Electronic spectra of 3dn transition ions in an octahedral ligand surrounding have been studied using the modified crystal field approach (MCFA), which includes a relativistic spin–orbital interaction. A new variable parameter, the effective nuclear charge Zeff of a metal ion that allows accounting implicitly the covalence degree of a metal–ligand bond, has been introduced. Energy diagrams similar to the Tanabe–Sugano ones have been calculated. To study the spin state evolution of the metal ion in an arbitrary distorted octahedral complex, a spin state diagram approach has been proposed. The intermediate-spin (IS) state problem for 3d4, 3d5, and 3d6 metal ions has been considered and conditions for the IS state realization have been formulated. The regions of the mixed high-, intermediate-, and low-spin states have been found. The possibility of coexistence of the different spin states of 3d ions in the octahedral complexes has been considered using crystallography data for the YBaCo2O5.5 layered cobaltite.