Color of Transition Metal Ions in Water Solution Dr Dragica Minić August 17, 2005

Position of Transition Metals in the Periodic Table The elements in the periodic table are often divided into four categories: The transition metals are the metallic elements that serve as a bridge, or transition, between the two sides of the table.

Let me consider the first transition metal series, elements from scandium (Z=21) to copper (Z=29). This metals either have incompletely filled d subshells or readily give rise to cations that have incompletely filled d subshells. Along this series the added electrons are placed in the 3d orbitals according to Hund's rule: to . 1. irregulation: electron configuration of chromium (Z=24) is and not ; 2. irregulation: electron configuration of copper (Z=29) is and not .

The reason is that slightly greater stability is associated with the half-filled and completely filled subshells. Electrons in the same subshells have equal energy but different spatial distribution. Consequently, their shielding of one another is relatively small and the electrons are more strongly attracted by the nucleus when they have the configuration. Consequently the orbital diagram for Cr is and orbital diagram for Cu is

Transition metals have a distinct tendency to form complex ions often with neutral molecules water. Many transition metal ions and complex ions and anions containing transition metals are distinctively colored. The origin of the color is electronic transition involving d electrons. In anhidrous form is blue but in the hydrated form is pink.

Colors of some of the first-row transition metal ions in water solution:

Colors of aqueous solutions of compounds containing vanadium in different oxidations states (V, IV, III and II).

For explanation of the color transition metals ions in water solution existing in complex form, we need to consider the bonding in complex ions of transition metals; There are three theories: The valence-bond theory The crystal field theory The ligand-field theory.

Crystal Field Theory This theory tried to describe the effect of the electrical field of neighboring ions on the energies of the valence orbitals of an ion in a crystal. Question: What effect will the surrounding ligands have on the energies five d orbitals of metal’s atoms? Answer: d orbitals of ligands have various orientations and in absence external disturbance have the same energy. When such metal ion is in the center of octahedron surrounding by six lone pair of electrons of ligands, two types of electrostatic interaction exist: The attraction between positive metal ion and negatively charged ligand (this force holds the ligands to metal in complex). The repulsion between lone pairs on ligand and electron in d orbitals metals.

Electron in and orbitals lying along z, x, and y axes, experience a greater repulsion from ligands than electrons in other orbitals:

and as result the equality energy of 5 d orbitals is nullified to give two high-lying levels and three low-lying energy levels. The energy difference between two sets d orbitals, is called crystal field splitting; its magnitude depends on the metal and the nature of the ligands.

A substance appears colored because it absorbs light at one or more wave-lengths in the visible part of the electromagnetic spectrum (400 to 700 nm) and reflects or transmits the others. Each wavelength of light in this region appears as a different color. A combination of all colors appears white, absence of lightwaves appears black.

Wavelenght absorbed, nm Relationship between wavelenght absorbed and color observed Wavelenght absorbed, nm Color observed 400 (violet) 450 (blue) 490 (blue-green) 570 (yellow green) 580 (yellow) 600 (orange) 650 (red) Greenish yellow Orange-yellow Red Purple Dark blue Blue Green When we say that the hydrated cupric ion is blue, we mean that each ion absorbs a photon a wavelength of about 600 nm (orange light), the transmitted light appears blue to our eyes.

Quantum-mechanical description Absorption of light may occur when the frequency of the incoming photon, multiplied by the Plank constant, is equal to the difference in energy between these two levels.

Example: Hydrated cupric ion absorbs photon whose frequency is about Hz or 600 nm. The energy change involved in the electron transition that occurs in the cupric ion is: When we say that the hydrated cupric ion is blue, we mean that each ion absorbs a photon wavelength of about 600 nm (orange light), the transmitted light appears blue to our eyes.

Example Ti has an outer configuration of , so that will be a ion. This means that in its ground state, one electron will occupy the lower group of d orbitals, and the upper group will be empty, after absorption of energy the lower groups d orbitals will be empty.

Crystal field splitting is: ion absorbs light in the visible region; the wavelenght corresponding to maximum absorption is 498 nm. Crystal field splitting is: This is the energy required to excite one ion.                                                                                                                                                                                                                                                                                                      

The d-orbital splitting in this case is 240 kJ per mole which corresponds to light of blue-green color; absorption of this light promotes the electron to the upper set of d orbitals, which represents the exited state of the complex. If we illuminate a solution of with white light, the blue-green light is absorbed and the solution appears violet in color.

Position of Transition Metals in the Periodic Table The elements in the periodic table are often divided into four categories:                                                                                                                                                                                   The transition metals are the metallic elements that serve as a bridge, or transition, between the two sides of the table.