Spin-crossover molecules are systems of great interest due to their behavior as molecular level switches, which makes them promising candidates for nanoscale memory devices, among other applications. In this paper, we report a computational study for the calculation of the transition temperature (T(1/2)), a key physical quantity in the characterization of spin-crossover systems, for the family of tetracoordinated Fe(II) transition-metal complexes of generic formula [PhB(MesIm)3FeNPR1R2R3]. Our calculations correctly reproduce the experimentally reported decrease in the T(1/2) with an increasing size of the phosphine and allow for the prediction of the T(1/2) in new members of the family that are not reported so far. More importantly, further insight into the factors that control the fine-tuning of the T(1/2) can be obtained by direct analysis of the underlying electronic structure in terms of the relevant molecular orbitals.