The influence of cis/trans isomerism on the physical properties of a cyano-bridged dinuclear mixed valence complex

The cyano-bridged complexes cis-[(LCoNCFeII)-Co-14-N-III(CN)(5)](-) and cis-[(LCoNCFeIII)-Co-14-N-III(CN)(5)] (L-14 = 6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine) are prepared and characterised spectroscopically, electrochemically and structurally: Na{cis-[(LCoNCFeII)-Co-14-N-III(CN)(5)]}.9H(2)O, monoclinic space group P2(1)1c, a = 14.758(3), b = 10.496(1), c = 19.359(3) Angstrom, beta = 92.00(2)degrees, Z = 4; cis-[(LCoNCFeIII)-Co-14-N-III(CN)(5)].4H(2)O, orthorhombic space group P2(1)2(1)2(1). a = 9.492(1), b = 14.709(2), c = 18.760(3) Angstrom, Z = 4. In both complexes, the pendant amine is cis to the bridging cyanide ligand. An analysis of the metal-to-metal charge transfer (MMCT) transition in these systems with Hush theory has been carried out. This has revealed that the change in the configuration of the macrocycle both decreases the redox isomer energy difference (DeltaE(1/2)) and increases the reorganisational energy (lambda) of the cis-[(LCoNCFeII)-Co-14-N-III-(CN)(5)](-) complex with respect to the trans-[(LCoNCFeII)-Co-14-N-III(CN)(5)](-) complex, the result being that both isomers display an MMCT transition of similar energy. The variation in redox isomer energy differences of the configurational isomers has been related to strain energy differences by molecular mechanics analysis of the [(CoLCl)-Cl-14](2+/+) precursor complexes.