Two N‐salicylidene‐N′‐acylhydrazines tridentate ligands containing heterocycles were prepared and utilized for the synthesis of mononuclear acentric complexes of CuII and PdII and pyridine serves as an additional ligand (MLxPy, M = metal, Lx = ligand). Characterization also includes single‐crystal X‐ray diffraction analysis. These complexesshow high second‐order nonlinear optical activity (μβ = 620 × 10–48esu for both CuLIIPy and PdLIIPy and μβ = 1600 × 10–48 and 1400 × 10–48 esu for CuLIPy and PdLIPy, respectively, at incident wavelength of 1907 nm). The properties of polymers (PMLx) obtained by grafting poly(4‐vinylpyridine) with fragments of the complexes are also reported. The polymers exhibit good thermal stability, high glass‐transition temperatures and the absence of crystallinity. The second‐order nonlinear optical (NLO) properties of thin, transparent poled films, prepared by spin …