Ten 1:1 and 2:1 complexes of [Mn(CO)(3)](+) and [Re(CO)(3)](+) with [Nb6O19](8-) and [Ta6O19](8-) have been isolated as potassium salts in good yields and characterized by elemental analysis, O-17 NMR and infrared spectroscopy, and single-crystal X-ray structure determinations. Crystal data for 1 (t-Re2Ta6): empirical formula. K4Na2Re2C6Ta6O35H20, monoclinic, space group, C2/m, a = 17.648(3) Angstrom, b = 10.056(1) Angstrom. c = 13.171(2) Angstrom, beta = 112.531(2)degrees, Z = 2. 2 (t-Re2Nb6): empirical formula, K6Re2C6Nb6O38H26, monoclinic, space group, C2/m, a = 17.724(1) Angstrom, b = 10.0664(6) Angstrom, c = 13.1965(7) Angstrom, beta = 112.067(1)degrees, Z = 2.3 (t-Mn2Nb6): empirical formula, K6Mn2C6Nb6O37H24, monoclinic, space group, C2/m, a = 17.812(2) Angstrom, b = 10.098(1) Angstrom, c = 13.109(3) Angstrom, beta = 112.733(2)degrees, Z = 2.4 (c-Mn2Nb6): empirical formula: K6Mn2C6Nb6O50H50, triclinic. space group, P (1) over bar, a = 10.2617(6) Angstrom, b = 13.4198(8) Angstrom, c = 21.411(1) Angstrom, alpha = 72.738(1)degrees, beta = 112.067(1)degrees, gamma = 83.501(1)degrees Z = 2.5 (c-Re2Nb6): empirical formula, K6Re2C6Nb6O54H58, monoclinic, space group, P2(1)/c, a = 21.687(2) Angstrom, b = 10.3085(9) Angstrom, c = 26.780(2) Angstrom, beta = 108.787(1)degrees, Z = 3. The complexes contain M(CO)(3) groups attached to the surface bridging oxygen atoms of the hexametalate anions to yield structures of nominal C-3v (1:1), D-3d (trans 2:1), and C-2v (cis 2:1) symmetry. The syntheses are carried out in aqueous solution or by aqueous hydrothermal methods, and the complexes have remarkably high thermal, redox, and hydrolytic stabilities. The Re-containing compounds are stable to 400-450 degreesC, at which point CO loss occurs. The Mn compounds lose CO at temperatures above 200 degreesC. Cyclic voltammetry of all complexes in 0.1 M sodium acetate show no redox behavior, except an irreversible oxidation process at similar to1.0 V vs Ag/AgCl. In contrast to the parent hexametalate anions that are stable only in alkaline (pH > 10) solution, the new complexes are stable, at least kinetically, between pH 4 and pH similar to 12.