The interaction of Mn-II Fe-II, and Co-II with the ligand taci (=1,3,5-triamino-1,3,5-trideoxy-cis-inositol) was studied in the solid state and in aqueous solution. Magnetic susceptibility measurements revealed a high-spin electron configuration for the Mn and the Fe complexes. Crystal data : [Mn(taci)(2)](NO3)(2) . 2H(2)O (1), C12H34MnN8O14, monoclinic, space group P2(1)/c, a = 7.107(5) Angstrom, b = 8.742(2) Angstrom, c = 18.527(6) Angstrom, beta = 100.20(4)degrees, Z = 2; [Fe(taci)(2)]Cl-2 . 6H(2)O (2), C12Cl2FeH42N6O12, monoclinic, space group C2/m, a = 11.420(2) Angstrom, b = 9.300(2) Angstrom, c = 13.330(3) Angstrom, beta = 111.97(3)degrees, Z = 2; [Co(taci)(2)](NO3)(2) . 1.5H(2)O (3), C12CoH33N8O13.5, monoclinic, space group P2(1)/c, a = 7.081(1) Angstrom, b = 8.743(1) Angstrom, c = 18.181(2) Angstrom, beta = 99.49(1)degrees, Z = 2. All three structures exhibited distorted octahedral MN6 coordination. The stability constants for the complexes [M(taci)](2+) (M = Mn, Fe, Co) and [M(taci)(2)](2+) (M = Fe, Co) in aqueous media have been evaluated by potentiometric titration. Comparison with other divalent cations showed that the order of stability increases Mn-II < Cd-II approximate to Fe-II < Zn-II < Co-II < Cu-II < Ni-II. In alkaline aqueous media, the complexes 1-3 are readily oxidized by dioxygen. The redox properties were analyzed by a series of polarographic and cyclic voltammetric measurements. A new isomeric form of a bis-taci Co-III complex was formed, where one of the taci ligands coordinates the metal cation by one nitrogen and two oxygen atoms. The complex was isolated in crystalline form as [Co(taci)(Htaci)](NO3)(4).0.5H(2)O (4), C12CoH32N10O18.5, monoclinic, space group C2/c, a = 30.541(3) Angstrom, b = 9.015(1) Angstrom, c = 17.852(2) Angstrom, beta = 95.63(1)degrees, Z = 8. [Co(tach)(H2O)(x)(NO3)(3-x)](x+) (tach = all-cis-1,3,5-triaminocyclohexane, 0 less than or equal to x less than or equal to 3) was used as a model to demonstrate that, for Co-III, the substitution of the three monodentate coligands by st taci molecule resulted generally in an asymmetric binding. An N,O,O and an N,N,O coordination mode were observed for [Co(taci)(tach)](NO3)3 . 3H(2)O (5), and [Co(H(-1)taci)(tach)](NO3)(2) .(3-x)H2O . xMeOH, 0.2 less than or equal to x less than or equal to 0.5 (6), respectively. Crystal data : 5 (C12CoH36N9O15), monoclinic, P2(1)/n, a = 8.942(5) Angstrom, b = 30.07(3) Angstrom, c = 9.354(5) Angstrom, beta = 109.25(4)degrees Z = 4; 6 (x approximate to 0.2, C12.2CoH35.4N8O12), monoclinic, P(2)1/n, a = 7.337(2) Angstrom, b = 19.847(4) Angstrom, c = 15.226(3) Angstrom, beta = 93.59(3)degrees, Z = 4.