Transition Metals

Definition of transition metal

d-block element that forms one or more stable ions with a partially filled d subshell (zn and sc are NOT transition metals)

atomic radii / 1st IE

Relatively constant

• nuclear charge increases

• electrons added to inner PQS, shielding effect increases

• NA remains constant

Melting point

Higher than s block

• strength of metallic bond is proportional to number of delocalised electrons

• transition metals contribute 3d and 4s electrons to delocalised electrons due to small energy difference

Density

denser than s block elements

• smaller atomic size

• more atoms per unit volume

Why can it exhibit variable oxidation states?

• close similarity in energy of 3d and 4s orbitals

• hence both 3d and 4s electrons can be removed to form stable ions

Coordination number + shape

2 — linear

4 — tetrahedral / square planar

6 — octahedral

How to find ligand complex

H2O ligand → always 6

Non-transition metal cation → 4

+1 cation → 2

Cu²⁺ cation → 4

other TM cation → 6

Acidity of aqua complexes

Some metals with high charge densities (eg Fe³⁺, Cr³⁺) have high polarising power, polarises electron cloud of H₂O and weakens O—H covalent bond, hence water undergoes hydrolysis to give acidic solution

Why are there colours of TMs

• in the presence of ligands, electronic repulsion between lone pair of electrons of ligands and d orbitals causes degenerate d orbitals of TM to split into 2 different energy levels with a small energy gap ∆E

• as 3d subshell is partially filled, electrons in lower energy d orbitals can absorb light corresponding to energy gap ∆E and be promoted to a higher energy d orbital

• colour observed is complement of colour absorbed

Factors affecting colour

1. number of d electrons present

   1. different degrees of repulsion with lone pairs in ligands, splitting 3d orbitals with different energy gaps

   2. different wavelength is absorbed, different colour observed

2. nature of ligands

   1. different ligands cause different degrees of repulsion with electrons in 3d orbitals, splitting the 3d orbitals with different energy gaps ∆E

   2. different wavelength absorbed, different colour observed

Why is Zn²⁺ and Cu⁺ colourless?

3d subshell is fully filled, no d-d transition possible

Why is Sc³⁺ colourless?

no electrons in 3d subshell, hence d splitting does not occur

Hetero VS Homogeneous catalysts

• hetero: due to presence of partially filled d subshell, TM can act as electron acceptor from reactants

• homo: due to ability to exhibit variable oxidation states, enables TM to provide reactions with alternative pathway of lower Ea