The three-fold symmetric, four-coordinate iron(II) phosphoraminimato complexes PhB(Meslm)(3)-Fe N=PRR'R" (PRR'R" = PMePh2, PMe2Ph, PMe3, and (PPr3)-Pr-n) undergo a thermally induced S = 0 to S = 2 spincrossover in fluid solution. Smaller phosphoraminimato ligands stabilize the low-spin state, and an excellent correlation is observed between the characteristic temperature of the spincrossover (T-1/2) and the Tolman cone angle (theta). Complexes with para-substituted triaryl phosphoraminimato ligands (p-XC6H4)(3)P=N- (X = H, Me and OMe) also undergo spincrossover in solution. These isosteric phosphoraminimato ligands reveal that the low-spin state is stabilized by more strongly donating ligands. This control over the spin state provides important insights for modulating the magnetic properties of four-coordinate iron(II) complexes.