The effect of the proximal ligand on the coordination of the nitrite ligand to the heme model systems Fe(Por)(η¹-ONO) (Por = meso-tetraarylporphyrinato dianion) was investigated by FTIR and UV-vis spectra in solvent free, low temperature, porous layered solids and by density functional computations. The reaction of the five-coordinate complex Fe(Por)(η¹-ONO) with the ether tetrahydrofuran gives a mixture of the O-nitrito and N-nitrito isomers Fe(Por)(THF)(η¹-ONO) and Fe(Por)(THF)(NO₂), respectively. This observation is in contrast to earlier studies with nitrogen donor Lewis bases where the N-nitrito isomers were clearly the more stable of the six-coordinated complexes. The adduct formation is reversible; the five-coordinate O-nitrito complexes Fe(Por)(η¹-ONO) were largely restored upon warming under vacuum pumping.