Catecholates and 2-amidophenoxides are prototypical "noninnocent" ligands which can form metal complexes where the ligands are best described as being in the monoanionic (imino)semiquinone or neutral (imino)quinone oxidation state instead of their closed-shell dianionic form. Through a comprehensive analysis of structural data available for compounds with these ligands in unambiguous oxidation states (109 amidophenolates, 259 catecholates), the well-known structural changes in the ligands with oxidation state can be quantified. Using these correlations, an empirical "metrical oxidation state" (MOS) which gives a continuous measure of the apparent oxidation state of the ligand can be determined based on least-squares fitting of its C-C, C-O, and C-N bond lengths to this single parameter (a simple procedure for doing so is provided via a spreadsheet in the Supporting Information). High-valent d(0) metal complexes, particularly those of vanadium(V) and molybdenum(VI), have ligands with unexpectedly positive, and generally nonintegral, MOS values. The structural effects in these complexes are attributed not to electron transfer, but rather to amidophenoxide- or catecholate-to-metal π bonding, an interpretation supported by the systematic variation of the MOS values as a function of the degree of competition with the other π-donating groups in the structures.

Reaction of (HBpz3)ReO(Ph)(OTf) with oxygen atom donors leads to oxidation of the phenyl group [HBpz3 = hydrotris(1-pyrazolyl)borate, OTf = triflate, OSO2CF3]. Reaction with Me2SO gives the adduct [(HBpz3)ReO(Ph)(OSMe2)]OTf, which undergoes phenyl-to-oxo migration at 25 °C to give the phenoxide complex [(HBpz3)ReO(OPh)(OSMe2)]OTf and Me2S. The Me2SO adduct readily and reversibly loses Me2S (k = 2.9(4) s-1 at 25 °C) as indicated by isotope exchange reactions and magnetization transfer. The Me2SO adduct also slowly oxidizes Me2SO to Me2SO2. These reactions all proceed via an intermediate rhenium(VII) dioxo complex, [(HBpz3)ReO2(Ph)]OTf. This dioxo complex can be observed at low temperature on reaction of (HBpz3)ReO(Ph)(OTf) with pyridine N-oxide. It rearranges at 0 °C by phenyl-to-oxo migration to give phenoxide products and the catecholate complex (HBpz3)ReO(O2C6H4). The kinetics of this migration have been measured (ΔH ⧧ = 14.8(7) kcal/mol, ΔS ⧧ = −20.5(25) eu). From these data and the activation parameters for reactions of [(HBpz3)ReO(Ph)(OSMe2)]OTf, a detailed free energy surface for the reactions of [(HBpz3)ReO2(Ph)]OTf is constructed. The dioxo complex reacts very rapidly with Me2S (1.7 × 105 M-1 s-1) with essentially no enthalpic barrier, consistent with the action of a highly electrophilic oxo ligand. Electrophilicity of the oxo groups is suggested to be a critical factor in facilitating the phenyl-to-oxo migration. A general explanation for the relative ease of various organometallic migration reactions, based on analogies with organic [1,2]-shifts, is presented.

Neural networks are being used to make new types of empirical chemical models as inexpensive as force fields, but with accuracy similar to the ab initio methods used to build them. In this work, we present a neural network that predicts the energies of molecules as a sum of intrinsic bond energies. The network learns the total energies of the popular GDB9 database to a competitive MAE of 0.94 kcal/mol on molecules outside of its training set, is naturally linearly scaling, and applicable to molecules consisting of thousands of bonds. More importantly, it gives chemical insight into the relative strengths of bonds as a function of their molecular environment, despite only being trained on total energy information. We show that the network makes predictions of relative bond strengths in good agreement with measured trends and human predictions. A Bonds-in-Molecules Neural Network (BIM-NN) learns heuristic relative bond strengths like expert synthetic chemists, and compares well with ab initio bond order measures such as NBO analysis.

The rhenium(V) complex [(HCpz3)ReOCl2]+ ([1]+), the tris(pyrazolyl)methane analogue of the known tris(pyrazolyl)borate complex (HBpz3)ReOCl2 (2), has been prepared. The two complexes are strikingly similar, as are the phosphine oxide adducts [(HCpz3)ReCl2(OPPh3)]Cl ([3]Cl) and (HBpz3)ReCl2(OPPh3) (4), which have been characterized by X-ray crystallography. Comparison of the bimolecular reduction of [1]BF4 and 2 by triarylphosphines reveals a pronounced charge effect, with the cationic species being reduced by PPh3 about 1,000 times faster than its neutral analogue in CH2Cl2 at room temperature. Ligand substitution of the adducts [3]+ and 4 is dissociative, with the cationic complex dissociating phosphine oxide about 56 times more slowly than the neutral compound. The relative impact of charge on ground and transition states in atom transfer reactions is discussed.

A procedure for generating the ruthenium hydride complex Ru(H)(H2)Cl(PCy3)2 in 95% yield is presented. Following a novel insertion−elimination pathway, this hydride can react with propargyl or vinyl halides to make metathesis-active vinyl and alkyl carbene species with the general formulas (PCy3)2Cl2RuCH−CHCR2 and (PCy3)2Cl2RuCHR, respectively. Tertiary propargyl chlorides work best, giving Ru−vinyl carbenes in extremely high yield.

The monomeric titanium(IV) hydroxide complex, LTi(OH)(LH(3)= tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine), which is sterically inhibited from condensation to a mu-oxo dimer, cannot be prepared by hydrolysis of the alkoxide, with K(eq)= 0.012 for hydrolysis of the titanium methoxide in THF.

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The rhenium(V) oxo oxalate complex (HBpz3)ReO(C204) (HBpz3 = hydridotris(l-pyrazolyl)borato) has been synthesized in three steps from potassium perrhenate. It has been characterized spectroscopically and its molecular structure determined by X-ray crystallography. When irradiated with UV light, the oxo oxalate complex undergoes an internal redox reaction, predominantly losing carbon dioxide and generating a reactive rhenium complex. The characterization of the transient photoproduct as the rhenium(III) oxo complex (HBpz3)Re( 0) is inferred from its reactions with trapping reagents: for example, photolysis in the presence of phenanthrenequinone gives a rhenium-(V) oxo catecholate complex in good yield. (HBpZ3)Re(0) also reacts with 62, yielding the rhenium(VII) complex (HBpz3)Re03, formally the result of four-electron oxidation. Labeling studies show that only one oxygen atom in the product comes from O2, with the second deriving from an oxalate ligand. That unimolecular four-electron reduction of oxygen does not occur readily in this system, despite its great exothermicity, may be due to a general symmetry-imposed barrier to cleavage of 02 at a single metal center. Crystal data for (HBpz3)ReO(C2O4)-0.5C6H6: a = 8.082 (2) A, b = 9.125 (3) A, c = 13.219 (3) A, a = 84.03 (2)°, 0 = 74.26 (2)°, y = 72.47 (2)°, triclinic, P\, Z = 2.