Reaction of (Ph4C4CO)(CO)3Ru with diethylamine in a toluene-water mixture at room temperature affords the complex (Ph4C4CO)(CO)2Ru(NHEt2) (3). Its structure was determined by X-ray diffraction analysis and found to possess a pseudotetrahedral geometry [monoclinic; a = 10.386 (2) Å, b = 19.953 (3) Å, c = 14.664 (7) Å; β = 103.07 (4)°; V = 2960.1 Å3; Z = 4; Dcalcd = 1.379 g.cm-3; F(000) = 1264; space group P21/c; S (goodness-of-fit) = 1.31 e; R = 0.042 (3604 reflections; I > 3σI)]. The cyclopentadienone (CPD) ligand is found in a η4 coordination, and its carbonyl is twisted by 6° from the ring plane. Its oxygen atom and the ruthenium coordinated nitrogen atom are hydrogen bonded (NH—O) with O. N. and O. H. distances of 2.848 and 1.98 A, respectively. The H atom (N-H) was unambiguously located in the difference Fourier map. Experimental evidence indicates that the NH. O. hydrogen bond is the major contributing factor to the thermodynamic stability of this type of coordinated amine complexes. A comparison of structural parameters and spectral data of 3 with two other structurally related complexes is included.