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Organometallic ChemistryCHEM 20312

Dr. Mark Whiteley 4.02g

Mark.Whiteley@manchester.ac.uk

Office Hour (weeks 9-12) Wednesday 9.00am

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Recommended Texts

• M Bochmann, 'Organometallics 1', Oxford Chemistry Primers, No. 12.

• M Bochmann, 'Organometallics 2', Oxford Chemistry Primers, No. 13.

• C. Elschenbroich, 'Organometallics', 2nd or 3rd eds, VCH.

• Housecroft and Sharpe 4th Edition Chapters 24 and 25

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What defines an Organometallic Complex?

[As2(CH3)4][Fe(η-C5H5)2]

Ferrocene

[RhI2(CO)2]-

They ALL have a DIRECT metal carbon bond

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Definition of Organometallic Compounds

• Organometallic chemistry: chemistry of molecules containing Metal-Carbon bonds.

• This course considers d-block (transition) metals but main group organometallics of Li, Mg (Grignard) Hg and Sn also very important synthetic reagents.

• M is often in low oxidation state (+2, +1, 0 or even negative), hence electron-rich.

• Stabilised by electron-acceptor ligands.

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Importance of Organometallic Chemistry:

(i) Catalysis• Monsanto process:

MeOH/CO/ Rh cat. MeCO2H (acetic acid)• Hydroformylation:

RCH=CH2 / CO/H2/ Rh cat. RCH2CH2CHO (aldehyde)• Ziegler-Natta process: CH2=CH2 Ti cat. polyethylene

All work because the organic molecules are coordinated and activated by the metal catalyst. Homogeneous systems have advantages of mild operating conditions and high product specificity.

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(ii) Organic Synthesis:

• 1. Isolation of reactive organic molecules as metal-coordinated species

eg. carbene, cyclobutadiene. • 2. Reactivity modification via transition metal

coordination eg activation of alkenes and arenes to

nucleophilic addition.

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Metal Carbonyl Complexes M(CO)n

• Carbon Monoxide (CO) acts as a good ligand to d-block transition metals.

• The CO ligand has zero formal charge and donates an electron pair (2 electrons) to the metal centre.

• CO acts as a strong π-acceptor ligand and therefore stabilises complexes with metals in a low oxidation state

• Metal carbonyl complexes are important as starting materials for a wide range of organometallic complexes via ligand substitution reactions

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Synthesis of Metal Carbonyl Complexes

1. Direct reaction of CO with the metal powder

Ni + CO

Fe + CO

Ni(CO)4

Fe(CO)5

1 atmosphere CO, 25°C

100 atmosphere (bar) CO, 150°C

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2. Reduction of a metal halide in a CO atmosphere

VCl3 + 3Na + CO

WCl6 + 2Et3Al + CO

[V(CO)6]−

W(CO)6

Diglyme, 300 bar

70 bar, 50°C

Benzene

Di(2-methoxyethyl) etherDiglyme =

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Structures of first row d block (3d) homoleptic metal carbonyls

V

CO

CO

CO

OC

OC CO

-

Mn

CO

CO

CO

OC

Mn CO

CO

CO

OC

OC

CO

Cr

CO

CO

CO

OC

OC CO

CO

Fe

CO

CO

OC

OC

Co

CO

CO

CO

Co

OC

OCOC

OC

OC

Ni

CO

CO

OC

CO

Terminal CO

Bridgingμ2-CO

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Trends in metal carbonyl structuresThe 18-electron rule

Cr

CO

CO

CO

OC

OC CO

CO

Fe

CO

CO

OC

OC

Ni

CO

CO

OC

CO

Cr group 6Cr(0) d6

6 CO = 12 e−

Total 18 e−

Fe group 8Fe(0) d8

5 CO = 10 e−

Total 18 e−

Ni group 10Ni(0) d10

4 CO = 8 e−

Total 18 e−

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The 18-electron rule

• Metal valence d electrons + ligand electrons donated to metal sum to 18 (filled s, p and d orbitals on metal)

• Applies to complexes with good acceptor ligands (eg CO)

• Some exceptions especially 17- and 16- electron systems (see later).

• Can be used to predict structures of organometallics.

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Examples of electron counting:1. Cr(CO)6

CO

Cr

COOC CO

COOCGroup Number of e-

Cr0 6

CO x 6 12

Total 18

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2. Mn2(CO)10 (Metal-metal bonding) (Mn d7 has an uneven (odd) number of valence d

electrons)

CO

Mn

COOC

CO

OC

CO

Mn

COCO

OC

CO

Mn0 7

CO x 5 10

Mn – Mn 1

Total 18Formation of a Mn-Mnmetal-metal bond adds an extraelectron to the count at each metal

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3. Fe2(CO)9 (Bridging carbonyl ligands)

Fe Fe

OC

CO

COOC

OC

CO

CO

OC CO

Fe0 8

COterminal x 3 6

CObridging x 3 3

M – M 1

Total 18Terminal COBridging CO

μ2-bridging CO ligands provide 1 electron to each metal centre.Bridging CO usually observed for smaller metal centres:(i) First row (3d) metals (ii) metals on right hand side of d block eg Co not Mn

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4. Na[Mn(CO)5] (Charged species)

Total

Mn 7

CO x 5 10negative charge 1

•In Na[Mn(CO)5] the anion [Mn(CO)5]− is present.•Count by adding one extra electron for the negative charge.

Mn CO

C

C

C

C

O

O

O

O

-

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5. [V(CO)6] (An ‘anomalous’ case)

Total

v 5

CO x 6 12

• V(CO)6 is a 17-electron radical (has an unpaired electron and does not obey the 18-electron rule).•Readily reduced to 18-electron [V(CO)6]−

•V-V bonded bimetallic not formed

C

C

C

C

O

O

O

O

V

C

C

O

O

17

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6. [Ru3(CO)12] (A metal cluster)

Total

Ru 8

CO x 4 8

• Metal cluster: more than 2 metals in complex•Ru group 8 (like Fe) d8

•All CO terminal (larger 4d metal reduces possibility of bridging)

Ru

Ru

Ru

C

CC

C

CC

C

C

C

C

C

CO

O

O

O

O

O

O O

O

O

O

O Ru-Ru x 2 2

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7. Mo(CO)n (Use of the 18-electron rule to predict molecular formula)

• Mo is a group 6 metal (like Cr)• Mo(0) d6

• Mo(CO)n obeys 18-electron rule. Each CO provides 2 electrons to Mo

• 6 (Mo) + n x 2 = 18• n = 6. Formula is Mo(CO)6