Lecture 4- LIGANDS

Report
LECTURE 4
TRANSITION METAL ORGANOMETALLICS
LIGANDS
TRANSITION METALS
2
Early
Late
Middle
EARLY TRANSITION METALS
Groups 3, 4
3




Strongly electrophilic and oxophilic
Few redox reactions (exception: Ti)
Nearly always < 18e
Polar and very reactive M-C bonds
(to alkyl and aryl)
EARLY TRANSITION METALS

Groups 3, 4
Few d-electrons:
for "hard" s-donors (N/O/F)
 weak complexation of p-acceptors (olefins, phosphines)
 preference

Typical catalysis: Polymerization
Me
M
Me
M
M
etc
Me
"MIDDLE" TRANSITION METALS
5




Many accessible oxidation states
Mostly 18e
Ligands strongly bound
Strong, not very reactive M-C bonds
Groups 5-7
"MIDDLE" TRANSITION METALS


Groups 5-7
Preference for s-donor/p-acceptor combinations
(CO!)
Typical catalysis: Alkene and alkyne metathesis
CH2
CH2
M
CH2
CH2
M
CH2
CH2
M
CH2
CH2
CH2
LATE TRANSITION METALS
Groups 8-10 (and 11)
7


Many accessible oxidation states
Mostly 18e or 16e
16e common for square-planar complexes



Easy ligand association/dissociation
Weak, not very reactive M-C bonds
Even weaker, reactive M-O/M-N bonds
LATE TRANSITION METALS
Groups 8-10 (and 11)


Preference for s-donor/weak p-acceptor ligands
(phosphines)
Typical catalysis: Hydroformylation
H
H
M
M
O
M
CO
CO
O
M
O
O
H
M
H
H2
M
H
H2
M
ROWS
9
1st row
2nd row
3rd row
Transition-metal Organometallics
GOING DOWN
10
1st row:

often unpaired electrons

different spin states (HS/LS) accessible

"highest possible" oxidation states not very stable

MnO4- is a strong oxidant
2nd/3rd row:

nearly always "closed shell"

virtually same atomic radii (except Y/La)

highest oxidation states fairly stable


ReO4- is hardly oxidizing
2nd row often more reactive than 3rd
THE CARBONYL LIGAND
11
• In 1884 Ludwig Mond found his nickel valves were
being eaten away by CO. An experiment was
designed where he deliberately heated Ni powder in
a CO stream thus forming the volatile compound,
Ni(CO)4, the first metal carbonyl. It was also found that
upon further heating Ni(CO)4 decomposes to give pure
nickel. This Ni refining process still used today is known
as the Mond process.
• Having no net dipole moment, intermolecular forces are
relatively weak, allowing Ni(CO)4 to be liquid at room
temperature.
THE CARBONYL LIGAND
CO groups have a high tendency to stabilize M−M bonds; not
only are CO ligands relatively small but they also leave the metal
atom with a net charge similar to that in its elemental form
(electroneutrality principle).
“Stable complexes are those with structures such that each atom has
only a small electric charge. Stable M-L bond formation generally
reduces the positive charge on the metal as well as the negative
charge and/or e- density on the ligand. The result is that the actual
charge on the metal is not accurately reflected in its formal
oxidation state”

12
- Pauling; The Nature of the Chemical Bond, 3rd Ed.;1960, pg. 172.
CARBONYL
13


CO also has the ability to stabilize polyanionic species
by acting as a strong p acceptor and delocalizing the
negative charge over the CO oxygens.
Na4[Cr(CO)4] has the extraordinarily low ν(CO) of
1462 cm−1, the extremely high anionic charge on the
complex, and ion pairing of Na+ to the carbonyl
oxygen contribute to the reduced CO bond order by
favoring the MC−ONa resonance
THE CARBONYL LIGAND
14


As the CO ligand is small and strongly bound, many
will usually bind as are required to achieve
coordinative saturation, e.g. V(CO)7
Metal carbonyls, in common with metal hydrides,
show a strong preference for the 18e
configuration.
METAL CARBONYLS – STRUCTURE and BONDING
15

CO is an unsaturated ligand, by virtue of the CO
multiple bond.
This contrasts to hard ligands, which are
σ donors, and often p donors, too.
• CO can act as a spectator or an actor
ligand.

CO is classed as a soft ligand because it is capable
of accepting metal dp electrons by back bonding,
i.e. it is a s-donor p-acceptor ligand.
Overview of Organometallic Chemistry
METAL CARBONYLS – STRUCTURE and BONDING
M
C
O
The CARBONYL LIGAND
17


In the CO molecule both the C and the O atoms are
sp hybridized.
The singly occupied sp and pz orbitals on each atom
form a σ and a p bond, respectively.
Frontier orbitals of free CO showing the
polarization of the pz orbital.
THE CARBONYL LIGAND
18

This leaves the C py orbital empty, and the O py
orbital doubly occupied, and so the second p bond is
formed only after we have formed a dative bond by
transfer of the lone pair of O py electrons into the
empty C py orbital.
• This transfer leads to a C−−O+ polarization of the
molecule, which is almost exactly canceled out by a
partial C+−O− polarization of all three bonding
orbitals because of the higher electronegativity of
oxygen.
• The free CO molecule therefore has a net dipole
moment very close to zero.
CARBONYL LIGAND
20





