David N. Hendrickson

Effect of Lattice Dynamics on Intramolecular Electron-Transfer Rates in Mixed-Valence Complexes

Abstract It is shown that the rates of intramolecular electron transfer for mixed-valence complexes in the solid state are not determined only by the electronic and vibronic coupling in the molecular complex. Lattice dynamics associated with the motion of ligands, solvate and counterions, which sometimes are manifested as phase transitions, frequently influence the electron-transfer rate.

Effects of cooperative intermolecular interactions on the electronic delocalization in mixed‐valence biferrocenium trihalide compounds

Intramolecular electron transfer in the mixed‐valence complexes in the solid state shows qualitatively different behavior from that in an isolated complex. The ‘‘extra’’ electron in a given mixed‐valence complex in the solid state may be localized at lower temperature and delocalized at higher temperature, even though the extra electron may be delocalized in the isolated molecule at any temperature. The solid state environment has an essential effect on the intramolecular electron‐transfer rate in mixed‐valence complexes. In order to clarify the mechanism of any possible environmental effects, a model is proposed for binuclear mixed‐valence biferrocenium trihalides. A binuclear mixed‐valence biferrocenium cation has two localized electronic states, [FeIIAFeIIIB] and [FeIIIAFeIIB], which are coupled to an out‐of‐phase combination of symmetric ligand–metal stretching modes, one on each metallocene unit. An intramolecular electronic interaction α induces the electron transfer between the two vibronic states ...

Manganese-histidine cluster as the functional center of the water oxidation complex in photosynthesis

The recent model of Kambara and Govindjee for water oxidation [Kambara T. and Govindjee (1985) Proc. Natl. Acad. Sci. U.S.A., 82:6119–6123] has been extended in this paper by examining all the data in order to identify the most likely candidate for the ‘redox-active ligand’ (RAL), suggested to operate between the water oxidizing complex (WOC) and Z, the electron donor to the reaction center P680. We have concluded that a very suitable candidate for RAL is the imidazole moiety of a histidine residue. The electrochemical data available on imidazole derivatives play heavily in this identification of RAL. Thus, we suggest that histidine might play the role of an electron mediator between the WOC and Z. A model of S-states in terms of their plausible chemical identity is presented here.

Mechanism of phase transitions affecting intramolecular electron transfer in trinuclear mixed‐valence transition‐metal compounds

Intramolecular electron delocalization in discrete mixed‐valence transition metal complexes in the condensed phase depends not only on the electronic structure of a single complex but also sensitively on the details of the packing arrangement [D. N. Hendrickson, S. M. Oh, T.‐Y. Dong, T. Kambara, M. J. Cohn, and M. F. Moore, Comments Inorg. Chem. 4, 329 (1985)]. The problem of how the cooperative properties of mixed‐valence complexes in the solid state depend on the electron localization and/or delocalization in a single complex is studied theoretically. A phenomenological intermolecular interaction which depends on the sense and the magnitude of molecular distortion arising from the electron localization is introduced. A theoretical model is developed based on molecular field theory in order to show what types of phase transitions relating to the electron delocalization are possible in the trinuclear mixed‐valence compounds and how the electronic structure of constituent molecules determines the type of p...

Low-temperature (4.2°K) study of the 2E1u←2E2g band system in the electronic spectra of various ferricenium compounds

The 2E1u←2E2g (16200cm^−1) band system for the three ferricenium salts [Fe(C5H5)2]PF6,[Fe(C5H5)2]BF4, and [Fe(C5H5)2](CCl3CO2H)2(CCl3CO2−) has been studied at 4.2°K. Analysis of the observed vibrational structure indicates that the 2E1u excited state is split into two Kramers doublets, with the extent of splitting being a function of the anion. Several ferricenium 2E1u vibrational frequencies have been identified and compared with corresponding values for ground state ferrocene. It appears from these comparisons that the iron 4px and 4py orbitals are minimally involved in the iron-ring bonding.

