Redox active molecular orbitals

Figure 3 contains drawings of the redox active molecular orbital in the oxidized and reduced states of the W complex and a Walsh diagram of the orbital energy changes attending the reaction. The redox active orbital is the bsu a orbital, which arises from an antibonding interaction between the dx2-y2... 

Fig. 30.2 Schematic pictures of the Ou and ti redox active molecular orbitals (RAMOs) (a) and the o and states (b)...

Fig. 30.4 o and redox active molecular orbitals (RAMOs) of the 1V54B and 1V540 models of the Cua site in the gas phase. AU isovalue surfaces are set at 0.03 (e/A ). Molecular structures are shown in thin lines... 

Valence PES data for [FeCy have been used to probe the MO structure of the ferrous site (Figure 9). The general shape of the photoelectron spectrum is quite similar to that for the ferric species except for a new peak on the low binding-energy side (labeled redox-active molecular orbital (RAMO)). That peak corresponds with the extra electron in the ferrous species, the lone electron in the 3 manifold. Although the overall shapes of the spectra are similar, the variable photon energy behavior is quite different in the ferrous relative to the ferric site. In the ferrous... 

In summary, all bis(dithiolene) complexes are redox active most of them undergo two or three reversible, one-electron redox reactions. The dithiolene ligand itself is also redox active, which contributes significantly to the redox properties of the metal complex. Molecular orbital pictures derived from quantum mechanical calculations are consistent with the observed redox potential data. 

Concerted two-electron transfer and reversible metal-metal bond cleavage in phosphine-bridged dimers have been investigated, and extended Hiickel molecular orbital calculations have shown that the redox-active orbital is a metal-metal antibonding orbital. A Ru-Ru-bonded dimeric cation [Ru(Cp)2]2++ has been prepared and characterized electrochemically. The electrochemistry of these dimers may give insight into more complex clusters and polymeric metals. 

EPR spectroscopic studies have uncovered various redox states of the Ni hydrogenases (Scheme 1). The oxidized state of the [Ni]-hydrogenases exhibit EPR signals (called Ni-A and Ni-B) that disappear on reduction and have been attributed to the nP+ oxidatiou state, with a (d ) grouud state (see Splitting, Crystal Field Molecular Orbital). The states of the enzyme that ehcit these signals are called Form A and Form B. Neither Form A (an already oxidatively inactivated form) nor Form B is catalytically active, and neither is sensitive to inactivation by O2, indicating that both are oxidized forms. 

In contrast to the HOMO and LUMO, the singly occupied molecular orbital (SOMO) can be correlated both qualitatively and quantitatively with experimentally measurable EPR hyperfine couplings (hfcs). As a result, host-guest systems that have redox-active guests that are stable as radicals provide excellent tools for studying the effects of noncovalent interactions on redox properties. 

EPR spectrochemical experiments yield information about (i) the site of redox activity in the compound (ii) the contribution of various nuclei to the molecular orbital occupied by the unpaired electron that may then be checked against the results of theoretical calculations and (iii) the half-wave potentials of systems where direct measurement is difficult (usually as a result of slow electron-transfer rates). Examples of each of these have been taken from our investigations and are detailed below. Initially, however, the cell used in these experiments is described followed by a review of some other cell designs. 

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