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olor of Transition

Metal Ions in Water

Solution

Dr Dragica Mini August 17, 2005

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P o s i t i o n o f Tr a n s i t i o n M e t a l s i n t h e P er i o d i c Ta b l e

The elements in the periodic table are often divided into four categories:

The transition metals are the metallic elements that serve as abridge, or transition, between the two sides of the table.

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Let me consider the first transition metal series, elements fromscandium (Z=21) to copper (Z=29). This metals either haveincompletely 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 .

1 5[Ar] 4s 3d

2 4[Ar] 4s 3d

1 10[Ar] 4s 3d

2 9[Ar] 4s 3d

2 1[Ar]4s 3d1 9[Ar]4s 3d

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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 isrelatively small and the electrons are more strongly attracted by thenucleus when they have the configuration.

Consequently the orbital diagram for Cr is

and orbital diagram for Cu is

53d

103d

53d

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Transition metals have a distinct tendency to form complex ionsoften with neutral molecules water. Many transition metal ionsand complex ions and anions containing transition metals aredistinctively colored. The origin of the color is electronictransition involving d electrons.

In anhidrous form is blue

but in the hydrated form is pink .

2CoCl

2 2CoCl 6H O

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

3+ 3+ 2+ 2+ 2+ 2+ 2+Ti , Cr , Mn , Fe , Co , Ni , Cu .

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

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For explanation of the color transition metals ions in watersolution existing in complex form, we need to consider thebonding in complex ions of transition metals;

There are three theories:

The valence-bond theory

The crystal field theory

The ligand-field theory .

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Crystal Field Theory

This theory tried to describe the effect of the electrical field ofneighboring ions on the energies of the valence orbitals of an ion in acrystal.

Question:What effect will the surrounding ligands have on the energies five dorbitals of metals atoms?

Answer:d orbitals of ligands have various orientations and in absenceexternal disturbance have the same energy.

When such metal ion is in the center of octahedron surrounding bysix lone pair of electrons of ligands, two types of electrostaticinteraction exist:

The attraction between positive metal ion and negatively chargedligand (this force holds the ligands to metal in complex).

The repulsion between lone pairs on ligand and electron in d

orbitals metals.

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Electron in and orbitals lying along z, x, and y axes,

experience a greater repulsion from ligands than electrons in otherorbitals:

x2 23d

y2x3d

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and as result the equality energy of 5 d orbitals is nullified to givetwo high-lying levels and three low-lying energy levels. The energy

difference between two sets d orbitals , is called crystal fieldsplitting ; its magnitude depends on the metal and the nature of theligands .

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A substance appears colored because it absorbs light at one or morewave-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. Acombination of all colors appears white, absence of lightwavesappears black.

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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 thateach ion absorbs a photon a wavelength of about 600 nm(orange light), the transmitted light appears blue to our eyes.

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Quantum-mechanicaldescription

Absorption of light may occurwhen the frequency of theincoming photon, multiplied bythe Plank constant, is equal tothe difference in energy betweenthese two levels.

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Example:

Hydrated cupric ion absorbs photon whose frequency is aboutHz or 600 nm.

The energy change involved in the electron transition thatoccurs in the cupric ion is:

When we say that the hydrated cupric ion is blue , we mean thateach ion absorbs a photon wavelength of about 600 nm(orange light), the transmitted light appears blue to our eyes.

34 14 -1 -19(6.63 10 J s)(5 10 s ) 3 10 J E h

2+2 6Cu[H O]

145 10

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Example

Ti has an outer configuration of , so that will be aion . This means that in its ground state, one electron will

occupy the lower group of d orbitals , and the upper group will beempty, after absorption of energy the lower groups d orbitals will beempty.

3+2 6Ti[H O]

2 24s 3d3+Ti

1d

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ion absorbs light in the visible region; the wavelenghtcorresponding to maximum absorption is 498 nm.

Crystal field splitting is:

This is the energy required to excite one ion.

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(6.63 10 Js)(3 10 m/s)3.99 10 J=240 kJ/mol

498 10 mhc

h

3+2 6Ti[H O]

3+2 6Ti[H O]

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The d -orbital splitting in this case is 240 kJ per mole whichcorresponds to light of blue-green color; absorption of this lightpromotes the electron to the upper set of d orbitals , whichrepresents the exited state of the complex. If we illuminate asolution of with white light, the blue-green light isabsorbed and the solution appears violet in color.

3+2 6Ti[H O]

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P o s i t i o n o f Tr a n s i t i o n M e t al s i n t h e Pe r i o d i c Tab l e

The elements in the periodic table are often divided into four categories:

The transition metals are the metallic elements that serve as abridge, or transition, between the two sides of the table.