The mononitrosyls of the transition metals of the third row of the periodic table were calculated by means of the density functional theory. The geometries and frequencies of the MNO complexes in different states are reported. A bent geometry is found fur the systems with higher multiplicities from vanadium through copper and linear geometries for lower multiplicities. The binding mechanism is described through an analysis of the molecular orbitals (MO). The bent geometry is obtained when the sigma antibonding MO resulting from the interaction of the 5 sigma MO of NO and the d(sigma), orbital of the metal center is occupied. The electronic configuration of all complexes studied is presented in a diagram that allows one to predict qualitatively whether the complex is bent according to its multiplicity. The interaction of the transition metals with other diatomic molecules is briefly discussed.