Coordination compounds, charge separation

Typical rules include converting nitro groups to the uncharged form (6.8), expanding shortcuts (6.9), replacing dative bonds in coordination compounds with separated charges (6.10), and neutralising zwitterions (6.11). 

Azo compound decomposition is much less susceptible to polar substituent effects, and so probably has less charge separation in the transition state,75 but is more sensitive to geometrical restrictions. Bridgehead azo compounds decompose at rates lower than expected on the basis of their tertiary nature, whereas peresters are much less strongly affected.70 The difference can be rationalized by the proposal that the transition state comes farther along the reaction coordinate in azo decomposition, so that the nonplanarity forced on the incipient radical by the ring system is felt more strongly there. 

In molecules with heteroatoms there is always some charge separation. In these cases, and more importantly with compounds that include charged metal ions, electrostatic effects may be critical. The problem is that, apart from some recent and, at least in the area of coordination chemistry, not fully tested methods,8 there are no simple and accurate methods to calculate partial charges empirically. 

From a functional point of view, this important property can be readily built into low molecular weight chromophore assemblies acting as artificial reaction centers (coordination compounds, the population of CT states is directly related to the concept of light-induced charge separation in photosynthesis. Whenever such CT states are photoreactive and lead to the formation of the same kind of permanent redox products as observed in photosynthesis, the most essential features of the primary light reactions have been successfully duplicated. In a more strict sense, this is of course only true, if actinic red or NIR-light of comparable wavelength is absorbed by both the natural and artificial photosynthetic systems. 

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