General Properties of Transition Metals

General Properties

Transition elements have an incomplete d-subshell that can form at least one stable ion with an incomplete d-subshell

Scandium and zinc do not fit the definition:

Elements of the 1st transition series are therefore titanium to copper

Typical metals but show the following additional properties:

• Variable oxidation states

• Complex ion formation

• Coloured compounds

• Catalysts

Variable Oxidation States

Transition metals can form more than one positive ion – have variable oxidation states. Roman numerals denote the oxidation state on the metal ion.

Iron (Fe) can form Fe2+ (Fe(II)) and Fe3+ (Fe(III)) ions

Complex ions

This is a molecule or ion, with a central metal atom/ion, with a number of molecules/ions (ligands) surrounding it. A variety of ligands, of different number, can form bonds

chromium(III) can form [Cr(NH3)6]3+, [Cr(OH)6]3- and [Cr(H2O)6]3+ complex ions

Coloured compounds

For example, the colour of the [Cr(OH)6]3- complex (OS of Cr is +3) is dark green. The [Cr(NH3)6]3+ complex (3) is purple.

Catalysts

The transition element can change to various oxidation states by gaining electrons or donating electrons from reagents within the reaction

Substances can also be adsorbed onto their surface and are activated in the process

Electron Configuration

Electrons start to fill the lowest energy sub-shells first (Aufbau Principle)

The 4s overlaps with the 3d subshell (similar energy) so 4s fills first

For chromium and copper, an electron is promoted from the 4s to the 3d:

Cr: [Ar] 3d5, 4s1

Cu: [Ar] 3d10, 4s1

The half-full/full d configurations are energetically more stable

Cu(ll): [Ar] 3d9 so incomplete d sub-shell – compounds are blue

How to… Write the full electronic configuration of the manganese(lll) ion.

Write out the electron configuration of the atom first:

Mn atomic number = 25 therefore 25 protons & 25 electrons

1s2, 2s2, 2p6, 3s2, 3p6, 3d5, 4s2

Remove electrons - start from the 4s subshell

Mn(lll) = 3+ ion = 25 – 3 = 22 electrons: 1s2, 2s2, 2p6, 3s2, 3p6, 3d4

Ligands

A ligand is a molecule/ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons to the bond (a Lewis base). It acts as a nucleophile

The number of dative (co-ordinate) bonds:

one dative bond – monodentate ligand

two dative bonds – bidentate ligand

more than 2 – multidentate ligand

Ligand Examples

Monodentate:

Water, ammonia, chloride, cyanide (CN–), thiocyanate (SCN–)

Bidentate:

1,2-diaminoethane (H2NCH2CH2NH2) or (‘en’), Ethanedioate ion (C2O42- )

Multidentate:

EDTA4–

Monodentate Ligand Examples

Bidentate Ligand Examples

Multidentate Ligand Examples

EDTA4- is a hexadentate ligand forming 6 dative covalent bonds

Complexes with water & ammonia

Water and ammonia molecules are neutral ligands - they have a lone pair of electrons able to form a dative covalent bond. Water and ammonia are small – 6 can fit around a central metal ion, each donating a lone pair of electrons, forming 6 dative bonds.

6 dative bonds = coordination number of 6

Overall charge is the sum of the charge on the central metal ion, and the charges on each of the ligands

cobalt(II) or chromium(II) complexes, with water or ammonia as ligands, will have an overall charge of 2+ (remember that water and ammonia are neutral molecules).

Complexes with hydroxide & chloride

Hydroxide and chloride ions are negatively charged ligands. Hydroxide ligands are small, so 6 of them can fit around a central metal ion and the complex formed will have a coordination number of 6

Chlorides are large so only 4 of them will fit around the metal ion. Complexes with 4 chloride ligands will have a coordination number of 4

Example

A complex with chromium(III) as a central metal ion and hydroxide ions as ligands, will have an overall charge of 3-...

The central metal ion has a charge of 3+

Each hydroxide ligand has a charge of 1- = 6 (overall negative charge is 6-)

The overall positive charge is 3+

The overall charge on the complex is 3–