The spin crossover effect in iron(II) materials containing the di-2-pyridylamine functional group has been investigated for the new nitrile-functionalised ligand DTAC (2,2 ,2 ,2 -((6-(di(pyridin-2-yl)amino)-1,3,5-triazine-2,4 diyl)bis (azanetriyl))tetra acetonitrile). This ligand has successfully been incorporated into a family of materials of the general formula trans-[Fe(DTAC)(2)(anion)(2)], wherein we have systematically varied the trans- nitrogen donor anion from NCS, NCSe, N(CN)(2) (dca; dicyanamide) to NCBH3 - thus forming the four mononuclear materials trans-[Fe(DTAC)(2)(NCS)(2)]center dot 6MeCN (1), trans-[Fe(DTAC)(2)(NCSe)(2)]center dot 6MeCN (2), trans-[Fe(DTAC)(2)(N(CN)(2))(2)] (3) and trans-[Fe(DTAC)(2) (NCBH3)(2)]center dot 3MeCN (4)). We find that the materials with a weaker crystal field strength anion remain high spin over all temperatures (1 and 2) whereas the materials containing stronger crystal field strength anions undergo a thermally induced spin crossover (3 and 4). Structural analysis revealed that the packing interactions in the solid state and the degree of solvation also play a large role in the observed magnetic behaviour. Indeed, aged or rapidly precipitated samples of 2 show a spin transition above room temperature.