutions at ambient temperatures, the principal fluxionality involving the two benzyl ligands exchanging their modes of attachment to the metal centers. C-13 NMR spectroscopy also establishes that the low-temperature limiting structures of the complexes in solution are identical with their solid-state molecular structures. Interestingly, the tungsten bis(benzyl) complexes exhibit the more facile fluxional processes in solution, this increased nonrigidity apparently rendering the tungsten complexes more reactive to Lewis bases. Crystal structure analyses of the five bis(benzyl) nitrosyl complexes have been performed in order to establish their solid-state molecular structures. Crystals of CpMo(NO)(CH2Ph)2 (1a) are orthorhombic, Pbca, with a = 10.122 (1) angstrom, b = 15.625 (2) angstrom, c = 20.770 (2) angstrom, and Z = 8; the structure was solved by conventional heavy-atom methods and was refined by full-matrix least-squares procedures to R = 0.024 and R(w) = 0.031 for 2057 absorption-corrected reflections with I > 3-sigma(I). Crystals of CpMo(NO)(2,4,6-Me3CH2C6H2)2 (1b) are triclinic, P1BAR, with a = 9.4113 (19) angstrom, b = 9.3499 (18) angstrom, c = 15.4533 (18) angstrom, alpha = 98.004 (18)-degrees, beta = 100.408 (15)-degrees, gamma = 110.511 (16)-degrees, and Z = 2; R = 0.031 and R(w) = 0.039 for 3983 reflections. Crystals of Cp*Mo(NO)(CH2Ph)2 (2) are monoclinic, P2(1)/c, with a = 13.969 (5) angstrom, b = 16.125 (3) angstrom, c = 9.778 (5) angstrom, beta = 110.67 (4)-degrees, and Z = 4; R = 0.029 and R(w) = 0.038 for 2775 reflections. Crystals of CpW(NO)(CH2Ph)2 (3) are orthorhombic, Pbca, with a = 10.110 (1) angstrom, b = 15.614 (2) angstrom, c = 20.654 (2) angstrom, and Z = 8; R = 0.025 and R(w) = 0.028 for 2192 reflections. Crystals of Cp*W(NO)(CH2Ph)2 (4) are monoclinic, P2(1)/c, with a = 13.916 (4) angstrom, b = 16.101 (3) angstrom, c = 9.757 (3) angstrom, beta = 110.08 (2)-degrees, and Z = 4; R = 0.021 and R(w) = 0.030 for 3471 reflections. All the complexes are monomeric and possess distinctive Cp'M(NO)(eta-1-CH2Ph')(eta-2-CH2Ph') four-legged piano-stool molecular structures in the solid state. The most chemically interesting feature of each structure involves the eta-2-benzyl ligand, which is characterized by the following six distinctive parameters: (a) its M-CH2 bond length is significantly shorter (i.e. by 0.04-0.06 angstrom) than that involving the eta-1-CH2Ph' group; (b) its CH2-C(ipso) bond length is also significantly shorter than that seen for the eta-1-CH2Ph' group; for instance, C(11)-C(12) = 1.445 (6) and 1.457 (6) angstrom and C(21)-C(22) = 1.472 (4) and 1.497 (5) angstrom in complexes 1a and 2, respectively; (c) its M-C(ipso) distance (ranging between 2.433 (3) and 2.510 (4) angstrom) is well within the range expected for a normal group 6 metal-carbon single bond; (d) its angle at the methylene carbon atom (i.e. C(12)-C(11)-M) is well below the value expected for a tetrahedral sp-3-carbon environment, being typically in the range 81-85-degrees; (e) its two M-C(ortho) distances (ranging between 2.81 and 3.26 angstrom) are longer than the range commonly observed for strong group 6 M-C single bonds; and (f) its four phenyl C-C distances from C(13) to C(17) are essentially identical, and the observed bond lengths of approximately 1.38 angstrom suggest the presence of delocalized multiple bonding extending over these carbons. These metrical parameters indicate that the eta-2-benzyl groups may be viewed to function as formal three-electron ligands via C-M sigma bonds involving their methylene and ipso carbon atoms. This bonding rationale is also consistent with the physical properties exhibited by the bis(benzyl) nitrosyl complexes.
