The effect of pressure to 175 kbar has been measured on the spin state of Fe(II) in bis- and trisphenanthroline complexes, using Mössbauer resonance and optical absorption. The bis complexes can be classified according to the degree of back bonding to the nonphenanthroline ligands. Compounds with halide ligands are high spin at low pressure. They convert to low spin with increasing pressure, but the rate of conversion with pressure decreases above 80 kbar. Compounds with intermediate back bonding (e.g., cyanates, thiocyanates, selenocyanates, or azides) are high spin at low pressure. With increasing pressure they tend to convert to low spin, but above ∼ 30–40 kbar there is a net low-to-high spin conversion. The biscyanide is low spin at low pressure. At high pressure significant conversion to high spin is observed. All tris complexes are low spin at low pressure and exhibit measurable conversion to high spin at high pressure. The optical absorption data indicate large red shifts for the ligand π–π* transitions and smaller red shifts for the metal-to-ligand charge transfer peaks. These results are explained in terms of reduced back donation due to thermal occupation of the ligand n* orbitals by ligand π electrons. The behavior is compared to that of the ferrocyanides. Similar experimental results are presented and briefly discussed for an intermediate-spin phenanthroline complex and for several bipyridyl compounds.