Transition Metals

Transition metals are d-block elements that have their d-orbitals partially filled. Zinc and Scandium are not considered transition metals, even though they're d-block elements. They do not have partially filled d-orbitals. Zn has the electronic configuration [Ar]4s²3d¹⁰. Sc has the electronic configuration [Ar]4s²3d¹.

Properties of transition metals:
1. They have high melting and boiling temperatures.
In their metallic structure, 3d as well as 4s electrons are available for delocalisation. Since there are more electrons within the sea of electrons, there is a greater electrostatic attraction - hence the higher melting temperatures.
2. They have variable oxidation states - except Scandium (Sc) and Zinc (Zn)
3. They form coloured compounds and ions - except Sc and Zn form white compounds and colorless ions
4. They show catalytic activity

Sn can only form Sn ³⁺ ion which has the same electronic configuration as [Ar].
Zn loses both of its 4s electrons to form only Zn²⁺ ions (with the electronic configuration of
[Ar]3d¹⁰).

Electronic Configurations:

In d block, electrons are added to an inner d-oribital and this shields the outer 4s electrons from the increased nuclear charge (i.e. they're easier to lose). Therefore, the atomic radius only decreases slightly and electronegativity and ionisation energies increase only slightly.

Examples of some electronic configurations:
Sc ----> [Ar]4s²3d¹
Zn ----> [Ar]4s²3d¹⁰
Cu ----> [Ar]4s¹3d¹⁰
Cr ----> [Ar]4s¹3d⁵

In Cr and Cu, the 3d orbitals fill first before the 4s orbitals. This is due to the increased stability offered by full and half-filled shells.
Moreover,
Fe²⁺ (3d⁶) is readily oxidised to Fe³⁺ (3d⁵)
Mn²⁺ (3d⁵) is not readily oxidised to Mn³⁺ (3d⁴)

When transition metals form ions, they lose electrons from the 4s sub-shell first before 3d sub-shell. Fe²⁺ has the eletronic structure [Ar]3d⁶  rather than [Ar]4s²3d⁴.
This occurs because of the repulsion forces on the 4s electrons, repelling them further away from the nucleus, by the 3d electrons. Therefore, 4s electrons are pushed to a higher energy level, higher than 3d, thus electrons in 4s sub-shell are lost before 3d sub-shell.

What are complex ions and ligands?

Complex ion has a metal ion at its centre with a number of other molecules or ions surrounding it that are attached to the central ion by dative covalent (coordinate) bonds.

Ligands are the molecules or ions surrounding the central ion, for example: water, ammonia and chloride ions. These ligands have active lone pairs of electrons in their outer energy level which are used in the dative covalent bond with the metal ions.

All ligands are lone pair donors. Therefore, all ligands function as lewis bases.

Dative covalent bonds are:
covalent bonds in which both electrons come from the same atom, unlike in a simple covalent bond where each atom supplies one electron. Remember: in covalent bonds two atoms share a pair of electrons. Both atoms are held together due to the forces of attraction between the positive nuclei and the shared pair of electrons.

Transition Metals - Formation of complex ions

Today, we will discuss the fomation of complex ions, specifically aqua ions.

Aqua ions occur when d-block cations dissolve in water and become hydrated (i.e. surrounded by water molecules). The oxygen atom in the water molecule has a lone pair of electrons that forms a bond with an empty 3d or 4p orbital in the metal ion.

The theory behind the exact nature of the bonding is beyond the scope of A-level. All you need to know is that a dative covalent bond forms with the oxygen atom as the donor atom.
There are other theories that tend to explain the bonding such as: the electrostatic forces of attraction between the delta negative oxygen and the positive metal ion.

An example of an aqua ion is the hydrated chromium (III) ion, [Cr(H₂O)₆]³⁺
The water molecules are called ligands

One of the lone pairs of electrons on the oxygen atom of each water molecule forms a dative covalent bond with an empty orbital in the Cr³⁺ ion. Six dative bonds form so the hydrated ion has the coordination number 6.

The ion, with its water molecules bonded to the central metal ion, is called a complex ion.

The coordination number is the number of near neighbouring atoms that are bonded to the central ion.

Ligands are organic molecules that donate the necessary electrons to form dative (coordinate) covalent bonds with metallic ions. (e.g. H₂O, NH₃)

Note: When drawing the hydrated ions make sure that the dative covalent bonds all start from the O of the H₂O and not from the H.

There are six dative bonds, each containing a pair of electrons. These six pairs of bonding electrons repel each other to the position of maximum separation and minimum repulsion. The shape is therefore a octahedral.

All complex ions with coordination number 6 are octahedral.

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Copper:

In the solid state, hydrated copper (II) ions have four water molecules arranged in a plane around Cu²⁺ion. However, in aqueous solution, two more water molecules are weakly bonded at right angles, forming an octahedron. The two non planar water molecules are further from the copper ion than are the four planar water molecules. Because of this, the formula of the hydrated ion in solution is sometimes written as [Cr(H₂O)₄]²⁺ instead of [Cu(H₂O)₆]²⁺