The spin state of Fe(II) poly(pyrazolyl)borate complexes is highly dependent upon substituents on the ligand molecule. While [B(pz)(4)]Fe-2 (1, pz = 1-pyrazolyl) and [PhB(pz)(3)]Fe-2 (2) are in a low-spin state in CHCl3 at ambient temperature, [HB(pz)(3)]Fe-2 (3) is in a spin-crossover state and [HB(3,5-Me(2)pz)(3)]Fe-2 (4) is in a high-spin state. Here, we present the first rational explanation of spin-crossover caused by substituents. X-ray structures of low-spin 1 (the triclinic space group P $($) over bar$$ 1 with a = 11.943(3) Angstrom, b = 12.310(3) Angstrom, c = 9.628(2) Angstrom, alpha = 96.12(2)degrees, beta 101.22(1)degrees, gamma = 100.02(2)degrees, V = 1352.7(5) Angstrom(3), and Z = 2) and 2 (the orthorhombic space group Pca2(1) with a = 18.046(2) Angstrom, b = 8.894(3) Angstrom, c 18.309(4) Angstrom, V = 2938(1) Angstrom(3), and Z = 4) were determined and compared with the reported structures of low-spin 3 and high-spin 4. All the complexes had a trigonally distorted geometry, and the Ligands were tridentate. H-1-NMR suggested that the solution structures of the complexes were similar to the X-ray structures. The key to the issue was the size of the Fe(II) ion. The fourth substituents on the boron atom in 1 and 2 forced a narrow arrangement on the tripod of the coordinated pyrazolyl groups and favored low-spin complex formation with a small Fe(II) ion. For 4, the methyl group at the 3-position of the pyrazolyl ring brought about severe interligand contact around the metal ion and prohibited low-spin complex formation. These contacts were ascertained by means of molecular mechanics calculations. Consequently, poly(pyrazolyl)borates can control the electron; configuration of Fe(II) ion through intra- and interligand contact.