The Continuous Symmetry Measure (CSM) methodology has been used to search for the existence of possible correlations between the degree of quantitative symmetry of the first coordination sphere of copper(II) complexes and various properties of these compounds. Several novel correlations were indeed found, employing both four- and six-coordinated copper complexes, including Blue Copper Proteins (BCPs). These include the relation between symmetry and the location of the d–d electronic transition maximum, the probability of electronic transitions, the ESR g∥ values and structural responses to temperature changes. The trend of the identified correlations agrees with classical predictions which are based on specific geometric parameter changes, but are now expressed more generally in terms of quantitative symmetry, which, unlike the specific geometric parameters, is a global structural descriptor. Thus, the d–d electronic transition is blue shifted in tetrahedral complexes as the tetrahedricity measure increases (relating to more distorted tetrahedra), and likewise the g∥ values decrease in response to this symmetry change; and the electronic transition becomes more allowed in octahedral complexes as the degree of octahedral distortion increases. Differences between the symmetry behaviour of CuCl42− and Cu within BCPs were identified and are analysed. It was found that temperature can affect symmetry in opposite ways, either increase or decrease it, and this phenomenon is analysed and interpreted using two different copper complexes.
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