Transition Metal | NEB Class 12 Chemistry | NEPALI EDUCATE

Unit: 6

Transition metal

# Introduction

The elements of which the differentiating electron enters d-orbital in their electronic configuration are called d-block elements. They are also called transition element as their properties are in between s-block and p-bloc elements. The elements of group 3 to 12 (IIIB, IVB, VB, VIB, VIIB, VIII, IB and IIB) are transition elements.

# Characteristics of Transition Elements:

1. Periodic position:

They occupy the position in between highly electropositive s-block elements and electronegative p-block elements. These elements are grouped into IIIB, IVB, VB, VIB, VIIB, VIII, IB and IIB

2. Electronic configuration:

The general electronic configuration of d-block elements is ns1-2 (n-1) d1-10.

3. Variable valency/ Variable oxidation number:

They show variable valency and oxidation number as electrons of both valence shell as well as penultimate shell takes part in bonding.

4. Metallic character:

All the transition elements are metals. They are hard, malleable, ductile, lustrous and are good conductors of heat and electricity.

5. Density, melting and boiling point:

They have high density usually greater than 5g/cm3. They have high melting and boiling point

6. Magnetic property:

Compounds of most of transition metals are paramagnetic in nature since metals contains unpaired in the d-orbitals of their ions. Paramagnetic substances are are attracted by the magnetic field.

7. Formation of complex compounds and ions:

Transition metals have capacity of forming the complex ions or complex compounds due to small sized of atoms and vacant d-orbitals. The ions or compounds in which ligands are bonded with central transition metal/metal ion are called complex ions or complex compounds.

For example

Hexacyanoferrate (III) ion [Fe (CN)6]3-

Tetraminecoper(II) ion [Cu(NH3)4]2+

8. Colored complex ions/compounds formation:

Most of the compounds of transition metals or metal ions are coloured. This is because of d-d transition of electrons in their compounds.

9. Catalytic property:

Most of the transition metals and their compounds show catalytic property.

10. Ionization energy and electronegativity:

The ionization energy and electronegativity of transition elements are in between that of s-block and p-block elements. This is due to the fact that the differentiating electron enters in inner d-orbitals that can’t not provide high shielding effect.


# Oxidation state of transition metals

This is the most significant distinguishing characteristics of transition metals that keeps them different from other s-block and p-block elements. Transition elements exhibit various oxidation states in their combine state. This is because the s-orbital of valence shell (ns) and d-orbitals of penultimate shell (n-1)d are quite closer to each other in term of energy and hence electrons of ns orbital as well as (n-1)d orbitals involve in bonding.

For example,

Here:

₂₁Sc ₁₈[Ar]4s²3d¹

₂₁S²⁺ = ₁₈[Ar]4s²3d¹               Oxidation state: +2    

₂₁S³⁺ ₁₈[Ar]4s⁰3d⁰                   Oxidation state: +3

Similarly, Titanium (Ti) has Ti⁺², Ti⁺³ and Ti⁺⁴ oxidation state as it loses two electrons from 4s with one electron from 3d orbital respectively as shown below.

₂₂Ti            =          ₁₈[Ar]4s²3d²

₂₂ Ti²⁺         =          ₁₈[Ar]4s⁰3d²     Oxidation state= +2

₂₂ Ti³⁺         =          ₁₈[Ar]4s⁰3d¹     Oxidation state= +3

₂₂Ti⁴⁺         =          ₁₈[Ar]4s⁰3d⁰     Oxidation state= +4

#      Some generalization

•Lowest oxidation state is equal to total number of electron of ns orbital.

• Highest oxidation state is equal to the sum of number of electrons of ns and (n-1)d orbitals.

• Generally, lower oxidation states form ionic compound and higher oxidation state forms covalent compound This is due to high polarity of the higher oxidation state (for eg, CrO is an ionic compound where as CrO7 is a covalent compound)

 

 

 

#   Complex ions and Metal Complexes (Coordinate complex)

The ions or the complex compounds which are formed by bonding of a transition metal/metal ion with suitable number of ligands, electron pair donor species, are called complex ion or metal complex or coordination complex. These are addition compounds that are formed by combining two or more molecules or ion and retain their property in both solid as well as solution. In such complex, constituent ions lose their identity and do not dissociate in to all constituent ions when dissolve in water. 

