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An obituary for inorganic chemist Malcolm H. Chisholm is presented.

From the reactions between Mo2(O2CtBu)4 and each of terephthalic acid and 4,4′-azodibenzoic acid, the compounds [Mo2(O2CtBu)3]2(μ-O2CC6H4CO2) (1) and [Mo2(O2CtBu)3]2(μ-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 Ill [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. [ABSTRACT FROM AUTHOR]

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 δ → δ* 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 δ 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(μ-O2C-X-C02)+ (X = conjugated spacer). [ABSTRACT FROM AUTHOR]

The formation of [(η5-Ind)Ru(CO)2H] from [(η5-Ind)Ru(CO)3]+ was investigated using both 1H and 2H NMR spectroscopy and GC−mass spectrometry. Both NMR and GC-MS data support a water-gas shift reaction mechanism as the cause for formation of [(η5-Ind)Ru(CO)2H]. The formation of [(η5-Ind)Ru(CO)2H] from [(η5-Ind)Ru(CO)3]+ was investigated using both 1H and 2H NMR spectroscopy and GC−mass spectrometry. Both NMR and GC-MS data support a water-gas shift reaction mechanism as the cause for formation of [(η5-Ind)Ru(CO)2H]. [ABSTRACT FROM AUTHOR]

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 Cs 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. [ABSTRACT FROM AUTHOR]

The electronic structures of D4h-M2(O2CH)4and the oxalate-bridged complexes D2h-[(HCO2)3M2]2(µ-O2CCO2) and D4h-[(HCO2)2M2]4(µ-O2CCO2)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 {d4-d4}, TC, RU {d6-d6}, and Rh {d7-d7}). Significant changes in the ordering, interactions, and electronic occupation of the molecular orbitals (MOs) arise through both the progression from d4 to d7 metals and the change from second to third row transition metals. For M = Mo and W, the highest-occupied orbitals are δ based, while the lowest-unoccupied orbitals are oxalate π* based; for M = Tc, the highest-occupied orbitals are an energetically tight δ*-based set of MOs, while the lowest-unoccupied orbitals are MM-based π*. For both Ru and Rh, the highest-occupied MOs are the MM π* and δ*, respectively, while the lowest-unoccupied MOs, in both instances, are MM-based σ*. With the exception of M = Ru, all of the complexes are closed shell. From the progression M2 → [M2]2 → [M2]4, we can envision the nature of bandlike structures for a 2-dimensional square grid of formula [M2(µO2CCO2)]∞. 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. [ABSTRACT FROM AUTHOR]

The oxidation state (OS) concept is arguably one of the most useful formalisms in chemistry. OSs are used to explain a variety of phenomena at transition metal centers, from chemical reactivity to spectroscopic properties. Attempting to define a theoretical method of evaluating this construct, however, has resulted in a broad debate among chemists, particularly inorganic chemists. With this in mind we propose a simple method for determining the oxidation states of transition metal centers using Natural Bond Orbital (NBO) theory. A description of the wavefunction (or electron density in the case of density functional theory, as presented in this investigation) is obtained from quantum chemical calculations. The 5 × 5 d -orbital Natural Atomic Orbital (NAO) occupation matrix is then obtained, and diagonalized. The resulting eigenvalues deliver the d -orbital occupations, from which the oxidation states can be inferred. The NBO-driven method also allowed us to probe “ambiguous” cases where a strong π-acid is involved in bonding (in our case, CO). The scope of the method is described, along with promising future applications. [ABSTRACT FROM AUTHOR]

The effective conjugation of ortho and ortho-altpara-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 derealization 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. [ABSTRACT FROM AUTHOR]