The reaction of Fe(III) with Na+ and K+ salts of the trivacant [alpha-SiW9O34](10-) ligand have been investigated at pH 6 and pH 1. A new dimer, [(alpha-SiFe3W9(OH)(3)O-34)(2)(OH)(3)](11-) (1), is synthesized by reacting Na7H3[alpha-SiW9O34] or K-10[alpha-SiW9O34] with exactly 3 equiv of Fe(III) in a 0.5 M sodium acetate solution (pH 6). The structure of 1, determined by single-crystal X-ray diffraction (a = 22.454(2) Angstrom, b = 12.387(2) Angstrom, c = 37.421(2), beta = 100. 107(8)degrees, monoclinic, C2/c, Z = 4, R-1 = 5.11% based on 12739 independent reflections), consists of two [alpha-SiFe3W9(OH)(3)O-34](4-) units linked by three Fe-mu-OH-Fe bonds. Reaction of K-10[alpha-SiW9O34] with 3 equiv of Fe(III) in water (pH 1) yields [(alpha-Si(FeOH2)(2)FeW9(OH)(3)O-34)(2)](8-) (2). The structure of 2 was also determined by single-crystal X-ray diffraction (a = 36.903(2) Angstrom, b = 13.9868(9) Angstrom, c = 21.7839(13) Angstrom, beta = 122.709(1)degrees, monoclinic, C2/c, Z = 4, R-1 = 4.57% based on 11787 independent reflections). It consists of two [alpha-Si(FeOH2)(2)FeW9(OH)(3)O-34](4-) Keggin units linked by a single edge. The terminal ligand on Fe1 in each trisubstituted Keggin unit becomes a mu(2) OXO ligand bridging to a [WO6](2-) moiety. The UV-vis spectra of both complexes show the characteristic oxygen-tometal-charge-transfer bands of polyoxometalates as well as an Fe(III)-centered band at 436 nm (epsilon = 146 M(-1)cm(-1)) and 456 nm (epsilon = 104 M(-1)cm(-1)) for complexes 1 and 2, respectively. Differential scanning calorimetry data show that complex 1 decomposes between 575 and 600degreesC whereas no decomposition is observed for complex 2 up to temperatures of 600degreesC.