Inorganic Chemistry/Transition metals

Inorganic Chemistry | Transition Metals | Catalysis

---

In the extended form of periodic table, the elements have been grouped into four blocks, the s, p, d, and f-blocks. The elements in groups 3-12 are called d-block or transition elements. The properties of these elements are unlike those of s-block and p-block elements. D-block elements represent a change or transition between the s- and p-block elements, hence why they are called transition elements. Previously transition elements were regarded as those which possessed partially filled penultimate d-subshell in their ground state or in one of their commonly occurring oxidation states. However the above definition doesn't cover the elements of group 12 i.e. Zinc metals(Zn,Cd and Hg) as these elements do not have partially filled (n-1)d-subshells in their ground state and neither too in their ionic state. However Zinc metals showing similarity with other transition metals were included in the transition metals. So forty elements belong to the transition metals family, 10 each in 3rd, 4th, 5th and 6th period. The transition elements include the most common elements used. These include iron, aluminium, titanium , which form the base of the construction and manufacturing industry, metals that are valued for their beauty i.e. gold, silver, and platinum and metals such as palladium, ruthenium and osmium which have catalytic properties. These contain the densest metals like osmium(d=22.49 gm cm-3), the metals with the highest and lowest melting points, tungsten(mpt. 3683K) and mercury(mpt. 234K). Certain transition metals are also important in living organisms. Iron, Fe, is present in the protein hemoglobin which is responsible for the transport of the oxygen in the living organisms. Cobalt, Co, present in vitamin B12 help in the metabolism of carbohydrates and fats in the human body. Copper and Zinc are important in biological catalysis. Cobalt, nickel, manganese, and molybednum are essential components of certain enzymes.

Transition metals all share some general characteristics, they:

• form complexes
  
• form ions which result in a coloured solution
  
• have variable oxidation states
  
• show catalytic activity

Atomic Electronic structure

Scandium	1s2 2s2 2p6 3s2 3p6 3d1 4s2
Titanium	1s2 2s2 2p6 3s2 3p6 3d2 4s2
Vanadium	1s2 2s2 2p6 3s2 3p6 3d3 4s2
Chromium	1s2 2s2 2p6 3s2 3p6 3d5 4s1
Manganese	1s2 2s2 2p6 3s2 3p6 3d5 4s2
Iron	1s2 2s2 2p6 3s2 3p6 3d6 4s2
Cobalt	1s2 2s2 2p6 3s2 3p6 3d7 4s2
Nickel	1s2 2s2 2p6 3s2 3p6 3d8 4s2
Copper	1s2 2s2 2p6 3s2 3p6 3d10 4s1
Zinc	1s2 2s2 2p6 3s2 3p6 3d10 4s2

Complexes

When ligands form co-ordinate bonds with transition metals ions a complex is formed. Common ligands are H₂O and NH₃. The transition metal ion acts as a Lewis Acid and the ligand acts as a Lewis base. The number of co-ordinate bonds in the complex is the co-ordination number of that complex. Example of complexes are the tetrachlorocobaltate(II) ion, [CoCl₄]^2- and the hexaqaua-iron(III) ion, [Fe(H₂O)₆]³⁺

Ligands

Ligands can be divided into three categories: unidentate, bidentate and multidentate. Unidentate ligands can form one co-ordinate bond with a central metal ion. Bidentate ligands can form two co-ordinate bonds with a central metal ion and so on. Another type of ligand is an ambidentate ligand. An ambidentate ligand contains two sites which can form a co-ordinate bond with a central metal ion, though they cannot both form a bond with the same metal ion at the same time. They can however simultaneously bond to separate ions creating a bridge between two complexes.

Ligand Type		Examples
Unidentate		CN-, Cl-, NH3
Bidentate		H2NCH2CH2NH2, C2O42-
Multidentate		EDTA4-
Ambidentate		SCN-

Table of Common Transition Metal Complexes

Ion	Colour
[Co(NH3)6] 3+	Dark Brown
[Co(NH3)6] 2+	Pale Yellow
[CoCl4] 2-	Blue
[Co(H2O)6] 2+	Pink
[Cr(NH3)6] 3+	Purple
[Cr(H2O)6] 3+	Blue / Green
[Cr(H2O)6] 2+	Blue
[Cr(H2O)2(OH)4] -	Green
[Cr2O7] 2-	Orange
[CrO4] 2-	Yellow
[Cu(NH3)4(H2O)2] 2+	Deep Blue
[CuCl4] 2-	Yellow Green
[Cu(H2O)6] 2+	Blue
[Fe(H2O)6] 3+	Yellow Orange
[Fe(H2O)6] 2+	Pale Green
[VO3] -	Colourless
[VO2] +	Orange
[VO(H2O)5] 2+	Blue
[V(H2O)6] 3+	Green
[V(H2O)6] 2+	Violet

Uses of Complexes & Ligands

EDTA

Antidote
EDTA reacts with metal ions to produce stable EDTA complexes. It can therefore be used to remove poisonous metal ions such as lead and mercury from the bloodstream. The complexes are then excreted.

Surgery
EDTA can remove calcium ions from bloodstream. The removal of these ions prevents the blood from clotting which is necessary during surgical procedures.

Water softener
EDTA can remove calcium ions from hard water, thereby softening it. This helps to prevent the formation of limescale.

Cisplatin

Cisplatin [Pt(NH₃)₂Cl₂] is a cytotoxic complex used during cancer treatment. It is usually used to treat ovarian, testicular, lung, bladder, gullet and stomach cancers.

Silver

Tollen's reagent
When silver nitrate is dissolved in aqueous ammonia [Ag(NH₃)₂]⁺ is formed. More commonly known as Tollen's reagent this complex is used in the test to distinguish ketones from aldehydes.

Electroplating
When a silver salt is dissolved in aqueous Potassium (or sodium) cyanide the [Ag(CN)₂] ^- is formed. The complex is used as the electrolyte in the silver-plating process.