D-block Elements | Introduction to Coordination Compounds

Coordination Compounds

The d-block elements are also known as transition elements or transition metals, which have the tendency to form coordination compounds. The d-block elements are lying between the s and p blocks elements of the periodic table. Those elements of the periodic table in which the last electron enters in d-orbital of the penultimate shell are called d-block elements. Penultimate mean inner to the outermost. In other words, the last electron goes to (n-1) d-orbital. These are four series of d-block elements as shown below:

d-block elements
Transition elements
Transition metals

These elements are either in their atomic state or in any of their common oxidation state have partially filled (n-1) d-orbitals. Thus, strictly according to the definition that we discuss above zinc, cadmium, Mercury and their divalent cations should be excluded from d-Block elements, since they do not have partially filled (n-1) d-orbitals. Similarly, copper, silver, gold, and their monovalent cations are also not considered to be transition metals or ions. In order to maintain a rational classification of elements, these elements are generally studied with d-block elements.

General Characteristics of d-block Elements

1. Metallic Character

All the transition elements are metal since the number of electrons in the outermost shell is very small being equal to 2. They are hard, malleable, and ductile. These metals are good conductors of heat and electricity.

2. Melting and Boiling Points

The transition metals have very high melting and boiling points as compared to those of s-block elements.

3. Ionization Potential

The ionization potential value of most of the d-Block elements lies in between those of s and p-block elements. This indicates that the d-block elements are less electropositive than s-block elements and more than p block elements. Thus, d-Block elements do not form ionic compounds so, readily as the alkali and alkaline earth metals do. Unlike s-block elements, d-Block elements form covalent compounds as well.

4. Variable Oxidation State

Most transition metals show several oxidation states in their compounds. These states are exhibited only in a few complexes and furthermore, these are unstable. The cause of showing different oxidation States is that in addition to using electrons in the outermost subshell nearly “ns” in compound formation, a variable number of inner (n-1) d electrons can also be used for this purpose. The energy difference between (n-1) d-orbitals and ns orbitals of transition metals is very small. As a result, the electrons may easily shift from (n -1) d to ns and from ns to (n-1) orbitals.

5. Atomic Volumes and Densities

The atomic volumes of the metals are much lower than those of alkali and alkaline earth metals. As the inner orbits get filled, the increased nuclear charge pulls the electronic cloud inward. The atomic volume, therefore, decreases. The decreases in an atomic volume increase the density. Accordingly, the densities of transition elements are very high as compared to those of alkali and alkaline earth metals.

6. Atomic and Ionic radii

The atomic and ionic radii values gradually decrease on moving from left to right along period. This is due to the fact that as the nuclear charge increases with an increased atomic number along period. The attraction of the electrons toward the nucleus increases and this increase in attraction leads to a decrease in the radii values across the period.

7. Paramagnetic behaviour

Many transition metals, atoms, and cations with partially filled d-subshell exhibit paramagnetic behavior i.e they are attracted into a magnetic field. The paramagnetism is caused by the presence of unpaired electrons. An electron has two types of motion spin motion (meaning of the electron on its axis) and orbital motion (moving of electron in an orbit). As we know that the movement of an electric charge produces a magnetic field.

Consequently, an electron spinning on its axis creates a magnetic moment which is called the spin moment of the electron. The orbital motion produces a magnetic moment just like an electric current traveling in a loop of wire. The magnetic moment produced in this way is called the orbital movement of the electron. The observed magnetic moment is the combination of these two movements. Both these two moments arise due to the presence of unpaired electrons.

8. Complex Formation

The cations of transition metals have a tendency to form complexes with several ligands. This tendency is due to the following reasons:

  1. The cations are relatively very small in size and hence have high positive charge density which makes it easy for the cations to accept the lone pairs of electrons from the ligands.
  2. The cations have vacant (n-1) d-orbitals, which accept lone pair of electrons from the ligands for bonding with them.

9. Catalytic Properties

Most of the transition metals and their compounds are used as catalysts. Some common examples are platinum Pt,  nickel Ni,  iron Fe, chromium Cr, vanadium pentoxide V2O5, etc.

10. Alloy Formation

The d-block elements form alloys with one another and with other metals, for example, brass (Cu-Zn), bronze (Cu-Zn-Sn), German Silver (Cu-Zn-Ni), etc.

Coordination Compounds: Introduction

Before we understand the concept of coordination compounds, only you have to take care of two things: what is salt and how is it formed? when an acid and base are combined or react, salts are formed.

