Slide 2/18

Schedule

• Lecture 4: Re-cap

• Lecture 5: -Acceptor Ligands and BiologyCO and O2 complexes

• Lecture 6: N2 and NO complexes, M-M bonds More on -acceptor ligands, introduction to metal-metal bonding

Slide 3/18

Summary of the Last Lecture

Metal-carbonyl complexes• Bonding is due to synergic OC M s-donation and M

CO p-back donation• Reduction in vco stretching frequency is related to the

extent of back-bonding• Number of vCO in IR and Raman can be used to work out

structure

O2 complexes• Haemoglobin and myoglobin bind weakly to O2 allowing

transport and storage of highly reactive molecule

Today’s lecture• N2 complexes and Metal-Metal bonding

Slide 4/18

2p

2p

2s

2p

2s

2p

2s

2p

Molecular Orbitals for N2 and CO

2pg

2pu

2pg

2pu

N NN2

2p

2s

O CCO

2p

2p

Slide 5/18

Molecular Orbitals for N2 and CO

• 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

C

O

• N2: bond order = 3 (N≡N triple bond) HOMO delocalized over both N (not a “lone pair”)

LUMOs are also shared equally:M

N

N

Slide 6/18

Metal-N2 Complexes

• N2 bonds to transition metals in a similar way to CO (see Lecture 5 Slide 5) σ-donation from N≡N: M -back donation from M: N≡N: (reduces N≡N bond strength) synergic weaker than for M-CO

Os

NH3

NH3

NH3NH3N

NH3N

2+

free N2: vN≡N = 2331 cm-1

M N N1900–2200 cm-1

M N N M~2100 cm-1

Ru

NH3

NH3

NH3NH3N

NH3N

4+

Ru

NH3

NH3

NH3H3N

NH3

Slide 7/18

Fixing Nitrogen

• Nature needs to obtain nitrogen from atmospheric N2

nitrogen content of soil is often the growth limiting factor around 17 million tonnes of NH3 per year industrially natural fixation about 2.5 times more important

• To use atmospheric N2, nature uses metal-enzymes: nitrogen fixation (nitrogenase) converts N2 to NH4

+

nitrification converts NH4+ to NO3

- which can be taken up by plants

• Nitrogenase faces a number of problems: thermodynamic stability of N≡N bond means reaction is unfavourable N2 is non-polar and unreactive: efficient enzymes/catalysts are required M-N2 bonds are weak

Slide 8/18

Nitrogenase – Fe protein

• Nitrogenase contains Fe and Mo and is made up of two proteins ‘Fe protein’ contains a pseudo-cubic [4Fe-4S] cluster:

Fe S

FeS

S

FeS

Fe

S(cysteine)

(cysteine)S

S(cysteine)

(cysteine)S

Cluster contains 2 × Fe2+ and 2 × Fe3+

Tetrahedral coordinate about Fe from 2 × S2 and 2 × S from cysteine

role: oxidation of Fe2+ to Fe3+ provides electrons

(energy for oxidation comes from hydrolysis of2 ATP molecules per electron)

Slide 9/18

Nitrogenase – FeMo protein

• Nitrogenase contains Fe and Mo and is made up of two proteins ‘FeMo protein’ contains two [8M-8S] clusters

Fe S

FeS

S

FeS

Fe

(cys)S

(cys)S

FeS

Fe SH

S

Fe S

Fe

S(cys)S(cys)

S(cys)

S(cys)

role: transfer of electrons from Fe protein

1. [8Fe-8S]

Slide 10/18

Nitrogenase – FeMo protein

• Nitrogenase contains Fe and Mo and is made up of two proteins ‘FeMo protein’ contains two [8M-8S] clusters

role: attachment and reduction of N2

2. [Mo7Fe-8S]

S

Fe

Fe

Fe

S

S Mo

S

SS

S S

FeFe S O

O(cys)S

Slide 11/18

S

Fe

Fe

Fe

S

S Mo

S

SS

S S

FeFe N2 O

O(cys)S

Nitrogenase – FeMo protein

• Site and mode of attachment of N2 to [Mo7Fe-8S] cluster is not known probably Fe rather than Mo possibly between two central Fe atoms:

role of Mo: asymmetry in cluster probably induces a small polarity in N2 bond, facilitating electrophilic

attack

Slide 12/18

2e and

2H+

H+ e and

H+

Nitrogenase – Reduction of N2

• Multiple stepwise addition of e and H+

e from Fe protein energy from ATP hydrolysis:

LM-N≡N LM=N=NHe and

H+LM≡N-NH2 LM≡N-NH3

+e and

H+

e and

H+

e andH+

LM LM-NH3 LM=NH LM≡N

NH3+

N2

NH3+

2p

2p

2s

2p

2s

2p

2s

2p

Molecular Orbitals for N2 and NO

2pg

2pu

2pg

2pu

N NN2

2p

2s

O NNO

2p

2p

bond order = ½ (8 – 3) = 2.5

Slide 14/18

Bonding in M-NO Complexes

• NO - two ways of bonding:

as NO+ - isoelectronic with CO 2e- -donation from O=N: M -back donation from M: NO

as NO - isoelectronic with O2

1e- -donation from O=N: M

M

N

O

+

M

N

O-

Slide 15/18

Fe2+

NO Synthase and Nitrophorin

• NO is used as a signalling molecule in mammals• NO synthase is a haem protein which produces NO upon receipt of a

signal• Nitrophorin is found in some bloodsucking parasites (“kissing bugs”)

• it is a haem protein that binds NO tightly pH = 5 (in saliva of bug)• it releases NO at pH = 7.4 (in blood of victim)• NO release signals to dilate blood vessels, aiding ‘blood donation’

HN

N

N

O

Fe2+

HN

N

N O

increase pH

decrease pH

Slide 16/18

2s

2p

2s

2p2pg

2pu

2pg

2pu

N NN2

Maximum Bond Orders – s and p-Block

1s 1s

H HH2

• maximum bond order for s-block is 1

• maximum bond order for p-block is 3

Slide 17/18

3d 3d

3dg

Maximum Bond Order – d-Block

3du

3du

3dg

3dg

3du

M M

2 ×

2 ×

dz2 + dz2

dx2-y2 + dx2-y2dxy + dxy

dxz + dxz

dyz + dyz

• the maximum bond order is 5

• but…complexes also contain ligands which use some of the d-orbitals reducing number of bonds

Slide 18/18

Summary

By now you should be able to....• Explain that why M-N2 bonding is much weaker than M-

CO bonding• Count electrons in NO complexes containing bent M-NO

(1 e- donor) or conraining linear M-NO (2 e- donor

Next lecture• Metal-Metal bonding in complexes