Schedule Lecture 4: Re-cap Lecture 5: -Acceptor Ligands and Biology CO, N 2 and O 2 complexes...

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)

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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

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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

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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

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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

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• 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

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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

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Mn(CO)6+: 2090 cm-1


• 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


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

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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


• 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


L: good -acceptor

L: poor -acceptor

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• M-CO – symmetry of the molecule: octahedral M(CO)6

dipole momentchange?

no yes no

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• 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


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]


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

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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

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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

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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

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Haemoglobin and Myoglobin - Function

• Unoxygenated protein contains high spin Fe(II) d6:

Fe2+

HN

N

HN

N



• 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

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Haemoglobin and Myoglobin - Function

Fe3+

HN

N

HN

N



OO

weak H-bond?

Fe2+

HN

N

HN

N



CO

enforcedbending

partial prevention of (irreversible) CO attachment

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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


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

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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

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Schedule Lecture 4: Re-cap Lecture 5: -Acceptor Ligands and Biology CO, N 2 and O 2 complexes...

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