Polymorphism is known to show a significant effect on the crystal packing structures and physical properties of molecular complexes. On the basis of our previous works on the spin-crossover (SCO) complex [Fe­(tpa)­(NCS)2] (tpa = tris­(2-pyridylmethyl)­amine), we have prepared the analogous complex [Fe­(tpa)­(NCSe)2] using a NCSe– coligand and obtained a series of crystalline polymorphs, namely SeI–SeIV, respectively, by controlled synthetic processes. Single-crystal X-ray diffraction studies reveal that these polymorphs show different packing modes and different intermolecular interactions. Various SCO behaviors are thus observed. Moreover, an obvious coligand effect on SCO behaviors is found by a comparison of the present study and the reported complex [Fe­(tpa)­(NCS)2]. The NCSe– complexes show a critical shift of T 1/2 values to a higher temperature range and a more gradual transition process in comparison to those for the isostructural complexes using NCS–, indicating a stronger ligand-field strength and a weaker cooperativity for the NCSe– coligand.