The novel trimolybdenum cluster [Mo-3(mu(3)-Br)(2)(mu-Br)(3)Br-6](2-) (1, {Mo-3}(9+), 9 d-electrons) has been isolated from the reaction of [Mo(CO)(6)] with 1,2-C2H4Br2 in refluxing PhCl. The compound has been characterized in solution by electrospray ionization mass spectrometry (ESI-MS), UV-vis spectroscopy, cyclic voltammetry, and in the solid state by X-ray analysis (counter-cations: (n-Bu)(4)N+ (1), Et4N+, Et(3)BzN(+)), electron paramagnetic resonance (EPR), magnetic susceptibility measurements, and infrared spectroscopy. The least disordered (n-Bu)(4)N+ salt crystallizes in the monoclinic space group C2/c, a = 20.077(2) angstrom, b = 11.8638(11) angstrom, c = 22.521(2) angstrom, alpha = 90 deg, beta = 109.348(4) deg, gamma = 90 deg, V = 5061.3(9) angstrom(3), Z = 4 and contains an isosceles triangular metal arrangement, which is capped by two bromine ligands. Each edge of the triangle is bridged by bromine ions. The structure is completed by six terminal bromine ligands. According to the magnetic measurements and the EPR spectrum the trimetallic core possesses one unpaired electron. Electrochemical data show that oxidation by one electron of 1 is reversible, thus proceeding with retention of the trimetallic core, while the reduction is irreversible. The effective magnetic moment of 1 (mu(eff), 1.55 mu(B), r.t.) is lower than the spin-only value (1.73 mu(B)) for S = 1/2 systems, most likely because of high spin-orbit coupling of Mo(III) and/or magnetic coupling throughout the lattice. The ground electronic state of 1 was studied using density functional theory techniques under the broken symmetry formalism. The ground state is predicted to exhibit strong antiferromagnetic coupling between the three molybdenum atoms of the core. Moreover, our calculated data predict two broken symmetry states that differ only by 0.4 kcal/mol (121 cm(-1)). The antiferromagnetic character is delocalized over three magnetic orbitals populated by three electrons. The assignment of the infrared spectra is also provided.