The manganese phosphorus cluster [Mn-5{N(SiMe3)(2)}{mu(4)-PSi/Pr-3}(2){mu-P(H)Si/Pr-3}(5)] (1) containing an unusual Mn5P7 core structure and three short Mn-Mn distances was obtained from the reaction of Mn{N(SiMe3)(2)}(2) with H2PSi/Pr-3. Quantum chemical investigations indicated the presence of comparably strong intramolecular antiferromagnetic interactions leading to a total S = 5/2 spin ground state, which was confirmed by magnetic measurements.