Metal ions in Biochemistry & Coordination Chemistry

Question 1

What are the roles of metal ions ?

Answer 1

• Charge carrier
• Osmotic balance
• Triggers (nerve signalling and muscle contraction)
• Structural (bones and proteins)
• Catalysts
• Transport and storage (i.e. O2)

Question 2

Bulk metals used by nature ?

Answer 2

• Make up 1% of the human body weight

- Na+, Mg2+, K+ and Ca2+

Question 3

Trace metals used by nature?

Answer 3

V, Cr, Mn, Fe, Co, Ni, Cu, ZN, Mo an W

Question 4

Specific role of Na and K ?

Answer 4

• Charge carrier

- Osmotic balance

Question 5

Specific role of Mg ?

Answer 5

• Structural role

- Hydrolytic cleavage

Question 6

Specific role of Ca ?

Answer 6

• Structural role
• Trigger
• Charge carrier

Question 7

Specific role of Zn ?

Answer 7

• Structural role

- Hydrolytic cleavage

Question 8

Specific role of Fe and Cu ?

Answer 8

• O2 transport, storage and/or activation
• Electron transport
• Superoxide (O3-) breakdown

Question 9

Specific role of Mo ?

Answer 9

• Oxo (O) transfer

- N2 activation

Question 10

Specific role of Ni ?

Answer 10

• Methane (CH4) production
• H2 activation
• Hydrolysis

Question 11

Specific role of Mn ?

Answer 11

• O2 evolution
• Peroxide breakdown
• Superoxide breakdown

Question 12

Specific role of Co ?

Answer 12

• Free radical reactions

- Nucleophilic metal ion

Question 13

What is the job of P and S ?

Answer 13

PO3R2- and SO3R- make organic (CHNO) compounds soluble in water as anions

Question 14

What is the job of Na+ and K+?

Answer 14

• Counterions and electrical carriers – can’t use Ca2+ or Al3+ as these would precipitate organic anions
• Also serve as messengers, activating Na+ or K+ specific ATPases

Question 15

What is the job of Mg2+ ?

Answer 15

Used as a weak Lewis acid inside cell and binds to anionic surfaces such as RNA and ribosomes

Question 16

What is the job of Ca2+ ?

Answer 16

Kept out of cell and is used as a temporary reflux trigger (i.e. nerve impulses)

Question 17

What does the number of available oxidation states help predict ?

Answer 17

Some of the roles of the metal

Question 18

Irving-Williams Series ?

Answer 18

• Relative stability of metal complexes
• Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II)
• The ionic radius is expected to decrease regularly for Mn2+ to Zn2+
• Stability increases in this order

Question 19

Metal ions do not freely exist in nature, they are always surrounded by ?

Answer 19

Ligands, from simple like H20 to more complex haems

Question 20

For instance, whenever metals dissolve in water, they are hydrated (coordinated) by ? to form ?

Answer 20

For instance, whenever metals dissolve in water, they are hydrated (coordinated) by water molecules to form a charged complex

Question 21

What does the water ‘complex’ contain ?

Answer 21

The water ‘complex’ has a positive charge (n+) and requires anions for charge
balance

Question 22

What is a Lewis acid and a Lewis base ?

Answer 22

• Lewis acid = Electron pair acceptor
  i. e. metal ions
• Lewis base = electron pair donor
  i. e. ligands

Question 23

What are Soft acids and bases?

Answer 23

Large species that are easily polarisable (think of a bean bag)

Question 24

What are Hard acids and bases?

Answer 24

Small species that are not easily polarised (think of a marble)

Question 25

L (Lewis base) must have electrons to ‘donate’ to ?

Answer 25

M (Lewis acid)

Question 26

What can a Lewis base either be ?

Answer 26

Neutral (uncharged) or charged (usually negative, e.g. CN-)

Question 27

In water, what is the most common ligand ?

Answer 27

Water

Question 28

What is a complex ion ?

Answer 28

When the coordination complex carries a charge

Question 29

In regards to the periodic table, where is the electronegativity the highest?

Answer 29

- Electronegativity is higher on the right side of the periodic table - Electronegativity is higher on the top of the periodic table

Question 30

Explain electronegativity in ionic bonds ?

Answer 30

Large electronegativity difference

Question 31

Explain electronegativity in covalent bonds ?

Answer 31

Small electronegativity difference

Question 32

What is meant by the coordination number and oxidation ?

Answer 32

- Coordination number: the number of ligands bound to the metal - Oxidation state: the charge of the ‘free’ metal ion

Question 33

What is the most common coordination environment ?

Answer 33

Octahedral (Coordination number: 6)

Question 34

What is important to note in regards to the octahedral geometry and it's ligands ?

Answer 34

In octahedral geometry, the ligands lie on the axis. Thus, the orbitals that are on the axis (dz2 and dx2-y2) are higher in energy than the ones that lie in-between the axis (dzx, dyz, and dxy). In a sense, the orbitals on the axis are essentially ‘repelled’ by the electrons of the ligand

Question 35

What is meant by Δo ?

Answer 35

- The energy difference between the two levels (ligand-field splitting) - The magnitude of Δo is ligand dependent

Question 36

What does the size of Δ depend on ?

Answer 36

Depends on L (Lewis base)

Question 37

What are ligands causing a small Δ called ?

Answer 37

Low field ligands (H20)

Question 38

What are ligands causing a large Δ called ?

Answer 38

High field ligands (CN-), forces electrons into lower levels

Question 39

What gives metal complexes magnetic properties ?

Answer 39

Unpaired electrons

Question 40

Addition of a cyanide solution to a colourless [Fe(H2O)6]2+ aquo solution results in ?

Answer 40

The appearance of a blue colour (technically this may be spin forbidden)

Question 41

The excitation of an electron in [Fe(CN)6]4- is responsible for ?

Answer 41

The blue colour of this complex

Question 42

What are colours in solution sometimes caused by ?

Answer 42

Excitation of lower energy to higher energy d orbitals

Question 43

The ability of ligands to spit the d orbitals varies according to ?

Answer 43

The spectrochemical series (ligands based on the strength of their interaction with metal ions)

Question 44

What is meant by weak field ligands ?

Answer 44

Ligands that cause a transition metal to have a small crystal field splitting, which leads to high spin

Question 45

What is meant by strong field ligands ?

Answer 45

Ligands that produce a large crystal field splitting, which leads to low spin

Question 46

What is the Chelate effect ?

Answer 46

Entropy favours multi-dentate ligands over monodentate ligands