Transition metals exhibit similarities within periods and groups due to their d-block electron configurations. They form coordination compounds where the metal ion is surrounded by ligands. Coordination compounds exist as complex ions and counter ions to balance charges. Isomers of coordination compounds can occur due to differences in ligand bonding, spatial arrangements of atoms, or inability to be superimposed on a mirror image. Crystal field theory describes the splitting of d-orbital energies based on ligand field strength. This influences the spin state and colors of transition metal complexes.