6. Transition metals

According to IUPAC, a transition element (or transition metal) is defined as:

This means:

• Elements like Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu are transition metals because they have partially filled d orbitals either as atoms or as ions.
• Zn, Cd, and Hg are not transition metals because they have completely filled d orbitals (d¹⁰) both as atoms and in their common +2 oxidation state. Yet they are d-block elements.

Why the word “transition” metals?

They fall between the s-block and p-block elements and show a gradual transition in properties from highly reactive metals on the left to less reactive metals on the right.

The general electronic configuration of d-block elements is:

(n−1)d^1–10 ns^1–2

The last electron in transition elements enters the (n−1)d orbital, which is in the penultimate shell.

Key concept for ion formation

When transition metals form ions, they lose the ns electrons first, then (n−1)d electrons.

Example:

Fe: [Ar] 3d⁶ 4s²

Fe²⁺: [Ar] 3d⁶  (loses 4s electrons first)

Fe³⁺: [Ar] 3d⁵  (loses one more from 3d)

Magnetic Properties

• Due to the unpaired electrons in the (n−1)d orbitals, transition metals are paramagnetic substances.
• These substances are attracted to the magnetic field.
• The paramagnetic character increases as the number of unpaired electrons increases.
• Paramagnetic substances show an increase in weight in a magnetic field, whereas diamagnetic substances show a decrease.
• The transition elements that contain only paired electrons are diamagnetic substances. These substances are repelled by the magnetic field.

μ = √n(n+2)  BM  (Bohr Magneton)

where n = number of unpaired electrons; μ = total magnetic moment of Mⁿ⁺

Metallic Character

• The transition elements form strong metallic bonds involving both ns and (n−1)d electrons.
• So, all the transition elements exhibit metallic characters, like hard, lustrous, malleable, ductile, high melting and boiling points, and good conductors of heat and electricity.
• As the number of unpaired d electrons increases, the strength of the metallic bond, hence the metallic character, also increases.

Ionization Energy

• The ionization energies of the first-row elements gradually increase due to increasing nuclear charge.
• The third ionization energy of Mn is much higher than the others, as the electronic configuration for Mn is [Ar]3d⁵4s², but after losing the 2 s-electrons, it becomes [Ar]3d⁵. All five d orbitals are half-filled in this case. Half-filled orbitals have extra stability.

Ionic Radii

• In the first-row transition elements, the ionic radii decrease slightly with increasing atomic number.
• The ionic radii also depend on the oxidation state of metals. As the oxidation states increase, the ionic radii decrease.

Fe > Fe²⁺ > Fe³⁺     Mn > Mn²⁺ > Mn⁷⁺

Alloy Formation

• Metals mix with other metals, and alloys are formed.
• As the d-block elements have similar atomic sizes, they can easily take up positions of one another in a crystal lattice. This causes alloy formation.
• E.g., Cr, V, Mn, etc., are present in alloy steels.

Variable Oxidation States

Most of the transition elements exhibit a variable oxidation state because:

• The energy difference between ns and (n−1)d orbitals is small.
• Both ns and (n−1)d electrons can participate in bonding.

Formation of Complexes

Transition metals form complex ions by accepting lone pairs from ligands due to:

• Small size of metal ions
• High ionic charge
• Presence of vacant d orbitals

Catalytic Property

The first-row transition elements exhibit catalytic properties due to:

• Variable oxidation states (can accept and donate electrons)
• Vacant d orbitals (can form intermediates)
• Surface adsorption (in heterogeneous catalysis)

• The EC of transition metals shows that these exhibit variable oxidation states, as both ns and (n−1)d electrons come into play. This is due to the small energy difference of the electrons in the ns and (n−1)d orbitals; both ns and (n−1)d electrons could be lost during the formation of compounds.
• The participation of ns electrons in bonding leads to a +2 oxidation state, which is a lower oxidation state. It is the most common oxidation state of the elements of the first transition series.
• The participation of (n−1)d electrons in bonding leads to higher oxidation states like +3, +4, +5, +6, etc.
• The oxidation state increases with atomic number. This increase is related to groups.

Oxidation states of first-row transition metals. Bold values represent the most common/stable oxidation states. Sc has only one common ion (+3); Zn has only one common ion (+2) — consistent with Zn not being a true transition metal.

Not all (n−1)d electrons participate in bonding; only the unpaired (n−1)d electrons take part.

• Ligand: An ion or molecule with at least one lone pair of electrons (e.g., H₂O, NH₃, Cl⁻, CN⁻).
• Ligands are bases and also nucleophiles.
• Coordination number (CN): The number of coordinate bonds formed between the central metal and the ligands.
• Transition metals form complex ions due to their small size and presence of vacant d-orbitals.
• E.g., K₃[Fe(CN)₆] contains simple K⁺ ion and [Fe(CN)₆]³⁻ as complex ion.
• They are electrically charged with a metal ion in the center, surrounded and linked by some neutral molecules or negative ions.
• All the first-row transition elements form complexes.

