Download Electronic spectroscopy • Lecture 5: π

Schedule
• Last Week: Electronic spectroscopy
Interelectron repulsion, covalency and spin-orbit coupling
• Lecture 4: Re-cap
• Lecture 5: π-Acceptor Ligands and Biology
CO, N2 and O2 complexes
• Lecture 6: M-M bonding
Multiple bonds and metal clusters
Summary of Course – week 5
Complexes of π-acceptor ligands
• be able to explain synergic (σ-donation, π-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
CO, N2 and NO complexes: 18 e- rule
Resources
• Slides for lectures 5-6
• Winter, Chapter 6.5-6.7 and 6.10-6.11(basic)
• Shriver and Atkins “Inorganic Chemistry” Chapter 21.1-5, 21.18 (4th Edition)
• Housecroft and Sharpe “Inorganic Chemistry” Chapter 23.2 (2nd Edition)
1
Summary of the Last Lecture
Electronic spectroscopy
• Be able to explain number of bands
• Be able to obtain Δoct 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 π-acceptor ligands
Slide 4/25
Molecular Orbitals for O2 and CO
2pσ
2pσ
2pπ
2pπ
2p
2p
2pσ
2p
2pπ
2pπ
2p
2pσ
2s
O
O2
2s
2s
O
O
2s
CO
C
JKB Lecture 5 slides 8-9
Slide 5/25
Molecular Orbitals for O2 and CO
•
O2:
¾ bond order = 2 (O=O double bond)
¾ Two singly occupied 2pπg antibonding orbitals
O
O
M
•
CO:
CO
¾ bond order = 3 (C≡O triple bond)
¾ HOMO is dominated by C 2pz (~ C “lone pair”)
O
¾ LUMOs are dominated by C 2px and 2py:
C
M
Slide 6/25
2
Metal Carbonyl Complexes
•
CO:
¾ bond order = 3 (C≡O triple bond)
¾ donation from HOMO into empty metal d-orbital:
increases e- density on metal
self-enhancing:
¾ back donation from filled metal orbitals into LUMOs
synergic
decreases e- density on metal
JKB Lecture 5 slide 10
Slide 7/25
Metal Carbonyl Complexes
•
M-CO:
¾ synergic: σ and π bonding are both weak in the absence of each other
¾ therefore requires d electrons on metal and non-contracted d-orbitals to
overlap with CO orbitals
carbonyls are found for low-oxidation state metals only (+2 or less)
carbonyls almost always obey the 18e rule
¾ σ-donation strengthens M-C bond
¾ π-back donation strengthens M-C bond and weakens C≡O
M
C
M
O
C
O
JKB Lecture 5 slide 10
Slide 8/25
Metal Carbonyl Complexes – Vibrations
•
M-CO – effect of bonding mode:
¾ σ-donation strengthens M-C bond
¾ π-back donation strengthens M-C bond and weakens C≡O
¾ C≡O stretching frequency is reduced from value in free CO
¾ more metals = more back donation:
free CO: vco = 2143 cm-1
O
O
C
C
M
M
O
C
M
1850–2120 cm-1 1750–1850 cm-1
M
MM
1620–1730 cm-1
Slide 9/25
3
Metal Carbonyl Complexes – Vibrations
•
M-CO – effect of charge:
¾ σ-donation strengthens M-C bond
¾ π-back donation strengthens M-C bond and weakens C≡O
¾ C≡O stretching frequency is reduced from value in free CO
¾ positive charge on complex contracts d-orbitals = less back bonding
¾ 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
Co(CO)4−:
1890
cm-1
Cr(CO)6: 2000 cm-1
V(CO)6−: 1860 cm-1
Fe(CO)42−: 1790 cm-1
Slide 10/25
Metal Carbonyl Complexes – Vibrations
•
M-CO – effect of other ligands:
¾ σ-donation strengthens M-C bond
¾ π-back donation strengthens M-C bond and weakens C≡O
¾ C≡O stretching frequency is reduced from value in free CO
¾ in LnM(CO)m complexes, weak π-acceptor ligands increase M Æ CO
back-donation
free CO: vco = 2143 cm-1
L: good π-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
L: poor π-acceptor
(CH3CN)3Mo(CO)3: 1783, 1915 cm-1
Slide 11/25
Metal Carbonyl Complexes – Vibrations
•
M-CO – symmetry of the molecule:
¾ octahedral M(CO)6
dipole moment
change?
