Coordination Chemistry

Question 1

Weak field and strong field case

Which has high spin and which has low spin?

Answer 1

Weak field case

• P > 10 Dq (so electrons go to eg orbitals before pairing)
• high spin (small energy gaps)

Strong field case

• P < 10 Dq (electrons pair in t2g orbitals before going to eg orbitals)
• low spin (large energy gaps)

Question 2

Order of spectrochemical series

Answer 2

Order of increasing field strength
Some weak field ligands: I- < Br- < SCN- < Cl- < OH- < H2O
Some strong field ligands: NCS- < NH3 < en < NO2- < PPh3 < CN- < CO

Question 3

Tetragonal elongation and tetragonal contraction

Answer 3

Tetragonal elongation - z goes down in energy

Tetragonal contraction - z increases in energy

Question 4

Define stability

Answer 4

The term is generally used in the thermodynamic sense, e.g. a complex is highly stable (i.e. its formation constant, Kf, is large).

Question 5

Define lability

Answer 5

The term is used in the kinetic sense, e.g. a complex is high labile (not inert).
* Note: complexes can be extremely stable but can undergo differing rates of ligand exchange due to it being labile or inert.

Question 6

Free-energies of stable complexes and inert complexes

Answer 6

Stable complexes have large negative free-energies of reaction (dG).
Inert complexes have large positive free-energies of activation (dG*).

Question 7

Define aquation and anation

Answer 7

Aquation: substitution of a ligand by H2O
Anation: substitution of H2O by an anion

Question 8

What are the two mechanisms of ligand substitution reactions?

Answer 8

Dissociative (L5M + X + Y)
- usual for octahedral complexes (no spare spots)
Associative (L5MXY)
- not common for octahedral complexes, but common for square plannar complexes (there is space to attack)

Question 9

Symmetry selection rules

Answer 9

s and d orbitals possess a centre of symmetry (gerade, g), but p-orbitals do not (ungerade, u).

• In complexes with a centre of symmetry, g –> u are allowed but g –> g are not.
• In complexes with no centre of symmetry (tetrahedral), d –> d transitions are allowed.
• Laporte forbidden –> colourless

Question 10

Spin selection rules

Answer 10

The promotion of an electron can only proceed if the orientation of the spin is the same in the excited state as in the ground state.
* Spin forbidden –> colourless

Question 11

What are three other types of electronic transitions?

Answer 11

Ligand-to-metal charge transfer (LMCT)
- metal is easily reduced (high oxidation state)
Metal-to-ligand charge transfer (MLCT)
- metal is easily oxidised
Intervalence charge-transfer transitions (IVCT)
- complexes containing metals that are 2 oxidation states can be coloured due to excitation of an electron from one metal ion to another
* all have intense colours