The complex formation of vanadium(IV) with 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (taci) and 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol (tdci) was studied in aqueous solution and in the solid state. The formation constants of [V(IV)O(taci)](2+), [V(IV)O(tdci)](2+), and [V(IV)(tdci)(2)](4+) and of the deprotonation product [V(IV)(tdci)(2)H(-)(1)](3+) were determined (25 degrees C, 0.1 M KNO(3)). Cyclic voltammetry measurements established a reversible one-electron transfer for the [V(IV)(tdci)(2)H(-)(m)]((4)(-)(m))/[V(III)(tdci)(2)H(-)(n)]((3)(-)(n)) couple (0 <or= n, m <or= 6) with a strongly pH-dependent E(1/2)(obs) (+0.15 V at pH 5, -0.57 V at pH 13.5). Slightly more negative potentials were measured for the taci complexes. An additional quasi-reversible electron transfer at strongly positive potentials (1.2-0.5 V) was assigned to the [V(V)(taci)(2)H(-)(m)]((5)(-)(m))/[V(IV)(taci)(2)H(-)(n)]((4)(-)(n)) couple. The structures of [V(IV)(taci)(2)](SO(4))(2).12H(2)O (1), [V(IV)(tdci)(2)][V(4)(V)O(12)].14.5H(2)O (2), and [V(IV)(tdci)(2)H(-)(1)]Cl(3).15H(2)O (3) were determined by single-crystal X-ray analysis. The cations of 1-3 exhibit a structure of approximate D(3)(d) symmetry. The vanadium centers have an almost regular octahedral geometry. The coordinated oxygen donors are deprotonated, and their protons are transferred to the amino groups which act as internal bases. For both ligands, EPR measurements support the formation of non-oxo V(IV) complexes, with a d(z)2 ground state, in aqueous solution above pH 7. The spectral features are indicative of hexacoordinated complexes with a geometry distorted toward a trigonal prism. Finally, a mechanism is proposed for the decomposition of [V(IV)L(2)H(-)(x)]((4)(-)(x)) (L = taci and tdci) in strongly alkaline media (pH approximately 13).