Density functional theory has been used to study the electronic spin-state properties of low-spin Fe[HB(PZ)(3)](2), lhigh-spin Fe[HB(3-MePZ)(3)](2), high-spin Fe[HB(3,5-Me(2)PZ)(3)](2), and high-spin Fe[HB(3,4,5-Me(3)PZ)(3)](2) complexes that exhibit very different iron(II) electronic spin-sate crossover behaviors with changing temperature and pressure. Excellent agreement is obtained between the experimentally,observed Mossbauer-effect quadrupole splittings and isomer shifts of these complexes and those calculated with the B3LYP functional and various different basis sets for both the high-spin and low-spin states of iron(II). The calculations for Fe[HB(PZ)(3)](2) that use the LANL2DZ, 6-31++G(d,p), and 6-311++G(d,p) basis sets for iron all lead to very similar electric field gradients and thus quadrupole splittings. The initial calculations, which were based upon the known X-ray structures, were followed by structural optimization, an optimization that led to small increases in the Fe-N bond distances. Optimization led to at most trivial changes in the intraligand bond distances and angles. The importance of the 3-methy I-H center dot center dot center dot H-3-methyl nonbonded intramolecular interligand interactions in controlling the minimum Fe-N bond distances and determining the iron(II) spin state both in Fe[HB(3-Mepz)(3)](2) and in the related methyl-substituted complexes has been identified.