Condensation of homochiral primary amines with 1-methyl-1H-imidazole-2-carbaldehyde affords the corresponding imidazolyl-imine compounds (L1-L3) which have been employed as ligands for the preparation of half-sandwich rhodium and iridium complexes of the formula [(η(5)-C5Me5)MClLn][SbF6]. Treatment of these chloride compounds with AgSbF6 renders dicationic aqua-complexes [(η(5)-C5Me5)MLn(H2O)] [SbF6]2 which act as catalysts for the Diels-Alder reaction between methacrolein and cyclopentadiene. Catalysis occurs with good exo:endo selectivity and poor enantioselectivity. All the compounds have been completely characterized by analytical and spectroscopic methods. Characterization includes the molecular structure determination of the complexes [(η(5)-C5Me5)MClLn][SbF6] (Ln = L1, M = Rh, (1) Ir (4); Ln = L3, M = Ir (6)) and [(η(5)-C5Me5)ML1(H2O)][SbF6]2 (M = Rh (7), Ir (10)) using X-ray diffraction. From the stereochemical properties of the organometallic precursors the catalytic outcome is discussed.