Schedule
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Transcript of Schedule
Schedule
• Lecture 4: Re-cap
• Lecture 5: -Acceptor Ligands and BiologyCO, N2 and O2 complexes
• Lecture 6: M-M bondingMultiple bonds and metal clusters
• Last Week: Electronic spectroscopy Interelectron repulsion, covalency and spin-orbit coupling
Summary of Course – week 5
Complexes of-acceptor ligands• be able to explain synergic (-donation, -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 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)
Slide 4/25
Summary of the Last Lecture
Electronic spectroscopy• Be able to explain number of bands• Be able to obtain oct 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 -acceptor ligands
Slide 5/25
2p
2p
2s
2p
2s
2p
2s
2p
JKB Lecture 5 slides 8-9
Molecular Orbitals for O2 and CO
2p
2p
2p
2p
O OO2
2p
2s
O CCO
2p
2p
Slide 6/25
Molecular Orbitals for O2 and CO
• O2: bond order = 2 (O=O double bond) Two singly occupied 2pg 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:
M
O
O
M
C
O
Slide 7/25
Metal Carbonyl Complexes
• CO: 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
JKB Lecture 5 slide 10
self-enhancing:
synergic
Slide 8/25
• M-CO: synergic: and 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
-donation strengthens M-C bond -back donation strengthens M-C bond and weakens C≡O
Metal Carbonyl Complexes
JKB Lecture 5 slide 10
carbonyls are found for low-oxidation state metals only (+2 or less)
carbonyls almost always obey the 18e rule
M C O M C O
Slide 9/25
Metal Carbonyl Complexes – Vibrations
• M-CO – effect of bonding mode: -donation strengthens M-C bond -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
M
C
O
1850–2120 cm-1
M
C
O
M
1750–1850 cm-1
M
C
O
MM
1620–1730 cm-1
Slide 10/25
Mn(CO)6+: 2090 cm-1
Metal Carbonyl Complexes – Vibrations
• M-CO – effect of charge: -donation strengthens M-C bond -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
Ni(CO)4: 2060 cm-1 Cr(CO)6: 2000 cm-1
V(CO)6: 1860 cm-1Co(CO)4
: 1890 cm-1
Fe(CO)42: 1790 cm-1
Slide 11/25
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
Metal Carbonyl Complexes – Vibrations
• M-CO – effect of other ligands: -donation strengthens M-C bond -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 -acceptor ligands increase M CO
back-donation
free CO: vco = 2143 cm-1
L: good -acceptor
L: poor -acceptor
Slide 12/25
Metal Carbonyl Complexes – Vibrations
• M-CO – symmetry of the molecule: octahedral M(CO)6
dipole momentchange?
no yes no
Slide 13/25
Metal Carbonyl Complexes – Vibrations
• M-CO – symmetry of the molecule: octahedral M(CO)6
vCO
1 IR2 Raman
rule of mutual exclusion: for molecules with a centre of inversion, no vibrations are both IR and Raman active
Metal Carbonyl Complexes – Vibrations
vco:4 IR (1 very weak)
4 Raman (1 very weak)some common bands
trans-[M(CO)4Cl2]
vco:1 IR
2 Ramanno common bands –
rule of mutual exclusion
• M-CO – symmetry of the molecule: cis-[M(CO)4Cl2]
Metal Carbonyl Complexes – Vibrations
vco:2 IR (which overlap)
2 Raman (which overlap)some common bands
vco:3 IR (1 week)
3 Raman (1 week)some common bands
• M-CO – symmetry of the molecule: fac-[M(CO)4Cl2] mer-[M(CO)4Cl2]
2p
2p
2s
2p
2s
2p
2s
2p
JKB Lecture 5 slides 8-9
Molecular Orbitals for O2
2p
2p
2p
2p
O OO2
2p
2s
O CCO
2p
2p
spin
inhibited
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
H-HH-H O=O OHH
OHH
spin-selection rules prevents “spin-flip” transition in O2 being important so reaction is not initiated by sunlightinitiation happens via a spark or a catalyst
Slide 18/25
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
Slide 19/25
O2 Transport Complexes
• In humans, transport system (haemoglobin) and storage system (myoglobin) are both Fe(II) complexes:
myoglobin
haemoglobin
muscle lungs
affinity of myoglobin > affinity of haemoglobin
affinity of haemoglobin increases as O2 pressure grows – cooperative effect
Slide 20/25
Haemoglobin and Myoglobin - Structures
• Haemoglobin consists of 4 haem groups, myoglobin consists of 1 haem group:
N
N N
N
Fe2+ Fe2+
HN
N
HN
N
proximal histidine residue
distal histidine residue
Slide 21/25
Haemoglobin and Myoglobin - Function
• Unoxygenated protein contains high spin Fe(II) d6:
Fe2+
HN
N
HN
N
proximal histidine residue
distal histidine residue
• Oxygenated protein contains low spin Fe(III) d5 and O2
:
• Unpaired electron on Fe(III) is weakly coupled to unpaired electron on O2
complex is diamagnetic
OO
Slide 22/25
Haemoglobin and Myoglobin - Function
Fe3+
HN
N
HN
N
proximal histidine residue
distal histidine residue
OO
weak H-bond?
Fe2+
HN
N
HN
N
proximal histidine residue
distal histidine residue
CO
enforcedbending
partial prevention of (irreversible) CO attachment
Slide 23/25
HN
N
Fe3+ NN
• Unoxygenated protein contain high spin Fe(II) d6:
• High spin ion has is too large to fit in haemring and actually sits slightly below it
• Oxygenated protein contains smallerlow spin Fe(III) d5 which fits into ring
Haemoglobin – Cooperative Effect
proximal histidine residue
OO
Fe2+NN
Fe3+ NN
•The motion of the proximal group is transferred through protein structure to the next deoxygenated haem group decreasing its activation energy for O2 attachment
Slide 24/25
Summary
By now you should be able to....• Explain that metal-carbonyl bonding is due to synergic
OC M -donation and M CO -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 25/25
Practice