Oxidation state Terms Links

The above multitude/diversity of metal-ligand bonds and of (bio-)catalytic transformations is given by terms (parameters) which are typical for the given metal ion (although c and x values change with oxidation states (Fe, Ce, V, etc.) and c x are linked by correlations for given oxidation states). 

The term electrochromism was chosen in 1961 on the model of thermochromism and photochromism [58]. It qualifies the ability of a material to change its optical properties when an electrical potential is applied across it [59]. The optical properties of an elec-trochromic material are linked to its oxidation state and hence can be manipulated by the oxidation—reduction process, ie, gain or loss of electrons. The required voltage for an electrochromic colour change is very low — only a few volts are necessary. The colour of an electrochromic material remains even when the current has ceased to flow (so-called memory effect [60]), and its colour change is reversible when the inverted potential is applied. Colour change can occur from a colourless to a coloured state or from one colour to another, and even in the infrared or UV ranges. 

Fig. 3.3. Tentative mechanism of reduction of dioxygen. The scheme shows some of the more significant reaction steps at the haem iron-Cug centre of cytochrome oxidase. The reaction may be initiated by delivery of dioxygen to the reduced enzyme (in anaerobiosis top of figure). An initially formed oxy intermediate is normally extremely short-lived, but can be stabilised and identified in artificial conditions (see Refs. 92, 99,129, 134). Concerted transfer of two electrons from Fe and Cu to bound dioxygen yields a peroxy intermediate. This, or its electronic analogue, is stabilised in the absence of electron donors (ferrocytochrome a and/or reduced Cu ), and has been termed Compound C [129,130,132). It may also be observed at room temperature, and is then probably generated from the oxidised state by partial oxidation of water in the active site, in an energy-linked reversed electron transfer reaction [29] (see also Refs. 92, 99). Also the ferryl intermediate [92,99,100] has been tentatively observed in such conditions [29]. In aerobic steady states the reaction is thought to involve the cycle of intermediates in the centre of the figure (dark frames). The irreversible step is probably the conversion of g = 6 (see Refs. 98, 133) to peroxy .

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