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Schedule. Last Week: Electronic spectroscopy Interelectron repulsion, covalency and spin-orbit coupling. Lecture 4: Re-cap. Lecture 5: p -Acceptor Ligands and Biology CO, N 2 and O 2 complexes. Lecture 6: M-M bonding Multiple bonds and metal clusters. Summary of Course – week 5.

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Schedule

  • Last Week: Electronic spectroscopy Interelectron repulsion, covalency and spin-orbit coupling

  • Lecture 4: Re-cap

  • Lecture 5: p-Acceptor Ligands and BiologyCO, N2 and O2 complexes

  • Lecture 6: M-M bondingMultiple bonds and metal clusters


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 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)


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


Molecular Orbitals for O2 and CO

2ps

2pp

2ps

2p

2pp

2s

2ps

2pp

2p

2p

2p

2pp

2ps

2s

2s

2s

O

CO

C

O

O2

O

JKB Lecture 5 slides 8-9


Molecular Orbitals for O2 and CO

  • O2:

    • 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:


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

self-enhancing:

synergic

JKB Lecture 5 slide 10


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


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


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


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


Metal Carbonyl Complexes – Vibrations

  • M-CO – symmetry of the molecule:

    • octahedral M(CO)6

dipole momentchange?

yes

no

no


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


Metal Carbonyl Complexes – Vibrations

  • M-CO – symmetry of the molecule:

    • cis-[M(CO)4Cl2]

  • trans-[M(CO)4Cl2]

vco:

1 IR2 Raman

no common bands –rule of mutual exclusion

vco:

4 IR (1 very weak)

4 Raman (1 very weak)

some common bands


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


Molecular Orbitals for O2

2ps

2pp

2ps

2p

2pp

2s

2ps

2pp

2p

2p

2p

2pp

2ps

2s

2s

2s

O

CO

C

O

O2

O

JKB Lecture 5 slides 8-9


Spin-Triplet O2

H-H

H-H

O=O

O

O

H

H

H

H

  • 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

spininhibited

  • 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


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


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


Haemoglobin and Myoglobin - Structures

  • Haemoglobin consists of 4 haem groups, myoglobin consists of 1 haem group:

distal histidine residue

proximal histidine residue


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-:

  • Unpaired electron on Fe(III) is weakly coupled to unpaired electron on O2-:

    • complex is diamagnetic

proximal histidine residue


Haemoglobin and Myoglobin - Function

weak H-bond?

enforcedbending

distal histidine residue

distal histidine residue

proximal histidine residue

proximal histidine residue

partial prevention of (irreversible) CO attachment


Haemoglobin – Cooperative Effect

  • 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

  • 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


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



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