Evidence of enhanced conjugation in ortho-arylene ethynylenes with transition metal coordination.

The effective conjugation of ortho and ortho-alt-para-arylene ethynylenes, with appropriately positioned pyridine and pyrazine heterocycles, increases upon binding to Ag(I) and Pd(II) cations. Significant bathochromic shifts in the electronic spectra, witnessed upon introduction of these metal bridges, are consistent with enhanced electron delocalization in the unsaturated backbone. Control studies suggest that this electronic behavior is attributable exclusively (in the case of Ag(I)) or partially (in the case of Pd(II)) to conformational restrictions of the conjugated backbones.

Benzoyl-dicarbon-yl(η-inden-yl)ruthenium(II).

In the title mol-ecule, [Ru(C(9)H(7))(C(7)H(5)O)(CO)(2)], the dihedral angle between the mean plane of the indene ring system and the phenyl ring is 86.28 (8)°. The crystal structure is stabilized by weak inter-molecular C-H⋯O and C-H⋯π(arene) inter-actions. The Ru-η(5)-cyclopentadienyl centroid bond length is 1.946 (11) Å

Patterned monolayers of neutral and charged functionalized manganese arene complexes on a highly ordered pyrolytic graphite surface.

Complex patterns: The arene manganese tricarbonyl complexes [Mn(eta(5)-2,5-didodecoxy-1,4-semiquinone)(CO)(3)] and [Mn(eta(6)-1,4-dioctyloxybenzene)(CO)(3)] BF(4) form patterned monolayers on the surface of highly ordered pyrolytic graphite (HOPG), as a result of hydrogen-bonding, hydrophobic, and electrostatic interactions, leading to an ordered 2D array of manganese atoms or ions.

The many ways to have a quintuple bond.

The existence and persistence of five-fold (quintuple) bonding in isomers of model RMMR molecules of quite different geometry are examined theoretically. The molecules studied are RMMR, with R = H, F, Cl, Br, CN, and CH3; M = Cr, Mo, and W. The potential energy surface of these molecules is quite complex, containing two, three, even four local minima. The structural preferences in these molecules are rationalized, and electronic factors responsible for these preferences are elucidated. The linear geometry is always a minimum, but almost never the global minimum; there is a definite preference in RMMR for either a trans-bent conformation or perturbations of the trans-bent isomer with at least one of the R groups in a bridging position about the MM bond. The potential energy surface of these RMMR molecules is relatively flat, the lowest energy conformation being that which for a given molecule attains the best compromise between maximization of the MM bonding and minimization of orbital interactions that are MR antibonding. A surprising low-symmetry C(s) structure is identified, which along with the trans-bent isomer is one of the two most popular choices for the global minimum. Regardless of what isomer of the RMMR molecule is preferred, the MM quintuple bond persists.

Concerning the electronic coupling of MoMo quadruple bonds linked by 4,4'-azodibenzoate and comparison with t2g 6-Ru(II) centers by 4,4'-azodiphenylcyanamido ligands.

From the reactions between Mo2(O2CtBu)4 and each of terephthalic acid and 4,4'-azodibenzoic acid, the compounds [Mo2(O2CtBu)3]2(mu-O2CC6H4CO2) (1) and [Mo2(O2CtBu)3]2(mu-O2CC6H4N2C6H4CO2) (2) have been made and characterized by spectroscopic and electrochemical methods. Their electronic structures have been examined by computations employing density functional theory on model compounds where HCO2 substitutes for tBuCO2. On the basis of these studies, the two Mo2 units are shown to be only weakly coupled and the mixed-valence ions 1+ and 2+ to be valence-trapped and Class II and I, respectively, on the Robin-Day classification scheme for mixed-valence compounds. These results are compared to t2g6-Ru centers linked by 1,4-dicyanamidobenzene and azo-4,4'-diphenylcyanamido bridges for which the mixed-valence ions [Ru-bridge-Ru]5+ have been previously classified as fully delocalized, Class III [Crutchley et al. Inorg. Chem. 2001, 40, 1189; Inorg. Chem. 2004, 43, 1770], and on the basis of results described herein, it is proposed that the latter complex ion is more likely a mixed-valence organic radical where the bridge is oxidized and not the Ru(2+) centers.

Oxalate-bridged dinuclear M2 units: dimers of dimers, cyclotetramers, and extended sheets (m = Mo, W, Tc, Ru, and Rh).

