Metal complexes—continued reduction

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

The corrosion inhibitor can be a complexing agent that stops the metal redeposition reaction (reduction) by eliminating free metal ions from the solution. Theoretically, this would consecutively stop the associated oxidation reaction. Due to parasitic reduction reactions, however, the metal oxidation can continue, even enhanced by the complexing agent effect. 

Bipyridyl (continued) as ligand, 12 135-1% catalysis, 12 157-159 electron-transfer reactions, 12 153-157 formation, dissociation, and racemization of complexes, 12 149-152 kinetic studies, 12 149-159 metal complexes with, in normal oxidation states, 12 175-189 nonmetal complexes with, 12 173-175 oxidation-reduction potentials, 12 144-147... 

The theory of electron transfer in chemical and biological systems has been discussed by Marcus and many other workers 74 84). Recently, Larson 8l) has discussed the theory of electron transfer in protein and polymer-metal complex structures on the basis of a model first proposed by Marcus. In biological systems, electrons are mediated between redox centers over large distances (1.5 to 3.0 nm). Under non-adiabatic conditions, as the two energy surfaces have little interaction (Fig. 5), the electron transfer reaction does not occur. If there is weak interaction between the two surfaces, a, and a2, the system tends to split into two continuous energy surfaces, A3 and A2, with a small gap A which corresponds to the electronic coupling matrix element. Under such conditions, electron transfer from reductant to oxidant may occur, with the probability (x) given by Eq. (10),... 

Enyl complexes (continued) reactivity, 8, 373 reductive elimination, 8, 380 structure and bonding, 8, 368 thermal decomposition, 8, 374 via transmetallation, 8, 367 transmetallation to metals, 8, 374 with niobium, 5, 87 with Pd 7i-complexes... 

One of the broadest and historically most important areas of dimetal chemistry is that of the simple carbonyl complexes (see Carbonyl Complexes of the Transition Metals). As mentioned above, Co2(CO)s, Fe2(CO)9, and Mn2(CO)io were among the first metal-metal bonded complexes characterized. To this day, these complexes continue to be involved in new chemistry, for example, Co2(CO)g found recent use in a one-pot synthesis of tricyclic 5-lactones .In the case of molybdenum, the zerovalent carbonyl is monomeric however, reduction gives a dinuclear metal carbonyl dianion in which the metal is in the [-1] oxidation state (equation 6). 

Repetitive square-wave potential techniques switch the potential continuously between the strongly reductive value necessary for the nucleation of the metal particles and a more positive one that is chosen to promote reoxidation of the CP material and thus recuperation of its conducting state, and/or unproved penetration of metal complex anions in the CP layer. Metal complex anions that are used as sources of metal reduction become partially consumed, but also expulsed as doping anions in the course of the reductive dedoping pulse. The size of the electrodeposited metal particles has been found to depend essentially on the frequency of the potential pulses [37,169] (Table 7.3). In fact, the data summarized in Table 7.3 show that by appropriate adjustment of the corresponding parameters, all of the currently exploited electrochemical techniques may result in the deposition of metal NPs in CPs. 

The nature and properties of metal complexes have been the subject of important research for many years and continue to intrigue some of the world s best chemists. One of the early Nobel prizes was awarded to Alfred Werner in 1913 for developing the basic concepts of coordination chemistry. The 1983 Nobel prize in chemistry was awarded to Henry Taube of Stanford University for his pioneering research on the mechanisms of inorganic oxidation-reduction reactions. He related rates of both substitution and redox reactions of metal complexes to the electronic structures of the metals, and made extensive experimental studies to test and support these relationships. His contributions are the basis for several sections in Chapter 6 and his concept of inner- and outer-sphere electron transfer is used by scientists worldwide. 

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