Transition Metal Coordination Compounds

2

Transition Metals• Valence electrons in a d subshell.• Form cations not anions.• Generally solids, except for Mercury (Hg) • Form coloured compounds

• A metal cation acts as a Lewis acid. A Lewis acid is a substance that can accept a pair of electrons from another atom to form a new bond.

• The electron pairs which are received come from surrounding groups which are called ligands.

• Ligands are typically either neutral or anionic atoms or molecules. • Ligands act as Lewis bases. A Lewis base is a substance that can donate a pair of

electrons to another atom to form a new bond.• The combination of a metal cation and all its ligands is called a coordination

complex

Coordination Complexes

M L

L

::

::

L

L

L:

:L

M = metal

Accepts electronsie. Lewis acid

L = Ligand

donates electronsie. Lewis base

• Consider the Lewis diagrams of the ligands H2O, Cl-, and CO

• One lone pair per donor atom can be donated to a “naked” metal ion, for example Fe2+ in water

• Copper(II) ions in the presence of a high concentration of chloride ion forms a chloro complex:

• Nickel when finely divided reacts readily with an atmosphere of gaseous carbon monoxide to form the liquid nickel tetracarbonyl:

• Note that coordination complexes may be overall neutral, cationic or anionic! • Ionic complexes require counter ions to form salts of complex ions

HO

H: : Cl: :

..

.. C O: :+-

Fe2+ + 6 H2O [Fe(OH2)6]2+

Cu2+ + 4 Cl- [CuCl4]2-

Ni + 4 CO [Ni(CO)4]

Examples of coordination complexes

HemoglobinThe Heme (the porphyrin in hemoglogin) molecule has chains branching off the porphyrin ring.

• Transitions elements are known to have many oxidation states• COMMON occurrences are in burgundy

• The elements in the middle are capable of existing in many oxidation states, while the ones at either end have fewer possibilities

• The flasks hold examples of Cr(III) (violet and green) and Cr(VI) (yellow and orange)

d electrons can be added or removed at relatively little cost

Variable Oxidation States

• A molecule such as the oxallate anion:

Two of its four oxygen atoms(red lone pairs in diagram) to donate toa metal, forming a ring

• This is called chelation, from the word for a crab’s claw• Whereas iron(III) has room in the primary coordination sphere for six water molecules in the

complex ion [Fe(OH2)6]3+, it only has room for three oxalate ions in the complex ion [Fe(C2O4)3]3-.

CuH2O

OH2

OH2

OH2

CuH3N

NH3

NH3

NH3

FeOH2

H2O

H2O

OH2

OH2

OH2

FeOO

O

O

O

O

C

C

C

C

O

O

O

O

O

O

C

C

2+ 2+ 3+

3-

Perspective line drawings of some coordination complexes encountered in Chemistry 2000

Chelating ligands

• The picture below gives three examples of six-coordinate metal ions complexed by three bidentate (= two toothed) ligands

• The one on the left is the [Fe(C2O4)3]3- ion that you will prepare and then crystallize as the K3[Fe(C2O4)3]·3H2O salt – the salt crystals have the beautiful luminescent green colour

• The crystals are light sensitive, and must be stored in the dark• The orange is an ethylenediamine complex of Co3+ that is overal cationic, while the burgandy

solid is the overall neutral triacetylacetonatochromium(III) complex

The Trioxallatoferrate(II) complex

• Transition metal complexes are very often coloured, whereas the metals and metalloids of the s and p blocks form colourless complexes.

• Consider the aqueous solutions of nitrate salts of Fe3+, Co2+, Ni2+, Cu2+ and Zn2+ shown in the following photograph.

• Why are four of them coloured, while the last is colourless?• Consider the electron configurations of the ions1. Fe3+

2. Co2+

3. Ni2+

4. Cu2+

5. Zn2+

2 6[ ]4 3Ar s d 5[ ]3Ar d2 7[ ]4 3Ar s d 7[ ]3Ar d2 8[ ]4 3Ar s d 8[ ]3Ar d1 10[ ]4 3Ar s d 9[ ]3Ar d2 10[ ]4 3Ar s d 10[ ]3Ar d

Electronic Structure and Colour of Transition Metal Coordination Compounds

• In the geometry octahedral the dx2-y2 and dz2 point directly at the ligands, the others do not

• While all the d electrons are repulsed by the ligand lone pairs, those that point directly at the ligands are repulsed more, leading to the octahedral ligand field splitting as follows:

D orbitals in an octahedral ligand field

• Consider our Fe(III) complex, with electron configuration• There are thus five d electrons, and there are two choices on how to distribute them, as

follows: high spin low spin

• When the donor atom is oxygen, the result is always high spin for reasons beyond this course

• Photons can be absorbed that promote electrons from the lower to the upper levels of this electron configuration

• When that happens, light is absorbed, and in fact more than one way of absorbing a photon is possible, leading to absorption at several different wavelengths

• The green colour of the complex is the net result from absorbing the other wavelengths of light

5[ ]3Ar d

Origin of colour

• A spectrophotometer (such as the Spectronic 20) measures absorption of light energy as a function of altered wavelength (dial on the instrument) and the results are graphed

• For the iron complex, this leads to:

Spectrophotometer