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The outer-sphere redox behaviour of a series of [L n Co IIINCFe II (CN) 5 ] − (L n = n-membered pentadentate aza-macrocycle) complexes have been studied as a function of pH and oxidising agent. All the dinuclear complexes show a double protonation process at pH ഠ 2 that produces a shift in their UV/Vis spectra. Oxidation of the different non-protonated and diprotonated complexes has been carried out with peroxodisulfate, and of the non-protonated complexes also with trisoxalatocobaltate ( oxidants, has been carried out as a function of pH, temperature, and pressure. As a whole, the values found for the activation volumes, entropies, and enthalpies are in the follow-

The cyano-bridged complexes cis-[L 14 Co III NCFe II (CN) 5 ] Ϫ and cis-[L 14 Co III NCFe III (CN) 5 ] (L 14 = 6-methyl-1,4,8,11tetraazacyclotetradecan-6-amine) are prepared and characterised spectroscopically, electrochemically and structurally: Na{cis-[L 14 Co III NCFe II (CN 1), b = 14.709(2), c = 18.760(3) Å, 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 (∆E 1/2 ) and increases the reorganisational energy (λ) of the cis-[L 14 Co III NCFe II -(CN) 5 ] Ϫ complex with respect to the trans-[L 14 Co III NCFe II (CN) 5 ] Ϫ 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 [CoL 14 Cl] 2ϩ/ϩ precursor complexes. DALTON

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The complex anions hexacyanoferrate(II) and hexacyanoferrate(III) are among the oldest known coordination compounds. The reaction of Fe 3+ aq and [Fe(CN) 6 ] 4-to form the intensely colored Prussian Blue (or Fe 2+ aq with [Fe(CN) 6 ] 3-to give the same products) was employed almost 300 years ago to generate pigments for use in inks and paints. In Prussian Blue, the main structure comprises arrays of high-spin Fe 3+ centers bridged by [Fe(CN) 6 ] 4-units, 2 where a metal-to-metal charge transfer (MMCT, intervalence) transition from [Fe(CN) 6 ] 4-to Fe 3+ gives rise to its intense blue color. If the mixed valence Prussian Blue is oxidized to its Fe 3+ /[Fe(CN) 6 ] 3-analogue (Prussian Brown) or reduced to the Fe 2+ /[Fe(CN) 6 ] 4-relative (Prussian White), the intense blue color is lost. This feature has found application in electrochromic materials, 3 where a system may be switched, reversibly, between two contrasting colored states by a simple electrochemical reaction. The strong coupling between metal centers bridged by cyano ligands has also been exploited in the development of molecular magnetic materials based on ferricyanide and other transition metal analogues. 4 Despite the multitude of cyano complexes in the literature, 5 and the intense degree of interest in the field of hexacyanoferrate complexes in particular, control over the assembly of oligonuclear arrays based on [Fe(CN) 6 ] 4-/3-anions still presents a major challenge. There are six potentially bridging sites at each [Fe(CN) 6 ] 4-unit, and polymeric arrays comprising linear (1D), square (2D), or cubic (3D) structures will generally result if either ferro-or ferricyanide is mixed with a labile metal ion or complex. Kinetic 6 and spectroscopic 7 studies of discrete dinuclear, cyanobridged complexes of hexacyanoferrates have been reported, but remarkably none has been structurally characterized. In several cases, mixed valence complexes containing ferro-or ferricyanide are thermodynamically unstable toward comproportionation 8 or exhibit photolability. 9 This has perhaps hampered attempts to produce X-ray-quality single crystals of these compounds.In order to make discrete dinuclear µ-cyano complexes of ferroor ferricyanide, one must employ a metal complex bearing a single reactive coordination site. We have achieved this goal through reaction 10 of the trans-chloropentaaminecobalt(III) complex of a known 11 macrocycle L 1 with [Fe(CN) 6 ] 4-. The proposed mechanism for the formation of [L 1 Co(µ-NC)Fe(CN) 5 ] -is given in steps (i)-(iv); and is based on the fairly well understood reactions between pentaaminecobalt(III) complexes and ferrocyanide. 12 Typically, in the reactions of acyclic pentaamine analogs, the Co II complex formed in step (iii) dissociates and a precipitate of Co II 3 [Fe III (CN) 6 ] forms unless a scavenging ligand such as EDTA is present. However, the Co II intermediate in the present system is stabilized by strong binding with the macrocycle L 1 . 13The solid state infrared spectrum of Na[L 1 Co(µ-NC)Fe(CN) 5 ] exhibits thre...

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