A series of linear trinuclear copper(II) complexes [Cu ₃ L(dmso) ₅ (H ₂ O)][ClO ₄ ] ₂ ·H ₂ O {H ₄ L = 2,6-bis[(5-substituted salicylidene)hydrazinocarbonyl]pyridine; L = L ¹ , R = H; L = L ² , R = Me ₃ ; L = L ³ , R = Bu ^t ; L = L ⁴ , R = OMe ₃ ; L = L ⁵ , R = Cl; L = L ⁶ , R = Br; dmso = dimethyl sulfoxide} have been synthesized and characterized on the basis of infrared and electronic spectra, electrochemical and variable-temperature (80–300 K) magnetic measurements. The crystal and molecular structure of [Cu ₃ L ¹ (dmso) ₅ (H ₂ O)][ClO ₄ ] ₂ ·H ₂ O has been established by X-ray diffraction. The structure consists of a Cu ^II Cu ^II Cu ^II trinuclear cation, unco-ordinated perchlorate anions, and a water molecule of crystallization. Each copper ion is in a square-pyramidal environment. The compartmental ligand L ¹ co-ordinates in two different modes: (i) as tridentate in the terminal compartment defined by the hydrazonic carbonyl, hydrazonic nitrogen and phenolic oxygen; (ii) tridentate in the central chamber defined by the pyridyl nitrogen and two hydrazonic nitrogens. Cryomagnetic investigations reveal a moderately strong antiferromagnetic spin exchange among adjacent copper(II) ions in each complex (J = -40 to -65 cm ^-1 ), showing that the compartmental ligands of the Cu ^II Cu ^II Cu ^II are good mediators for spin-exchange interactions. Cyclic voltammetry of complex 1 showed one oxidation wave attributed to Cu ^II Cu ^II Cu ^II →& -e ^- Cu ^II Cu ^III Cu ^II , a one-electron transfer reduction peak due to Cu ^II Cu ^II Cu ^II →& +e ^- Cu ^II Cu ^I Cu ^II and a two-electron transfer wave established as Cu ^II Cu ^I Cu ^II →+2e ^- Cu ^I Cu ^I Cu ^I .