A series of 1-azadiene ligands as well as the corresponding (η4-azadiene)Fe(CO)3 complexes have been synthesized and structurally characterized by means of single-crystal X-ray diffraction. At the molecular level, the 1-azadiene assumes a trans conformation as free ligand, while a cis coordination is found in the organometallic complexes. In the coordinated ligand the H(C3) hydrogen is found nonplanar with respect to the C−C−C−N plane. A molecular orbital analysis has been carried out by means of extended Hückel calculations. The electronic origin of the out-of-plane position of the 1-azadiene hydrogen atom H(C3) has been justified in terms of a better π*-back-bonding between the 1-azadiene and Fe(CO)3 molecular fragments. A comparison of cis and trans conformations of the ligand has also been performed. The organometallic crystals show the presence of a large number of hydrogen bond interactions of the C−H- - -O type involving CO-ligand acceptors. Hydrogen bonding of the azadiene moiety is always realized by the hydrogen atom at the C−N double bond and the hydrogen atom in α-position with respect to the imine group.