AbstractThe stepwise formation constants ofN‐antipyrinyl‐N′‐3‐phenyl‐2‐propenoyl‐thiourea (I) complexes with metal ions of the first transition series, Cu(II), Ni(II), Co(II), Mn(II) and Zn(II) have been determined in 70% (V/V) ethanol‐water medium. The formation constants for the chelates follow the Irving‐Williams series:Zn(II)Mn(II)The effect of ionic radius and electronegativity on the chelate formation are discussed. Complexes of Cu(II) and Ni(II) have been synthesised and characterised by elemental analysis, electrolytic conductance, IR spectra and magnetic susceptibility measurements. The ligand formsbis‐complexes with Cu(II) and Ni(II). The binding sites are oxy