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Study Notes: About Transition Metal Complexes

The range/type of transition metal complexes and their chemical properties is diverse and very interesting.

To summarise, a transition metal complex is made up of a centrally positioned transition metal atom or ion surrounded by, and bonded to, a group of other ions or neutral molecules (these are known as ‘ligands’). Examples of such ligands include NH3 and the organic compound pyridine. A 6 ligand octahedral arrangement is common (see below) but others such as the 4 ligand tetrahedral form also exist.

An octahedral structure of a transition metal complex showing the 6 metal-ligand bonds in 3 dimensions.

The complex may have an overall positive or negative charge (with an appropriate number of counter ions to balance the charge) or have no charge at all. The examples below show the complex enclosed by square brackets:

Neutral complex
[PtIV(NH3)2Cl4] and [Cr(CO)6]
Cationic complex [IrIII(pyridine)4Cl2]+Cl- and [CuII(NH3)4]2+SO42-
Anionic complex K2+[PtIVCl6]2- and Na3+[FeIII(CN)6]3-

Many compounds of transition metal complexes are water soluble and once in solution water molecules can compete with other ligands for places around the metal/ion.

Transition metals can exist in aqueous solution as the hexa-aqua (or hexa aquo) complex which is the common form for +2 and +3 cations.

[Cr(H2O)6]3+ and [Ni(H2O)6]2+

The majority of transition metal complexes, as a solid or in solution, are coloured ie they absorb wavelengths in the visible region. The chromium complex above, for example, is blue while the nickel complex is green. It should also be noted that transition metal complexes can also absorb in the ultraviolet and the near infrared regions depending upon the particular compound.

The UV-Vis spectra of transition metal complexes are similar in appearance to those of organic compounds in that they generally comprise a few broad absorption bands with little fine structure. Typically, the visible (and near infrared) region contain rather weak absorption bands (epsilonmax 1-102) whereas stronger absorption occurs in the near UV (epsilonmax 103-104).

Spectra of a cobalt transition metal complex with a strong absorption band in the UV and weaker bands in the visible region

The spectra can be used to provide a great deal of information about the structure and make up of a complex.

 

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