For examples, potassium ferrocyanide, K₄[Fe (CN)₆] is complex salt which does not give K+, Fe2+ and CN- ions rather it gives K+ and [Fe (CN)₆}^4- ions in the solution.

For example:                          

Hexamine Cobalt (III) chloride                                    [Co (NH₃)₆] Cl₃

Diamine Silver(I) chloride [Ag (NH₃)₂] Cl

Potassium ferrocyanide K₄[Fe (CN)₆]

Potassium argentocyanide      K [Ag (CN)₂]

            

#      Ligands:

The negative ions or the neutral molecules that have lone pair of electrons which can be shared with the central metal cation are called ligands.

For example,

NH₃, H₂O, NO, CO, CN-, Cl- etc. are ligands. 

The tendency of d-block elements for formation of complex is mainly due to the following two reasons.

1.      Positive ions (cations) are relatively small in size and have very high positive charge density which makes easy to accept pair of electrons from ligands.

2.      The cations have a mostly vacant (n-1)d orbital which are of approximate energy to accept lone pair of electrons from the ligands.

In contrast to d-block elements s and p block elements cannot form complex since they have no vacant d-orbital as well as their cations do not have tendency to accept lone pair of electrons from ligands due to their large size.

 

#      Double Salt (Lattice compound):

The molecule/ addition compounds which are formed by mixing two or more simple salts in stoichiometric proportion that retain their property in solid state but get dissociated completely in water giving all individual constituent ions, are called double salts. Double salts are stable in solid state only and they dissociate completely in to constituent ions.

For example,

Mohr’s salt, KCl.MgCl₂.6H₂O gets dissociated in water giving K⁺(aq.), Mg⁺²(aq.) and Cl^-(aq.)

Carnallite                        KCl.MgCl₂.6H₂O

Mohr’s salt                     FeSO₄.(NH₄)₂SO₄.6H₂O

Potash alum (fit Kiri)     K₂SO₄. Al₂(SO₄)₃.24 H₂O

 

#      Difference between Coordinate complex and Double salt

Double salt	Complex salt/ion
It is addition salt which is formed by mixing two or more salts in stoichiometric proportion	It is the well-organized addition salt which is formed by combining two or more simple molecules or ions
Constituent ions do not lose them identity	Constituent ions lose their identity.
It retains its identity in solid state only but lose identity in solution	It retains its identity in solid state as well as in solution.

 

#      Shape of complex ions (Octahedral complex): 

(Crystal field theory and d-orbital in complex ion)

 

Hans Bethe and John Hasbrouck, van Vleck (1930) proposed crystal field theory. CFT explains the interaction of transition metal with ligands and splitting of the d-orbitals. According to this theory, when ligands approach to the central metal cation, (n-1)d orbital gets spitted in to two different sets of energy due to electronic repulsion and ligands occupy positions around positively charged central cation.

The postulates of crystal field theory are summarized below:

1.      The CFT assumes that bonding between the metal cation and ligand is of electrostatic in nature. Thus the bonding between central metal and the surrounding ligands is an ionic bond.

2.      In absence of ligands, the d-orbitals of the metal ions are degenerated (having same energy). In presence of ligands, the repulsive interaction of the electrons of the central metal cation and the electrons of ligands causes the splitting of degenerated d-orbitals of the metal ion.

In octahedral complexes, d-orbitals are splitted in t_2s and e_g sets. The d_xy, d_yz, d_xz orbitals form lower energy set t_2g while d_x2-y2 and d_z2 orbitals form higher energy set eg. This splitting can be shown as figure below. 

              

Fig: Splitting of d-orbital in octahedral complex

3.      The crystal field splitting and the energy gap between the energy sets depend upon nature of metal ion, oxidation state of metal, coordination number of metal and the strength of the ligands. The strong field ligands like CO, CN, NH₃ etc. cause large crystal field splitting of orbitals (High ∆_o) and the weak field ligands like H₂O, OH^-, X^- etc. cause small crystal field splitting (low ∆_o).

