The reaction of Ru3(CO)12 with PPN(N3) was found to lead through several intermediates to PPN[Ru4(NCO)(CO)13]. The structure of this product is proposed to be analogous to [Ru4Cl(CO)13]- with the μ-NCO replacing the chloride. Spectroscopic characterization of the intermediates suggests that their formulations are [Ru3(NCO)(CO)11]- and [Ru3-(NCO)(CO)10]-. Protonation of the initial reaction solution containing these trimers allows the isolation of HRu3(NCO)(CO)10 in low yield. The tetranuclear cluster reacts with CO at room temperature to give [Ru3(NCO)(CO)11]-, and it reacts with H2 under the same conditions to give [H2Ru4(NCO)(CO)12]-. This anion reacts with CF3SO3H to give H3Ru4(NCO)(CO)12, which was characterized by a single-crystal X-ray crystallographic study [P21/n space group; a = 9.836 (2), b = 22.063 (5), c = 10.056 (2) Å; β = 97.93 (1)°; V= 2161 (1) Å3; Z = 4; final R = 0.024; final Rw = 0.028]. The structure contains a butterfly arrangement of ruthenium tricarbonyl groups with the isocyanate bridging the open edge. The hydrogen atoms were located and refined.