The complex formation of vanadium(IV) with cis-inositol (ino) and the corresponding trimethyl ether 1,3,5-trideoxy-1,3,5-trimethoxy-cis-inositol (tmci) was studied in aqueous solution and in the solid-state. With increasing pH, the formation of [VO(H-2L)], [(VO)(2)L2H-5](-), [VO(H-3L)](-) (L = ino) or [(VO)(2)L2H-6](2-) (L = tmci), [V(H-3L)(2)](2-), and [VO(H-3L)(OH)(2)](3-) was observed. For the vanadium(IV)/ino system, [(VO)(2)L2H-7](3-) was observed as an additional dinuclear species. The formation constants of these complexes were determined by potentiometric titrations (25 degrees C, 0.1 M KCl). In addition, the vanadium(IV)/ino system was investigated by means of UV-vis spectrophotometric methods. EPR spectroscopy and cyclic voltammetry confirmed this complexation scheme. EPR measurements indicated the formation of three distinct isomers of the non-oxo complex [V(H(-3)ino)(2)](2-) in weakly basic solution. This type of isomerism, which is not observed for the vanadium(IV)/tmci system, was assigned to the ability of ino to bind the vanadium(IV) center with three alkoxo groups having either a 1,3,5-triaxial or an 1,2,3-axial-equatorial-axial arrangement. The structures of [V(H(-3)ino)(2)][K-2(ino)(2)]center dot 4H(2)O (1) and [Na6V(H(-3)ino)(2)](SO4)(2)center dot 6H(2)O (2) were determined by single-crystal X-ray analysis. In both compounds, the coordination of each ino molecule to the vanadium(IV) center via three axial deprotonated oxygen donors was confirmed. The centrosymmetric structure of the coordination spheres corresponds to an almost regular octahedral geometry with a twist angle of 60 degrees. The crystal structure of the potassium complex 1 represents an unusual 1:1 packing of [V(H(-3)ino)(2)](2-) dianions and [K-2(ino)(2)](2+) dications, in which both K+ ions have a coordination number of nine and are bonded simultaneously to a 1,3,5-triaxial and an 1,2,3-axial-equatorial-axial site of ino. In 2, the [V(H(-3)ino)(2)](2-) complexes are surrounded by six Na+ counterions that are bonded to the axial alkoxo oxygens and to the equatorial hydroxy oxygens of the cis-inositolato moieties. The six Na+ centers are further interlinked by bridging sulfate ions. According to EPR spectroscopy, the D-3d symmetric structure of the [V(H(-3)ino)(2)](2-) anion is retained in H2O, in dimethylformamide, and in a mixture of CHCl3/toluene 60:40 v/v.