 

 

The two isomers for the [Co (NH₃)₄Cl₂] ⁺ complex ion as shown in figure below.

#      Color of Transition Metal Compounds (Explained by CFT)

The color of the transition metal compounds is mainly due to incompletely filled d-orbitals and d-d electronic transition within the same sub shell. According to CFT, in the isolated metal ion, the five d orbitals are degenerated (having equal energy) but when the ion is surrounded by anions or ligands in the solution or compound, this degeneracy is lost and d-orbitals get splitted in to t_2g and e_g sets. This is called crystal field splitting. The energy difference between t_2g and e_g level orbitals lie in the visible region. The absorption of visible radiation of definite wave length by compound from surrounding causes the d-d electronic transition from lower energy d-orbitals (t_2g) to higher energy d-orbitals (e_g) and transmits the complementary color to human eye in the visible region. This is the reason why the solution or compound of transition metal ions are usually colorful.

For example,

suppose the energy gap in the splitted up d-orbitals (t_2g and e_g) of the metal ion in the compound corresponds to the energy of yellow light, the yellow light would be absorbed that promotes d-d transition of electrons from t_2g to e_g   and transmits the complementary colour dark blue colour. The ions having d0 and d10 configuration do not show orbital splitting and hence they are colorless. Since s and p orbitals are symmetrical in geometry and their splitting does not occur, hence their compounds are colorless.

 

Example:

[Cu (H₂O)₆]²⁺ ion absorbs red colour and transmits blue color as blue is complementary colour of red. So, it appears blue.

Complementary colour
Absorbed	Transmitted
IR	White
Red/ Orange	Blue
Yellow	Indigo
Green	Violet
Violet	Green

 

Example:

d-d transition in [Fe (H₂O)₆] ⁺²

 

 

                       Fig: splitting of d-orbitals in [Fe(H₂O)₆]⁺²

Qn. Compound of Ti3+are coloured (purple) but compounds of Ti+4 are colourless. Why?

Ans: Ti3+ has one electron in d-orbital.  When its compound absorbs radiation of visible region of wavelength, the electron jumps from lower d- orbitals (t_2g) level to upper d-orbitals (e_g) within the same subshell (i.e., d-d transition) and transmits light of complementary colour in visible region. This is the reason that Compounds of Ti³⁺are coloured (purple) but Ti⁴⁺ has empty d-orbital (d⁰) and so, there is no d-d transition. Hence compound of Ti⁴⁺ are colorless.

₂₂Ti³ = ₁₈ [Ar]4s⁰3d¹                                                          ₂₂ Ti⁴⁺ = ₁₈[Ar]4s⁰3d⁰

It is noted that the transition metals ions containing full filled d- orbitals (d¹⁰) such as Zn⁺², Cd⁺², Hg⁺² etc. are white. Similarly, Sc⁺³, Ti⁴⁺, V⁵⁺ etc. are also white due to empty d- orbitals.

In such case, no d-d transition occurs.

#      Catalytic property of transition metals:

Many transition metals, their alloys and their compounds such as (metals: Fe, Pt, Pd and Ni), (alloys and amalgam: Fe/Mo, Zn-Hg), (Compounds: V₂O₅, MnO₂) behaves as a catalyst in the several chemical reactions.

Transition metals show catalytic property due to following reasons,

1.      Some transition metals and their compounds provide a large surface area on which the gaseous reactants be adsorbed and come closer for the reaction. This is called occlusion property. For eg, hydrogenation of alkene and alkyne

2.      These transition metals have vacant d-orbitals and show variable oxidation states. These elements can form an unstable intermediate compound with suitable reactants providing alternative paths with lower activation energy and therefore increase the rate of reaction. The intermediate compound readily decomposes to form the final product and catalyst gains original oxidation state.

       Reactants + Catalyst à [Intermediate compound (unstable)]  à Products + catalyst

 

#      Shape of the Octahedral complex:

The coordination number of the central metal ion in the coordination compounds determines the spatial arrangement of ligands around the central metal ion.In the octahedral complex or ion, central metal cation is bonded with six ligands, four ligand are in one plane while fifth and sixth ligands are above and below the plane.

Some examples of octahedral complexes are shown in figure.