no
yes
no
Slide 12/25
4
Metal Carbonyl Complexes – Vibrations
•
M-CO – symmetry of the molecule:
¾ 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
Slide 13/25
Metal Carbonyl Complexes – Vibrations
•
M-CO – symmetry of the molecule:
¾ cis-[M(CO)4Cl2]
vco:
4 IR (1 very weak)
4 Raman (1 very weak)
some common bands
¾ trans-[M(CO)4Cl2]
vco:
1 IR
2 Raman
no common bands –
rule of mutual exclusion
Metal Carbonyl Complexes – Vibrations
•
M-CO – symmetry of the molecule:
¾ fac-[M(CO)4Cl2]
vco:
2 IR (which overlap)
2 Raman (which overlap)
some common bands
¾ mer-[M(CO)4Cl2]
vco:
3 IR (1 week)
3 Raman (1 week)
some common bands
5
Molecular Orbitals for O2
2pσ
2pσ
2pπ
2pπ
2p
2p
2pσ
2p
2pπ
2p
2pπ
2pσ
2s
O
O2
2s
2s
O
O
2s
CO
C
JKB Lecture 5 slides 8-9
Spin-Triplet O2
•
O2 in the atmosphere is the result of continuous photosynthesis
¾ it is a potentially highly toxic in the presence of fuels (carbohydrates etc)
¾ however, it is metastable because of the 2 unpaired electrons (“triplet”)
2H2(g) + O2(g) Æ 2H2O(l)
ΔcombH = -484 kJ mol-1
spin
H-H H-H O=O
inhibited
H
O
H H
O
H
¾spin-selection rules prevents “spin-flip” transition in O2 being important
so reaction is not initiated by sunlight
¾initiation happens via a spark or a catalyst
O2 Transport Complexes
•
Almost all reactions between O2 and metal complexes are irreversible:
4Fe2+ + O2 + 2H2O + 8OH- Æ 4Fe(OH)3 Æ 2Fe2O3 + 6H2O
•
Transport system for O2 in animals must:
¾ carry O2 in its ground state form (with two unpaired electrons)
¾ capture gas phase O2
¾ transport it via the circulatory system
¾ release it completely to intermediate storage site
•
Transport system for O2 in animals must:
¾ not react irreversibly with O2
¾ be highly efficient and cope with changes in supply and demand
¾ have a lower affinity for O2 than the storage system
Slide 18/25
6
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
muscle lungs
Slide 19/25
Haemoglobin and Myoglobin - Structures
•
Haemoglobin consists of 4 haem groups, myoglobin consists of 1 haem
group:
N
HN
N
N
Fe2+
N
distal
histidine
residue
Fe2+
N
N
HN
proximal histidine residue
Slide 20/25
Haemoglobin and Myoglobin - Function
•
Unoxygenated protein contains high spin Fe(II) d6:
N
HN
O
O
•
Oxygenated protein contains low spin
Fe(III) d5 and O2−:
distal
histidine
residue
Fe2+
N
•
Unpaired electron on Fe(III) is weakly
coupled to unpaired electron on O2−:
¾ complex is diamagnetic
HN
proximal histidine residue
Slide 21/25
7
Haemoglobin and Myoglobin - Function
N
N
weak Hbond?
HN
distal
histidine
residue
O
O
enforced
bending
HN
distal
histidine
residue
O
C
Fe33+
Fe2+
N
N
HN
HN
proximal histidine residue
proximal histidine residue
partial prevention of (irreversible) CO attachment
Slide 22/25
Haemoglobin – Cooperative Effect
•
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
O
N
•
Fe2+
O
N
Oxygenated protein contains smaller
O
low spin Fe(III) d5 which fits into ring
N Fe 3+ N
N Fe3+ N
N
•The motion of the proximal group is
transferred through protein structure to the
next deoxygenated haem group decreasing
its activation energy for O2 attachment
HN
proximal histidine residue
Slide 23/25
Summary
By now you should be able to....
• Explain that metal-carbonyl bonding is due to synergic
OC Æ M σ-donation and M Æ CO π-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
Slide 24/25
8
Practice
Slide 25/25
9