NaOH + HCl → NaCl

So, how many kinds of salts are there? There are two kinds of salts:

  1. Simple salt
  2. Mixed salt

Simple Salts

Simple salts are those which has only one type of cation and one type of anion is present. For example:

  • Sodium chloride, NaCl is a simple salt because it has only one type of cation Na+ and one type of anion Cl.
  • Calcium hydroxide, Ca(OH)2 is a simple salt because it has only one type of cation Ca2+ and one type of anion OH.
  • KCl, NaI, MgCl, MgBr, CH3COONa etc is also an example of simple salts.

Mixed Salt

Mixed salts are those which has more than one type of cation and/or anion is present. For example:

  • K2SO4. Al2(SO4)3. 24H2O (Alum) is an example of mixed salt. But question is here, How many types of cation and anion are present in Alum? Cation: k+, Al3+ and Anion: SO42- only. Alum contain more tha one type of cation (K+, Al3+) so, it is mixed salt.
  • KCl. MgCl2. 6H2O (cernalite) also a mixed salt. Cation: k+, Mg2+ and Anion: Cl only. Cernalite contains more than one type of cation so, it is mixed salt.

As we discussed above, simple salts are formed when an acid or base reacts. But the question is here, How mixed salts are formed? when more than one simple salts are combined, mixed salt is formed. For example:

K2SO4. Al2(SO4)3. 24H2O is a mixed salt. How many simple salts are present in Alum? The answer is two. 1) K2SO4 is a simple salt because it is made up of only one type of cation (K+) and anion (SO42-). 2) Al2(SO4)3 is also a simple salt because it contains only one cation and anion Al3+ and SO42- respectively. So, Alum has two simple salts that’s why it is mixed salt.

These mixed salts are also known as addition or molecular compounds.

Addition Compounds

Two or more simple salts are mixed and then allowed to evaporate then as a result we get addition or molecular compound.

KCl + MgCl2 + 6H2O → KCl. MgCl2. 6H2O

Addition or molecular or mixed salts are also called hydrated salts due to the presence of water, H2O. These salts are separated by dots.

Examples of Addition Compounds

  • K2SO4 + Al2(SO4)3 + 24H2OK2SO4. Al2(SO4)3. 24H2O (Alum)
  • FeSO4 + (NH4)2SO4 + 6H2O → FeSO4. (NH4)2SO4. H2O (Mohr’s)
  • Fe(CN)2 + 4KCN → Fe(CN)2. 4KCN
  • CuSO4 + 4NH3 → CuSO4. 4NH3
  • AgCN + KCN → KCN. AgCN

Types of Addition compounds

Addition or molecular compounds has further divided into two categories:

  1. Double salts or lattice compound
  2. Coordination compounds or complex compounds

Double Salt

  1. Double salt is also called lattice compound.
  2. That addition or molecular compounds which completely breaks into a constitient ions when dissolved in water are called double salts.
  3. Double salts are stable in solid state because when react with water it completly dissocate into constituent ions.
  4. In double salts, metal atom or ion exhibit normal valency.
  5. They lose their identity on dissociation.

Examples of Double Salts

  • KCl + MgCl2 + 6H2O → K+ + Mg2+ + 3Cl + 6H2O
  • K2SO4 + Al2(SO4)3 + 24H2O → 2K+ + 2Al3+ + 4SO42- + 24H2O
  • FeSO4 + (NH4)2SO4 + 6H2O → Fe2+ + 2NH4+ + 2SO42- + 6H2O

Carnallite, Alum, and Mohr’s salt is mixed salt or addition compound or molecular compound or hydrated compound or double salt.

Coordination Compounds

  1. Coordination compound is also called complex compound.
  2. That addition or molecular compound which do not completely ionize into their respective ions are called coordination compounds.
  3. These are stable in solid as well as aqueous form.
  4. In complex, the metal or ion show higher valency rather than normal.
  5. They do not lose their identity on dissociation.

Examples of Coordination Compounds

  • Fe(CN)6 + 4KCN → Fe(CN)6. 4KCN → K4[Fe(CN)6] → 4K+ + [Fe(CN)6]4-

All these cations and anions are not separated when it is dissolved in water so, it is a coordination compound or complex compound.

  • CuSO4 + 4NH3 → CuSO4. 4NH3 → [Cu(NH3)]SO4 → [Cu(NH3)]2- + SO42-
  • AgCN + KCN → KCN. AgCN → K[Ag(CN)2] → K+ + [Ag(CN)2]
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