The electronic structures of D(4h)-M(2)(O(2)CH)(4) and the oxalate-bridged complexes D(2h)-[(HCO(2))(3)M(2)](2)(mu-O(2)CCO(2)) and D(4h)-[(HCO(2))(2)M(2)](4)(mu-O(2)CCO(2))(4) have been investigated by a symmetry analysis of their MM and oxalate-based frontier orbitals, as well as by electronic structure calculations on the model formate complexes (M = Mo and W {d(4)-d(4)}, Tc, Ru {d(6)-d(6)}, and Rh {d(7)-d(7)}). Significant changes in the ordering, interactions, and electronic occupation of the molecular orbitals (MOs) arise through both the progression from d(4) to d(7) metals and the change from second to third row transition metals. For M = Mo and W, the highest-occupied orbitals are delta based, while the lowest-unoccupied orbitals are oxalate pi based; for M = Tc, the highest-occupied orbitals are an energetically tight delta-based set of MOs, while the lowest-unoccupied orbitals are MM-based pi. For both Ru and Rh, the highest-occupied MOs are the MM pi* and delta*, respectively, while the lowest-unoccupied MOs, in both instances, are MM-based sigma. With the exception of M = Ru, all of the complexes are closed shell. From the progression M(2) --> [M(2)](2) --> [M(2)](4), we can envision the nature of bandlike structures for a 2-dimensional square grid of formula [M(2)(mu-O(2)CCO(2))](infinity). Only for Mo and W oxalates should good electronic communication between MM centers generate a band of significant width to lead to metallic conductivity upon oxidation.

Studies of oxalate-bridged MM quadruple bonds and their radical cations (M=Mo or W): on the matter of linkage isomers.

Electronic structure calculations employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) have been carried out on the model complexes {[(HCO2)3M2]2(mu-O2CCO2)}0/+(M=Mo or W) in D2h symmetry, where the oxalate bridge forms either five- or six-membered rings with the M(2) centres; the complexes are hereafter referred to as mu(5,5)0/+ and mu(6,6)0/+, respectively. The calculations predict that the neutral complexes should exist as the mu(5,5) linkage isomer, while the radical cations favour the mu(6,6) isomer by ca. 4-6 kJ mol-1. For the mu(5,5) isomers, the rotational barriers about the oxalate C-C bond have been calculated to be 15.9 and 27.2 kJ mol-1 for M=Mo and W, respectively. For the cationic mu(5,5)+ isomers the barrier is higher, being 36.8 and 50.6 kJ mol-1 for M=Mo and W, respectively. The calculated Raman and visible near-IR spectra for the mu(5,5)0/+ and mu(6,6)0/+ are compared with experimental data obtained for the {[(tBuCO2)3M2]2(mu-O2CCO2)}0/+ complexes, hereafter referred to as M4OXA0/+(M=Mo or W). The experimental data more closely correlate with that calculated for the mu(5,5)0/+ linkage isomers, and the 13C-NMR spectrum of the mixed metal complex Mo2W2OXA indicates the presence of the 5-membered oxalate-bridged species (J(CC)=100 Hz).

Cations M2(O2CtBu)4+, where M = Mo and W, and MoW(O2CtBu)4+. Theoretical, spectroscopic, and structural investigations.

With the aid of density function theory, the molecular and electronic structures of the molecules Mo2(O2CMe)4, MoW(O2CMe)4, and W2(O2CMe)4 and their single-electron oxidized radical cations have been determined; this includes calculated observables such as v(MM) and the delta --> delta* electronic transition energies. The calculated properties are compared with those for the corresponding pivalates, M2(O2CtBu)4 (M = Mo or W) and MoW(O2CtBu)4 and their radical cations prepared in situ by oxidation with Cp2FePF6. The EPR spectra of the radical cations are also reported. The EPR spectrum of the MoW(O2CtBu)4+ cation reveals that the unpaired electron is in a polarized MM delta orbital having 70% Mo and 30% W character. The MM stretching frequencies show good correlation with the MM bond lengths obtained from single-crystal X-ray diffraction studies of MoW(O2CtBu)4, W2(O2CtBu)4, and W2(O2CtBu)4+PF6- compounds, along with previously reported structures. These data provide benchmark parameters for valence trapped dicarboxylate bridged radical cations of the type [(tBuCO2)3M2]2(micro-O2C-X-CO2)+ (X = conjugated spacer).