Matthew A. Lynn

Nitro-substituted benzoates of dimolybdenum: the Mo24+ δ to ligand charge transfer band

The preparation and characterization of a series of Mo2(O2CAr′)4 compounds, where Ar′ is a nitrosubstituted phenyl group, are described. The introduction of the NO2 group in the 4- (para) position causes a significant red-shift of a metal-to-ligand charge transfer (MLCT) band such that these compounds are intensely purple in solution: λmax ca. 540 nm with e ∼ 15 000 M−1 cm−1. The cinnamate derivative (O2CCHCHPh-4-NO2) shows a further red-shift of this MLCT band while the 3-NO2 (meta) derivatives are only slightly red-shifted compared to the parent benzoate Mo2(O2CPh)4. The cyclic voltammograms for Mo2(O2CR)4, where R = Me, tBu, n-octyl, Ph, C6H4-4-NO2, and C6H4-3-NO2 reveal how substituents on R influence the ease of oxidation from the δ orbital. The structure of Mo2(O2CC6H4-3-NO2)4 was determined by a single crystal X-ray diffraction study as its pyridine adduct with additional pyridine in the lattice. Crystal data for Mo2(O2CC6H4-3-NO2)4(py)2·2py at −174°C: a = 10.982(2) A, b = 20.457(4) A, c = 10.615(2) A, α = 93.82(1)°, β = 90.49(1)°, γ = 98.31(1)°, Z = 2, dcalc = 1.65 g cm−3 and space group P1. The nature of the MLCT band is discussed in the light of Fenske-Hall MO calculations on Mo2(O2C6H5)4, Mo2(O2C6H4-4-NO2)4, Mo2(O2CC6H4-3-NO2)4, and Mo2(O2C6H3-2-Cl-4-NO2)4.

The non-perpendicular and non-parallel alkyne bridge in W2(μ-C2H2)(μ-OCH2tBu)2(OCH2tBu)61

Abstract The bonding in the ethyne adduct W 2 ( μ -C 2 H 2 )( μ -ONp) 2 (ONp) 6 (Np=CH 2 t Bu) has been examined by various computational methods [Extended Huckel (EHMO), Fenske–Hall, and Gaussian 92 RHF (Restricted Hartree–Fock) and density functional (Becke-3LYP) calculations] employing the model compound W 2 ( μ -C 2 H 2 )( μ -OH) 2 (OH) 6 . EHMO and Fenske–Hall calculations suggest, based on total orbital energy, that a μ -parallel ethyne geometry should have the lowest energy, although traditional frontier orbital arguments agree with the observance of a skewed acetylene bridge. Gaussian 92 computations reproduce the non-perpendicular/non-parallel μ -C 2 H 2 geometry in close agreement to that observed in the solid-state (X-ray) structure, which leads us to suggest that the distortion is not sterically imposed by the attendant alkoxide ligands. The observed geometry can be rationalized in terms of Jahn–Teller distortional stabilization from either the μ -parallel or μ -perpendicular mode, i.e., the geometry is favored on electronic grounds, though the potential energy surface is rather shallow. These results are discussed in terms of previous studies of the addition of alkynes to d 3 –d 3 dinuclear complexes of tungsten and in terms of relationships between d 2 -W(OR) 4 and d 8 -Os(CO) 4 fragments.

The electronic structure and bonding in W2(μ-H)2(OiPr)4(DMPE)2 and a comparison with Mo2(OiPr)4(DMPE)2

Abstract The bonding in the recently reported [1] W 2 (μ-H) 2 (O i Pr) 4 (DMPE) 2 (O i Pr=isopropoxide; DMPE=bis(dimethylphosphino)ethane) molecule is investigated via the computational method of Fenske and Hall and the results are compared with those of a previous study of the electronic structure of Mo 2 (O i Pr) 4 (DMPE) 2 by Bursten and coworkers [2] . In the dimolybdenum system, a Mo–Mo triple bond of configuration σ 2 π 4 δ 2 nb (nb=nonbonding) unites the two molybdenum atoms of oxidation states 0 and 4+. The introduction of the two bridging hydrides reduces the metal–metal bond order to two in W 2 (μ-H) 2 (O i Pr) 4 (DMPE) 2 and by analogy the bonding between the tungsten atoms can be described as σ 2 π 2 δ 2 nb . Although there is extensive mixing of the W–W and W–H σ bonds, an orbital can still be ascribed as a W–W σ bond. An analogy is made to the bonding in B 2 H 4 2− , an ethane-type molecule, and to B 2 H 6 .

Comparison of the Bonding of Benzene and C60 to a Metal Cluster: Ru3(CO)9(μ3-η2,η 2,η2-C6H6) and Ru3(CO)9(μ3-η2,η2,η2-C60)

The electron distributions and bonding in Ru3(CO)9(μ3-η2,η2,η2-C6H6) and Ru3(CO)9(μ3-η2,η2,η2-C60) are examined via electronic structure calculations in order to compare the nature of ligation of benzene and buckminsterfullerene to the common Ru3(CO)9 inorganic cluster. A fragment orbital approach, which is aided by the relatively high symmetry that these molecules possess, reveals important features of the electronic structures of these two systems. Reported crystal structures show that both benzene and C60 are geometrically distorted when bound to the metal cluster fragment, and our ab initio calculations indicate that the energies of these distortions are similar. The experimental Ru–Cfullerene bond lengths are shorter than the corresponding Ru–Cbenzene distances and the Ru–Ru bond lengths are longer in the fullerene-bound cluster than for the benzene-ligated cluster. Also, the carbonyl stretching frequencies are slightly higher for Ru3(CO)9(μ3-η2,η2,η2-C60) than for Ru3(CO)9(μ3-η2,η2,η2-C6H6). As a whole, these observations suggest that electron density is being pulled away from the metal centers and CO ligands to form stronger Ru–Cfullerene than Ru–Cbenzene bonds. Fenske-Hall molecular orbital calculations show that an important interaction is donation of electron density in the metal–metal bonds to empty orbitals of C60 and C6H6. Bonds to the metal cluster that result from this interaction are the second highest occupied orbitals of both systems. A larger amount of density is donated to C60 than to C6H6, thus accounting for the longer metal–metal bonds in the fullerene-bound cluster. The principal metal–arene bonding modes are the same in both systems, but the more band-like electronic structure of the fullerene (i.e., the greater number density of donor and acceptor orbitals in a given energy region) as compared to C6H6 permits a greater degree of electron flow and stronger bonding between the Ru3(CO)9 and C60 fragments. Of significance to the reduction chemistry of M3(CO)9(μ3-η2,η2,η2-C60) molecules, the HOMO is largely localized on the metal–carbonyl fragment and the LUMO is largely localized on the C60 portion of the molecule. The localized C60 character of the LUMO is consistent with the similarity of the first two reductions of this class of molecules to the first two reductions of free C60. The set of orbitals above the LUMO shows partial delocalization (in an antibonding sense) to the metal fragment, thus accounting for the relative ease of the third reduction of this class of molecules compared to the third reduction of free C60.