Some examples of complex ions:

Complex Formula	Central Ion	Ligands	Coordination Number
[Fe(H₂O)₆]³⁺	Fe³⁺	H₂O	6
[Cu(NH₃)₄]²⁺	Cu²⁺	NH₃	4
[Ni(CN)₄]²⁻	Ni²⁺	CN⁻	4
[Co(NH₃)₅Cl]²⁺	Co³⁺	NH₃, Cl⁻	6

IUPAC Naming of Coordination Complexes

1. Write the name of the cation followed by the anion, with a space in between (just like simple salts).
2. Write the name of the ligand first, kept in alphabetical order, followed by the metal.
3. A neutral coordination compound is given in one word.
4. The oxidation state of the metal is written in roman numerals in parentheses.
5. If the complex is an anion, the metal name ends in -ate (e.g., ferrate, cuprate).
6. For anionic ligands, use: chloro (Cl⁻), cyano (CN⁻), aqua (H₂O), ammine (NH₃).

Formula	Name
K₃[Fe(CN)₆]	Common: Potassium ferrocyanide IUPAC: Potassium hexacyanoferrate(III)
[Cu(NH₃)₄]SO₄	Common: Tetraammine cupric sulphate IUPAC: Tetraammine copper(II) sulphate
Fe(CO)₅	IUPAC: Pentacarbonyliron

In a complex compound, the metal ion is surrounded by ligands, forming a coordination sphere. The shape of a complex ion depends upon the coordination number.

Complex ions having a coordination number of 4 have either a tetrahedral or a square planar structure. Similarly, complex ions having a coordination number of 6 have an octahedral structure.

Ni(CO)₄ — coordination number = 4 — Tetrahedral shape

• Tetrahedral: ligands at four corners of a tetrahedron.
• Square planar: ligands at four corners of a square, metal in the center.
• Octahedral: ligands at six corners (up, down, front, back, left, right).

Why Different Shapes?

Depends on:

• Size and charge of the metal ion
• Type of ligand (strong vs. weak field)
• d-electron configuration (explained by Crystal Field Theory)

In an isolated metal atom/ion, all five d orbitals are degenerate, meaning they have the same energy.

The energy change of d-orbitals during the formation of a complex is explained by Crystal Field Theory (CFT), proposed by Bethe and Van Vleck.

During the formation of a complex ion, ligands approach the central metal atom/ion. The lone pair of electrons in the ligands exerts a repulsive force on the d electrons of the metal, which changes the energy of the d orbitals.

In an octahedral complex, the metal ion is at the center of an octahedron, and six ligands approach along the x, y, and z axes.

• Two of the d orbitals, d_x²−y² and d_z², being directed toward the axes, experience strong repulsion. Hence, their energy increases. These d orbitals are called the e_g set.
• d_xy, d_yz, and d_zx orbitals are directed between the axes. They experience less repulsion, meaning their relative energy is lower. These d orbitals are the t_2g set.

Thus, the octahedral field of the ligands removes the degeneracy of the d orbitals and splits them into two groups of different energy levels.

The energy difference between the e_g and t_2g sets is called Δ_o (crystal field splitting energy).

Have you ever wondered why Copper(II) sulfate is a brilliant blue, or Potassium permanganate is a vibrant purple? It all comes back to the Crystal Field Theory.

• Most transition metal ions form coloured compounds.
• When white light passes through a transition metal complex, some wavelengths are absorbed to excite an electron from t_2g to e_g orbital (d–d transition).
• The remaining light is transmitted or reflected, and we see the complementary colour.
• For example, copper(II) ions absorb light from the red end of the spectrum, so the complementary colour seen is a pale blue (called cyan).

Complementary pairs of colours:

Absorbed	Colour seen (complementary)
● red	● cyan
● yellow	● blue
● green	● magenta

As discussed in CFT, the degenerate d orbitals of transition metals split, and an electron is promoted from the lower (t_2g) to the higher (e_g) set upon absorbing light:

The transition elements exhibit catalytic properties mainly due to:

• Presence of vacant d orbitals
• Variable valencies (having more than one stable oxidation state)
• Ability to form complexes — vacant d orbitals that are energetically accessible and can form dative bonds with ligands

The catalytic properties of these elements are explained by:

a. Intermediate Compound Formation (Homogeneous Catalyst)

The catalyst forms an unstable intermediate with the reactant, which decomposes to give the product and regenerates the catalyst. This provides an alternative reaction path with lower activation energy.

b. Adsorption Theory (Heterogeneous Catalyst)

Transition metals adsorb reactants on their surface. This weakens the bonds in the reactant molecules, helping initiate the reaction and speed it up. E.g., Ni, Pd, Pt, etc., used in hydrogenation reactions.

Industrial Examples

Process	Catalyst	Reaction
Haber’s process	Finely divided Fe	N₂ + 3H₂ ⇌ 2NH₃
Contact process	V₂O₅	2SO₂ + O₂ ⇌ 2SO₃
Hydrogenation of oils	Ni	Unsaturated oil → saturated fat
Decomposition of KClO₃	MnO₂	2KClO₃ → 2KCl + 3O₂