2. Lecture 5: p-Acceptor Ligands and Biology CO, N2 and O2 complexes
Schedule
Last Week: Electronic spectroscopy Interelectron repulsion, covalency and spin-orbit coupling
Lecture 4: Re-cap
Lecture 5: p-Acceptor Ligands and Biology CO, N2 and O2 complexes
Demos: charles law (2.3), CO2 density (2.1), star wars (4.3)
Brief mention of 3 states of matter.
Lecture 6: M-M bonding Multiple bonds and metal clusters

3. Summary of Course – week 5
Complexes of p-acceptor ligands
be able to explain synergic (s-donation, p-back donation) model for bonding in M-CO and M-N2 complexes
be able to explain reduction in CO stretching frequency in complex
be able to explain changes in CO stretching frequency with metal charge and with ligands
electron counting in CO, N2 and NO complexes: 18 e- rule
Resources
Slides for lectures 5-6
Winter, Chapter and (basic)
Shriver and Atkins “Inorganic Chemistry” Chapter , (4th Edition)
Housecroft and Sharpe “Inorganic Chemistry” Chapter 23.2 (2nd Edition)

4. Summary of the Last Lecture
Electronic spectroscopy
Be able to explain number of bands
Be able to obtain Doct from spectrum for d1, d3, d4, d6, d7, d8 and d9
Selection rules
Be able to predict relative intensity of spin-allowed vs spin forbidden, octahedral vs tetrahedral and ligand-field vs charge-transfer transitions
Today
Bonding and vibrational spectroscopy in complexes containing p-acceptor ligands

5. Molecular Orbitals for O2 and CO
2ps
2ps
2pp
2pp 2p 2s 2p 2p 2p
2pp
2ps 2s 2s 2s O O2 O O CO C
JKB Lecture 5 slides 8-9

6. Molecular Orbitals for O2 and CO
bond order = 2 (O=O double bond)
Two singly occupied 2ppg antibonding orbitals
CO:
bond order = 3 (C≡O triple bond)
HOMO is dominated by C 2pz (~ C “lone pair”)
LUMOs are dominated by C 2px and 2py:

7. Metal Carbonyl Complexes
bond order = 3 (C≡O triple bond)
donation from HOMO into empty metal d-orbital:
increases e- density on metal
back donation from filled metal orbitals into LUMOs
decreases e- density on metal
self-enhancing:
synergic
JKB Lecture 5 slide 10

8. Metal Carbonyl Complexes
M-CO:
synergic: s and p bonding are both weak in the absence of each other
therefore requires d electrons on metal and non-contracted d-orbitals to overlap with CO orbitals
s-donation strengthens M-C bond
p-back donation strengthens M-C bond and weakens C≡O
carbonyls are found for low-oxidation state metals only (+2 or less)
carbonyls almost always obey the 18e rule
JKB Lecture 5 slide 10

9. Metal Carbonyl Complexes – Vibrations
M-CO – effect of bonding mode:
s-donation strengthens M-C bond
p-back donation strengthens M-C bond and weakens C≡O
C≡O stretching frequency is reduced from value in free CO
more metals = more back donation:
free CO: vco = 2143 cm-1
1850–2120 cm-1
1750–1850 cm-1
1620–1730 cm-1

10. Metal Carbonyl Complexes – Vibrations
M-CO – effect of charge:
s-donation strengthens M-C bond
p-back donation strengthens M-C bond and weakens C≡O
C≡O stretching frequency is reduced from value in free CO
positive charge on complex contracts d-orbitals = less back bonding
negative charge on complex expands d-orbitals = more back bonding
free CO: vco = 2143 cm-1
Mn(CO)6+: 2090 cm-1
Ni(CO)4: 2060 cm-1
Cr(CO)6: 2000 cm-1
Co(CO)4-: 1890 cm-1
V(CO)6-: 1860 cm-1
Fe(CO)42-: 1790 cm-1

11. Metal Carbonyl Complexes – Vibrations
M-CO – effect of other ligands:
s-donation strengthens M-C bond
p-back donation strengthens M-C bond and weakens C≡O
C≡O stretching frequency is reduced from value in free CO
in LnM(CO)m complexes, weak p-acceptor ligands increase M  CO back-donation
free CO: vco = 2143 cm-1
L: good p-acceptor
Mo(CO)6: 2005 cm-1
(PF3)3Mo(CO)3: 2055, 2090 cm-1
(PCl3)3Mo(CO)3: 1991, 2040 cm-1
(P(OMe)3)3Mo(CO)3: 1888, 1977 cm-1
(CH3CN)3Mo(CO)3: 1783, 1915 cm-1
L: poor p-acceptor

12. Metal Carbonyl Complexes – Vibrations
M-CO – symmetry of the molecule:
octahedral M(CO)6
dipole moment change? no
yes no

