The cluster anion [Ru3H(CO)11]– has been prepared in high yields by the reaction of [Ru3(CO)12] with Na[BH4] in tetrahydrofuran. The anion is isolated as the tetraethylammonium and the bis (triphenylphosphine) iminium salts. The salt [N(PPh3)2][Ru3H(CO)11] crystallizes in the monoclinic space group P21/c, with the cell dimensions a= 16.355(5), b= 17.057(4), c= 17.715(5)Å, β= 106.29(2)°, and Z= 4. The structure has been solved by a combination of multisolution sign-expansion and Fourier-difference methods, and refined to R 0.053 for 4 341 observed intensities. The three ruthenium atoms define an isosceles triangle, the shortest edge of which is symmetrically bridged by a carbonyl group and a hydride ligand. A variable-temperature 13C n.m.r. study of [NEt4][Ru3H(CO)11] reveals the anion to be fluxional in solution; the spectra are indicative of several dynamic processes occurring over the range –116 to 32 °C which are distinctly different from those proposed for the iron analogue [Fe3H(CO)11]–.