Aquacobalamin binds hydrogen peroxide reversibly to form a cobalt­(III) hydroperoxo adduct with a 0.25 mM dissociation constant, as evidenced by UV–vis absorption spectroscopy and corroborated by NMR, Raman spectroscopy, stopped-flow UV–vis measurements, and density functional theory calculations.

As opposed to heme, siroheme inhibits the charge transfer from the cubane via direct routes.

The nature of the blue color in the iodine-starch reaction is still a matter of debate. Some textbooks still invoke charge-transfer bands within a chain of neutral I2 molecules inside the hydrophobic channel defined by the interior of the amylose helical structure. However, the consensus is that the interior of the helix is not altogether hydrophobic—and that a mixture of I2 molecules and iodide anions reside there and are responsible for the intense charge-transfer bands that yield the blue color of the “iodine-starch complex”. Indeed, iodide is a prerequisite of the reaction. However, some debate still exists regarding the nature of the iodine-iodine units inside the amylose helix. Species such as I3-, I5-, I7- etc. have been invoked. Here, we report UV-vis titration data and computational simulations using density functional theory (DFT) for the iodine/iodide chains as well as semiempirical (AM1, PM3) calculations of the amylose-iodine/iodide complexes, that (1) confirm that iodide is a pre-requisite for blue color formation in the iodine-starch system, (2) propose the nature of the complex to involve alternating sets of I2 and Ix- units, and (3) identify the nature of the charge-transfer bands as involving transfer from the Ix- σ* orbitals (HOMO) to I2 σ* LUMO orbitals. The best candidate for the “blue complex”, based on DFT geometry optimizations and TD-DFT spectral simulations, is an I2-I5-I2 unit, which is expected to occur in a repetitive manner inside the amylose helix.

The complete series of hydrogen-rich six-vertex cyclopentadienyl dimetallaboranes Cp 2 M 2 B 4 H 8 (Cp = η 5 -C 5 H 5 ; M = Ir, Ru/Os, Re, Mo/W, and Ta), including the experimentally known Ir, Ru, and Re derivatives synthesized by Fehlner and co-workers, have now been examined by density functional theory. The nature of the central M 2 B 4 polyhedra in the lowest energy Cp 2 M 2 B 4 H 8 structures relates to the skeletal electron count as determined by the Wade−Mingos rules. Thus, the lowest energy Cp 2 Ir 2 B 4 H 8 structures with 16 Wadean skeletal electrons have central pentagonalpyramidal Ir 2 B 4 units similar to that of the known pentagonal-pyramidal B 6 H 10 . The lowest energy Cp 2 M 2 B 4 H 8 (M = Ru, Os) structures with 14 Wadean skeletal electrons have central capped-tetragonal-pyramidal rather than octahedral M 2 B 4 units. However, isomeric Cp 2 M 2 B 4 H 8 (M = Ru, Os) structures with central M 2 B 4 octahedra are found at energies starting at ∼15 kcal/ mol (M = Ru) and ∼10 kcal/mol (M = Os) above the capped-tetragonal-pyramidal global minima. The lowest energy electron poorer Cp 2 M 2 B 4 H 8 structures (M = Re, Mo, W, Ta) have central M 2 B 4 bicapped tetrahedra with the metal atoms at the degree 5 vertices. Higher energy Cp 2 Re 2 B 4 H 8 structures include capped-tetragonal-pyramidal structures with surface ReRe double bonds and a pentagonal-pyramidal structure with a surface ReRe triple bond. The lowest energy Cp 2 M 2 B 4 H 8 (M = Mo, W) structures appear to have surface MM double bonds and thus also the 12 skeletal electrons for their bicapped-tetrahedral structures. However, the lowest energy likewise bicapped-tetrahedral Cp 2 Ta 2 B 4 H 8 structure is best interpreted in having CpTa units with 16-electron rather than 18-electron tantalum configurations and a surface Ta−Ta single bond.