Organometallic compounds

Presented by :
Arvind Singh Heer
(M.Sc -Analytical Chemistry)
Organometallic CompoundsOrganometallic Compounds

Chemistry of compounds containing metal-carbon bonds.
In many complexes, both σ- and π-bonding exist between the
metal atom and carbon.
Types
Sandwich complexes, cluster compounds, and carbide clusters
(to name a few).

The 1st
– Ziese’s compound/salt (Sec. 13-1).
The organic molecule is attached to the metal via the π electrons of
the ethylene ligand.
Compounds with CO
Ni(CO)4 – Mond (purification of Ni).
The Big Boom in Organometallic Chemistry
Synthesis of ferrocene (Sec. 13-1).
Began the era of modern organometallic chemistry.

Organic Ligands andOrganic Ligands and
NomenclatureNomenclature
A number of ligands may bond through different number of atoms.
The number is indicated by η (eta) followed by a superscript.
Ferrocene – contains the pentahaptocyclopentadienyl ligand.
hapto means to fasten
Do a few others.

The 18-Electron RuleThe 18-Electron Rule
Total of 18 valence electrons on the central atom (there
are many exceptions). Table 13-1 (Sec. 13-3-1).
Cr(CO)6
(η5
-C5H5)Fe(CO)2Cl
(CO)5Mn-Mn(CO)5
(η3
-C5H5)(η5
-C5H5)Fe(CO)
In general, hydrocarbon ligands come before the metal.
HM(CO)5 The metal is in the 1st
row.

The 18-Electron RuleThe 18-Electron Rule
18 electrons represent a filled valence shell for a
transition metal.
Why do many complexes (if not most) violate the 18-
electron rule?
The 18-electron rule does not consider the type of bonding and
interactions. The interactions between the ligands and the
metal need to be identified to determine if the complex will
obey or violate the 18-electron rule. This treatment will also
identify why in many cases.

Interactions between theInteractions between the
Ligands and the MetalLigands and the Metal
Examine the MO diagram for Cr(CO)6.
This includes interactions between the d-orbitals and the σ-donor/π-
acceptor orbitals of the six ligands.
Understand this diagram in terms and strengths of the different types of
interactions.
18-electron is the most stable for this type of complex.
Assuming the d-orbitals to be at similar energy levels, which complex
would you predict to be the most stable?
Complexes that possess ligands that are both strong σ donors and π acceptors should
be the most likely to obey the 18-electron rule.

How about ligands that have different donor and acceptor
characteristics?
Ethylenediamine is a σ donor, but not as strong as CO. Why
affects does this have on the diagram studied previously?
The [Zn(en)3]2+
complex is stable. How many electrons?

How about TiCl6
2-
? It has 12 electrons. Can you justify this
with an interaction diagram?

Square-planar complexes (16-electron).
Examine Figure 13-11 (Section 13-3-3).
The ligand is a good σ donor and π acceptor.
Understand the interactions and influences on stabilization of the
complex.
The 16-electron square-planar complexes are mostly
encountered for d8
metals.
Oxidations states of +2 are common.

Ligands in OrganometallicLigands in Organometallic
Chemistry – Carbonyl ComplexesChemistry – Carbonyl Complexes
Examine the frontier orbitals
(HOMO and LUMO)
Synergistic effect
σ donor/π acceptor
Spectroscopic evidence?
Bond lengths are vibrational
frequencies.
Figure 5-14

How will the interaction diagram appear for a binary
octahedral compound?
HOMO – These will have the same symmetry characteristics as
a py orbital (previously).
Γred(HOMO) – A1g + Eg + T1u
LUMO – These will have the same symmetry characteristics as
the px and py orbitals (previously considered).
Γred(LUMO) – T1g + T2g + T1u + T2u

Bridging Modes of COBridging Modes of CO
CO can also form bridges
between two or more
metals.
Position of C-O stretching
mode. Why is there a general
decrease in frequency with
increasing metal centers?

Most binary carbonyl complexes obey the 18-electron rule.
Why?
Why doesn’t V(CO)6 form a dimer to obey the 18-electron
rule?
The tendency of CO to bridge transition metals decreases
going down the periodic table. Why?
No synthesis discussion.

Oxygen-bonded carbonyls
Occasionally, CO bonds
through the oxygen atom in
addition to the carbon atom.
Attachment of a Lewis acid to
the oxygen weakens the CO
bond.

Ligands Similar to COLigands Similar to CO
CS, CSe, CN-
, and N2
CN-
is able to bond readily to metals having higher oxidation
states.
CN-
is a good σ donor, but a weaker acceptor (cannot stabilize
metals of low oxidation state).
No NO complexes.

Hydride and DihydrogenHydride and Dihydrogen
ComplexesComplexes
Hydride complexes (e.g. [ReH9]2-
)
Only a 1s orbital of suitable energy for bonding
Must be a σ interaction (minimal basis set)
Co2(CO)8 + H2 → 2HCo(CO)4
Dihydrogen complexes
Ziese’s salt
What are the types of possible interactions? What happens to
the H-H bond? Extreme case?

