Bioinorganic and Transition Metals

Question 1

What are bulk elements? What are the trace/essential elements?

Answer 1

make up the majority of elemental composition of the body
C, H, O, N - 99% of the human body
Na, K, Mg, Ca, P, S, Cl - 0.9% of the human body

Mn, Fe, Co, Ni, Cu, Zn, Mo, B, Si, Se - essential to most but not all biological systems

V, Cr, F, I, As, Br, Sn - have essential function in either plants or animals

Question 2

What is dental flurosis and what is the cause?

Answer 2

hypomineralisation of the tooth enamel

caused by excessive ingestion of fluoride during enamel formation

Question 3

What is Wilson’s disease and the cause?

Answer 3

generic disorder caused by build up of copper in the body

caused by mutation of a protein that transports excess copper into bile

Question 4

Which elements are polluting in vivo?

Answer 4

Pb- lead 
TI- thallium 
Hg - mercury 
Cd - cadmium
Al - aluminium

Question 5

What orbitals do transition metals have?

Answer 5

give rise to 5 d orbitals - can hold maximum 10 electrons

can have different oxidation states

Question 6

How can you work out the number of electrons in an elements d orbital?

Answer 6

group number - oxidation state

example
Zn 2+ = 12 - 2 = 10 electrons in the d orbital

Question 7

What are the different d-orbitals?

Answer 7

5 d orbitals
eg orbitals - 2 orbitals
- electron density is spread along the axis
t2g orbitals - 3 orbitals
- electron density is spread between the axis

Question 8

What are the three terms involved in co-ordination chemistry?

Answer 8

complex or coordination compound
ligands
coordination number

positively charged central ion (metal, acceptor - electron pair acceptor) is surrounded by ligands (ions or molecules, nucleophile - electron pair donor)

Question 9

What are the different types of ligands?

Answer 9

monodentate/unidentate
- bonded to the central ion at one point
bidentate
- bonded to the central ion at two points
polydentate/multidentate
- bonded to the central ion at multiple points

Question 10

What is the crystal field theory?

Answer 10

simple theory that is used to explain the transition metal complex 
looks at 
- structure 
- spectroscopic properties
- magnetic properties

Question 11

What are the assumptions made in the crystal field theory?

Answer 11

1 - central metal ion - is a point positive charge
2 - ligands are regarded as dipoles or anions - negative charge
3 - ligands are considered point charges - negative charge
= no orbital overlap or electron sharing
4 - ionic lattice is built from the positive and negative point charges
5 - bonding energy - ionic electrostatic forces

Question 12

What is the octahedral crystal field energy?

Answer 12

metal ion starts at the centre of a sphere
- M does not feel electron density from ligands
- d-orbitals have the same energy = degenerate
energy rises
- repulsion occurs as the ligands get closer to the M
- electrons can feel each others density
orbitals spilt into different energy levels
- eg rises to higher level = feels greater electron density, electrons density is along the axis
- t2g drops to a lower level = feels less electrons density, electron density is between the axis

Question 13

What is the distance between the orbitals called in octahedral?

Answer 13

octahedral crystal field splitting energy

• energy of the higher orbital eg is +3/5 = +0.6
• energy of the lower orbital t2g is -2/5 = -0.4

Question 14

What is tetrahedral crystal field energy?

Answer 14

metal ion starts at the centre of a cube
energy rises as ligands get closer to the M
- orbitals are spilt due to poor orbital overlap between metal and ligands
- orbitals are directed onto the axis but ligands are not
orbital spilt into different energy levels
- t2g rises to higher energy = +2/5 = +0.4
- eg drops to lower energy level = -3/5 = -0.6

Question 15

What is the difference between the crystal field splitting energy of tetrahedrals and octahedrals?

Answer 15

crystal splitting field energy of tetrahedrals is lower than the crystal splitting field energy of octahedral

• attraction between ligands and the electron density in orbitals of tetrahedrals is smaller than that in octahedrals
• orbitals in tetrahedrals are not pointing directly towards the ligands

tetrahedral splitting energy is 4/9 octahedral splitting energy

Question 16

What are the factors influencing crystal field splitting?

Answer 16

type of ligands

• smaller, more localised charges interact more strongly with orbitals = I, Br, Cl, F
• smaller crystal field splitting energy

charge

• charge on metal ion = higher the charge, smaller radius on metal ion, negative charged ligands are are attracted to it
• larger crystal field splitting energy

orbital size

• orbital size increases down a group
• increased orbital size, greater overlap or orbitals and ligands
• larger crystal field splitting energy

oxidation state
- higher the oxidation state of the metal ion, the greater the crystal field splitting energy

Question 17

How does type of ligand affect the colour of transition metal complexes?

Answer 17

colour of TM complex depends on wavelength of light absorbed

crystal field splitting energy difference determines colour of the coordination complex

weak ligands
- have smaller crystal field splitting energy = weak field
- absorb lower energy light with longer wavelengths
strong ligands
- have larger crystal field splitting energy = strong field
- absorb higher energy light with shorter wavelengths

Question 18

What is the difference between strong and weak ligands?

Answer 18

weak ligands

• I, Br, Cl, F, OH
• attract/interact more strongly with orbitals
• smaller crystal field splitting energy - weak field

strong ligands

• CN, NH3, NCS, H20
• interact less strongly with orbitals
• larger crystal field splitting energy - strong field

Question 19

What is the difference between low spin and high spin?

Answer 19

pairing energy > CFSE
- electron joins higher orbital instead of pairing up = high spin
high spin has a small crystal field splitting energy - does not take much energy to put it in high spin = weak field
pairing energy < CFSE
- electrons pairs up in lower orbital instead of rising to higher orbital = low spin
low spin has a large crystal field splitting energy - takes too much energy to put it in high spin = strong field

Question 20

What does paramagnetic and diamagnetic mean?

Answer 20

diamagnetic
- no unpaired electrons
paramagnetic
- has unpaired electrons

Question 21

What spin are octahedral and tetrahedral complexes in?

Answer 21

tetrahedral complexes are always in high spin
- have lower crystal field splitting energy = does not take much energy for electrons to rise to higher orbital
octahedrals
- 2nd and 3rd row of the periodic table are always low spin
d4, d5, d6 and d7 orbitals of the first row of transition metals can be low or high spin = Cr, Mn, Fe and Co

Question 22

Why is zinc important for enzymes?

Answer 22

no crystal field stabilisation energy = cancels out to zero as it has 10 electrons
- no CFSE in tetrahedral or octahedral form

lack of CFSE favours

• substrate binding
• product release
• formation of bound intermediates

means that at the active site of an enzyme, zinc can change the geometric centre form from tetrahedral to octahedral