Application of 129I Mössbauer spectroscopy to a metal iodide: [Ru(cp)2I] I3

The 27.8 keV transition of 129I was used to study the bonding in [Ru(cp)2I]I3. The nuclear quadrupole coupling constants, isomer shifts relative to the source, asymmetry parameters, full widths at half‐maximum, intensities, and asymmetries in the recoilless fraction were obtained for each of the three inequivalent iodide sites. The Mossbauer parameters for the triiodide anion agree with both NQR and Mossbauer studies on other compounds containing the triiodide anion.

Biferrocenium tetrabromoferrate, [(C5H5)Fe(C5H4)- (C5H4)Fe(C5H5)]FeBr4. The X-ray structural characterization of a mixed-valence compound

Abstract Biferrocenium tetrabromoferrate, [(C5H5)Fe(C5H4)-(C5H4)Fe(C5H5)]FeBr4 (1) obtained as a by-product in the synthesis of biferrocenium trihalide salts, crystallizes in the noncentrosymmetric orthorhombic space group, P212121: (at 296 K) a 7.492(2), b 9.903 (2), c 31.604(9) A, V 2345(1) A3, and Z = 4. The cation, similar to other structurally characterized biferrocenium salts, adopts a trans-configuration, but, in contrast, possesses no crystallographically imposed symmetry relating the two ferrocenyl environments. Different average Fe-ring distances at the two environments: Fe(1), 2.02(2) and Fe(2), 2.08(2) A are typical of iron(II) and iron(III) states, respectively, indicating the presence of trapped oxidation states. Interionic contacts are both shorter and more numerous for the iron(III) ferrocenyl fragment than for the iron(II) fragment. Both ferrocenyl units have non-eclipsed ring configurations, with a staggering angle of 6.5(3)° at Fe(1) and 23.5(3)° at Fe(2). The FeBr4− counterion is tetrahedral and ordered.

Effect of pressure on the charge-transfer band of the [Fe(CN)6]4−·dimethyl viologen ion pair

The effect of pressure on the optical charge-transfer (CT) band of the ion pair (Fe(CN)/sub 6/)/sup 4 -//center dot/DMV/sup 2 +/ (DMV = dimethyl viologen) has been studied in aqueous solution. The most significant effects of pressure were twofold: (1) In the pressure range 0.001 to 10.0 kbar (1 kbar = 986.92 atm = 0.1 GPa) the CT band maximum shifted to higher energy by /approximately/1700 cm/sup -1/. (2) In this same pressure range the integrated intensity of the CT band increased by /approximately/35%. There were no changes in the peaks full-width at half maximum (FWHM). Qualitatively, these phenomena are in accord with a vibronic coupling model used to describe mixed-valence metal compounds and also with a general theory of absorption spectra of ions in solution, as well as with Mullikens theory of charge transfer. We also noted that the oscillator strength of the CT band decreased by over a factor of 2 at ambient pressure with increasing concentration of the ion pair (from 1.0 to 50 mM). This change correlates with the change of the ionic strength over the same concentration range. 28 refs., 3 figs.

Effect of pressure-induced freezing on the energy of the intervalence transfer electronic absorption band of binuclear mixed-valence complexes

Abstract The Marcus continuum model for the solvent reorientation contribution to the energetics of outer-sphere electron transfer has often been used to analyze the energy of the intervalence transfer (IT) electronic absorption band for a binuclear mixed-valence transition-metal complex. E op , the energy required to transfer an electron optically in a mixed-valence complex, was measured as a function of pressure for two binuclear mixed-valence complexes in different solvents which freeze at 25°C under pressures

Mixed-valence ferrocene systems temperature dependence in the mössbauer spectra of [Fe(II)Fe(III) [1.1] ferrocenophanes

Abstract 57Fe Mossbauer spectra (300 and 4.2°K) have been determined for dioxidized [Fe(III)Fe(III)] biferrocenylene; a large quadrupole splitting (2.89 mm/sec at 300°K) was interpreted in terms of extensive delocalization of e2g electrons resultant from an FeFe bonding interaction. Mossbauer and electronic absorption spectra have also been determined for two mono-oxidized [1.1] ferrocenophanes. Temperature dependence is noted in the [Fe(II)Fe(III)] ferrocenophane Mossbauer spectra and is tentatively associated with a phase transition wherein the FeFe distance is changed to permit greater FeFe exchange interaction.