13. Metal Carbonyl Complexes – Vibrations
M-CO – symmetry of the molecule:
octahedral M(CO)6
vCO
1 IR
2 Raman
rule of mutual exclusion: for molecules with a centre of inversion, no vibrations are both IR and Raman active

14. Metal Carbonyl Complexes – Vibrations
M-CO – symmetry of the molecule:
cis-[M(CO)4Cl2]
trans-[M(CO)4Cl2]
vco:
1 IR 2 Raman
no common bands – rule of mutual exclusion
vco:
4 IR (1 very weak)
4 Raman (1 very weak)
some common bands

15. Metal Carbonyl Complexes – Vibrations
M-CO – symmetry of the molecule:
fac-[M(CO)4Cl2]
mer-[M(CO)4Cl2]
vco:
3 IR (1 week) 3 Raman (1 week)
some common bands
vco:
2 IR (which overlap)
2 Raman (which overlap)
some common bands

16. Molecular Orbitals for O2
2ps
2ps
2pp
2pp 2p 2s 2p 2p 2p
2pp
2ps 2s 2s 2s O O2 O O CO C
JKB Lecture 5 slides 8-9

17. 2H2(g) + O2(g)  2H2O(l) combH = -484 kJ mol-1
Spin-Triplet O2
O2 in the atmosphere is the result of continuous photosynthesis
it is a potentially highly toxic in the presence of fuels (carbohydrates etc)
however, it is metastable because of the 2 unpaired electrons (“triplet”)
2H2(g) + O2(g)  2H2O(l) combH = -484 kJ mol-1
spin inhibited O H O H
H-H
O=O
spin-selection rules prevents “spin-flip” transition in O2 being important so reaction is not initiated by sunlight
initiation happens via a spark or a catalyst

18. 4Fe2+ + O2 + 2H2O + 8OH-  4Fe(OH)3  2Fe2O3 + 6H2O
O2 Transport Complexes
Almost all reactions between O2 and metal complexes are irreversible:
4Fe2+ + O2 + 2H2O + 8OH-  4Fe(OH)3  2Fe2O3 + 6H2O
Transport system for O2 in animals must:
carry O2 in its ground state form (with two unpaired electrons)
capture gas phase O2
transport it via the circulatory system
release it completely to intermediate storage site
Transport system for O2 in animals must:
not react irreversibly with O2
be highly efficient and cope with changes in supply and demand
have a lower affinity for O2 than the storage system
Very important equation - one that you need to remember
surely this should depend on the gas ? … No, as we’ll see soon

19. O2 Transport Complexes
In humans, transport system (haemoglobin) and storage system (myoglobin) are both Fe(II) complexes:
myoglobin
haemoglobin
affinity of myoglobin > affinity of haemoglobin
affinity of haemoglobin increases as O2 pressure grows – cooperative effect
Very important equation - one that you need to remember
surely this should depend on the gas ? … No, as we’ll see soon
muscle
lungs

20. Haemoglobin and Myoglobin - Structures
Haemoglobin consists of 4 haem groups, myoglobin consists of 1 haem group:
distal histidine residue
Very important equation - one that you need to remember
surely this should depend on the gas ? … No, as we’ll see soon
proximal histidine residue

21. Haemoglobin and Myoglobin - Function
Unoxygenated protein contains high spin Fe(II) d6:
distal histidine residue
Oxygenated protein contains low spin Fe(III) d5 and O2-:
Very important equation - one that you need to remember
surely this should depend on the gas ? … No, as we’ll see soon
Unpaired electron on Fe(III) is weakly coupled to unpaired electron on O2-:
complex is diamagnetic
proximal histidine residue

22. Haemoglobin and Myoglobin - Function
weak H-bond?
enforced bending
distal histidine residue
distal histidine residue
Very important equation - one that you need to remember
surely this should depend on the gas ? … No, as we’ll see soon
proximal histidine residue
proximal histidine residue
partial prevention of (irreversible) CO attachment

23. Haemoglobin – Cooperative Effect proximal histidine residue
Unoxygenated protein contain high spin Fe(II) d6:
High spin ion has is too large to fit in haem ring and actually sits slightly below it
Oxygenated protein contains smaller low spin Fe(III) d5 which fits into ring
Very important equation - one that you need to remember
surely this should depend on the gas ? … No, as we’ll see soon
The motion of the proximal group is transferred through protein structure to the next deoxygenated haem group decreasing its activation energy for O2 attachment
proximal histidine residue

24. Summary By now you should be able to....
Explain that metal-carbonyl bonding is due to synergic OC  M s-donation and M  CO p-back donation
Explain that the reduction in vco stretching frequency is related to the extent of back-bonding
Appreciate that the number of vCO in IR and Raman can be used to work out structure
Explain that haemoglobin and myoglobin bind weakly to O2 allowing transport and storage of highly reactive molecule
Next lecture
N2 complexes and Metal-Metal bonding

25. Practice