lecture 1 organometallics(1)
DESCRIPTION
inorganicTRANSCRIPT
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Organometallic ChemistryCHEM 20312
Dr. Mark Whiteley 4.02g
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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