Ligands Having ExtendedLigands Having Extended ππ
SystemsSystems
Linear π systems
Ethylene, allyl, and 1,3-
butadiene
Cyclic π systems
C3H3, C4H4, and Figure 13-
22.

Bonding InvolvingBonding Involving ππ SystemsSystems
Bonding between ethylene and a metal.
σ donation/π acceptance
If orbitals of appropriate symmetry are present (isolobal), an
interaction may occur (Fig. 13-23).
Construct an MO diagram.
π-allyl systems (trihapto ligand)
Examine Fig. 13-25, could construct MO interaction diagram.
[Mn(CO)5]-
+ C3H5Cl → (η1
-C3H5)Mn(CO)5 → (η3
-
C3H5)Mn(CO)4 + CO

CyclicCyclic ππ SystemsSystems
C5H5 (η1
, η3
, or η5
bonding modes (η4
can also be
observed)).
Ferrocene (η5
-C5H5)2Fe
Orbitals on the ligands and metal can interact if they have the
same symmetry.
Strongest interaction is between orbitals of similar energies.
What is the point group?
Let’s give it the treatment!!

Fullerene Complexes (anFullerene Complexes (an
immenseimmense ππ system)system)
Adducts to the oxygens of oxmium tetroxide
C60(OsO4)(4-t-butylpyridine)2
Complexes in which the fullerene itself behaves as a ligand
Fe(CO)4(η2
-C60), Mo(η5
-C5H5)2(η2
-C60)
Compounds containing encapsulated metals
UC60, Sc3C82

Fullerenes as LigandsFullerenes as Ligands
C60 behaves primarily as an electron deficient alkene. Bonds to
metals in a dihapto fashion through a C-C bond at the fusion of two
6-membered rings (Fig. 13-35).
[(C6H5)3P]2Pt(η2
-C2H4)+C60→[(C6H5)3P]2Pt(η2
-C60)
What affect does this have on the two carbon atoms?

Fullerenes ContainingFullerenes Containing
Encapsulated MetalsEncapsulated Metals
Cage organometallic
compounds
U@C60 and Sc3@C82

Complexes Containing M-C, M=C,Complexes Containing M-C, M=C,
and Mand M≡≡C BondsC Bonds

Alkyl Complexes (M-C)Alkyl Complexes (M-C)
Grignard reagents (Mg-alkyl bonds) and methyl lithium.
Grignard reagents can be used to synthesize organometallic
compounds containing an alkyl group
The interaction is largely through σ donation.
Metals containing only alkyl ligands are rare and usually unstable.

Carbene Complexes (M=C)Carbene Complexes (M=C)
Fisher-type and Schrock-type complexes.
What are the differences between the two different type of carbene
complexes (Table 13-6).

Bonding in Fisher carbene complexes.
σ donation and π back bonding (illustrate).
Complex is generally more stable if the carbene atom is
attached to a highly electronegative atom. The electronegative
atom participates in the π bonding.
Similar to a π-allyl system (illustrate, Fig. 13-41).
Can be represented as a hybrid structure.
What type of spectroscopic evidence would show the existence of
M=C?

Discuss the proton NMR of Cr(CO)5[C(OCH3)C6H5].
At high temperatures there is one signal from the methyl
protons and at low temperatures there is one signal. Why?

Carbyne (alkylidyne) ComplexesCarbyne (alkylidyne) Complexes
(M(M≡≡C)C)
Illustrate a compound.
Type of bonding
σ bond, plus two π bonds.
Neutral 3-electron donor.

Spectra Analysis andSpectra Analysis and
Characterization of OrganometallicCharacterization of Organometallic
CompoundsCompounds
X-ray crystallography
Infrared spectroscopy
NMR spectroscopy
Mass spectrometry
Elemental analysis
Others

Infrared (IR) SpectraInfrared (IR) Spectra
The number of IR bands depends on the molecular symmetry (IR
active modes).
Monocarbonyl complexes
Dicarbonyl complexes
Linear and bent
Three or more carbonyl on the complex (Table 13-7).
We will assume that all the IR active modes are visible and distinguishable.
Exercise caution when using this table.

Positions of IR BandsPositions of IR Bands
Terminal > doubly bridging > triply bridging
Why?
As π-acceptor ability increases, the C-O stretch decreases.
What may affect the ability to accept electron density into the π-
acceptor orbitals?

NMR SpectraNMR Spectra
Chemical shifts, splitting patterns, and coupling
constants are useful in characterizing environments of
atoms.
13
C NMR
Table 13-9 (unique carbon environments)
1
H NMR
Protons bonded to metals are strongly shielded (chemical
shifts)
Table 3-10
Ring whizzing
Using spectroscopy for identification.

ReferencesReferences
1. Organometallic Chemistry and Catalysis, Didier Astruc
2. Organometallic Chemistry, R.C. Mehrotra
3. Inorganic Chemistry: Principles of Structure and Reactivity, James
E. Huheey, Ellen A. Keiter, Richard L. Keiter, Okhil K. Medhi
4. Reaction Mechanisms of Inorganic and Organometallic Systems,
Robert B. Jordan; Professor of Chemistry, University of Alberta
5. http://www.chem.iitb.ac.in/~rmv/ch102/ic6.pdf
-Thank
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