The trinuclear manganese complex [In3O4(phen)(4)(H2O)(2)](NO3)(4) . 2.5H(2)O, 1 (where, phen = 1,10-phenanthroline), has been synthesized by the Ce(IV) oxidation of a concentrated solution of manganese(II) acetate and phen in 1.6 N nitric acid. The complex crystallizes in the triclinic space group with a = 10.700(2) Angstrom, b = 12.643(3) Angstrom, c = 20.509(4) Angstrom, alpha = 78.37(3)degrees, beta = 83.12(3)degrees, gamma = 82.50(3)degrees, and Z = 2. The structure was solved by direct methods and refined by least-squares techniques to the conventional R (R(w)) factors of 0.055 (0.076) based on 4609 unique reflections with F-o greater than or equal to 6.0 sigma(F-o). The structure of the cation consists of an ore-bridged Mn3O44+ core, with the geometry of the manganese atoms being octahedral. The coordination polyhedron of one of the manganese atoms (Mn(1)) consists of two mu oxo ligands and two pairs of nitrogen atoms of two phen moieties, whereas that of each of the remaining two manganese atoms consists of three mu-oxo Ligands, two nitrogen atoms of a phen moiety, and the oxygen atom of a water molecule. The complex represents the second example for water coordination to manganese(IV) centers in complexes with a Mn3O44+ core. Optical spectra in ligand buffer (pH 4.5) reveal complete conversion of the complex into a (MnMnIV)-Mn-III species. The observed room-temperature (298 K) magnetic moment of 3.75 mu(B) indicates the presence of strong antiferromagnetic coupling in the complex.