A metal orbital forms a s bond with HOMO orbital of CO.
The HOMO is a s orbital based on C (due to the higher electronegativity of
O its orbitals have lower energy).
The metal orbitals form a p bond with the CO p* LUMO (again polarized
toward C)
The metal HOMO, the filled M dp orbital, back donates to the CO LUMO
increasing electron density at both C and O because CO p* has both C and
O character.
The result is that C becomes more positive on coordination, and O becomes
more negative. This translates into a polarization of the CO on binding.
THE CARBONYL LIGAND
21




This metal-induced polarization chemically activates the CO ligand.
It makes the carbon more sensitive to nucleophilic and the oxygen
more sensitive to electrophilic attack.
The polarization will be modulated by the effect of the other ligands
on the metal and by the net charge on the complex.
In LnM(CO), the CO carbon becomes particularly + in character if
the L groups are good p acids or if the complex is cationic, e.g.
Mo(CO)6 or [Mn(CO)6]+, because the CO-to-metal s-donor electron
transfer will be enhanced at the expense of the metal to CO back
donation.
CARBONYL
22


If the L groups are good donors or the complex is anionic, e.g. Cp
W(CO) or 2[W(CO)5]2−, back donation will be encouraged, the CO
carbon will lose its pronounced + charge, but the CO oxygen will
become significantly −.
The range can be represented in valence bond terms the extreme in
which CO acts as a pure s donor, through to the extreme in which
both the p∗x and p∗y are both fully engaged in back bonding.
CARBONYLs and IR BANDS
23



The high intensity of the CO stretching bands (a result
of polarization on binding) means that IR spectroscopy
is extremely useful.
From the band position, we can tell how good the metal
is as a p base.
From the number and pattern of the bands, we can tell
the number and stereochemistry of the CO’s present.
CARBONYL LIGAND
24


We can tell the bond order of the CO ligand by recording the
M-CO IR spectrum. The normal range of the C-O stretching
frequency, (CO) is 1820–2150 cm−1. Free C-O stretch at
2143 cm-1. Lower energy for stretching mode means C-O bond
is weaker.
As the metal to CO p* back bonding becomes more important,
we populate an orbital that is antibonding with respect to the
C=O bond, and so we lengthen and weaken the CO bond, i.e.
the M−C p bond is made at the expense of the C=O p bond.
CARBONYL LIGAND
25


Strong s donor co-ligands or a negative charge on the metal result in CO
stretches at lower frequency. Why?
v(CO) cm-1
[V(CO)6]1859
Cr(CO)6
2000
[Mn(CO)6 ]+
2100
[Fe(CO)6 ]2+
2204
The greater the ability of a metal to donate electrons to the p* orbitals of
CO, the lower the energy of the C-O stretching vibration.
SAMPLE EXERCISE
26

On the basis of the carbonyl complexes in the table
shown, predict the approximate position (in cm-1) of
the C-O streching band in [Ti(CO)6]2-
CARBONYL LIGANDS
27



Carbonyls bound to very poor p-donor metals have very high
frequency ν(CO) bands as a result of weak back donation.
When these appear to high energy of the 2143 cm−1 band of free
CO, the complexes are sometimes called non-classical carbonyls.
Even d0 species can bind CO, for example, the nonclassical, formally
d0 Zr(IV) carbonyl complexes, [Cp2Zr(S2)(CO)] has a ν(CO) stretching
frequency of 2057 cm−1.
CARBONYL LIGANDS
28
The highest oxidation state carbonyl known is trans[OsO2(CO)4]2+ with ν(CO) = 2253 cm−1.
Carbonyls with exceptionally low ν(CO) frequencies are
found for negative oxidation states (e.g., [Ti(CO) ]2−; ν(CO)
= 1747 cm−1) or where a single CO is accompanied by non
p-acceptor ligands (e.g., [ReCl(CO)(PMe3)4]; ν(CO) = 1820
cm−1); these show short M−C and long C−O bonds.
CARBONYL LIGAND
29


One of the most extreme weak p-donor examples is
[Ir(CO)6]3+ with ν(CO) bands at 2254, 2276, and 2295
cm−1.
The X-ray structure of the related complex [IrCl(CO)5]2+
shows the long M−C [2.02(2)A° ] and short C−O
[1.08(2)A° ] distances expected.
Overview of Organometallic Chemistry
SYNTHESIS
30
1.
Direct reaction of a transition metal with CO.
Ni
+
4CO
Ni(CO)4
This method requires that the metal already be in a
reduced state because only p-basic metals can bind
CO.
SYNTHESIS
31
2. Reductive carbonylation (reducing agent plus CO gas):
CrCl3 + 6CO + Al
Re2O7 + 17CO
NiSO4
+ CO + S2O42-
Cr(CO)6 + AlCl3 (catalyzed by AlCl3)
Re2(CO)10
Ni(CO)4
+ 7 CO2 (CO as RA)
SYNTHESIS
32
3. Thermal or photochemical reaction of other binary
carbonyls.
Fe(CO)5
hv
Fe2(CO)9
Lesser known:
From organic carbonyls.
BRIDGING MODES
33
CO has a high tendency to bridge two metals (μ2-CO)
Electron count here is unchanged either side of equilibrium
In most cases the M−M bond accompanies the CO bridging group.
The CO stretching frequency in the IR spectrum falls to 1720–1850
cm−1 on bridging.
Type of CO
v(CO) cm -1
Free CO
2143
terminal M-CO
1850-2120
bridging CO
1700-1850
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BRIDGING
34



Consistent with the idea of a nucleophilic attack by a second
metal, a bridging CO is more basic at O than the terminal
ligand.
Thus a bridging CO ligand will bind a Lewis acid more
strongly than a terminal CO ligand.
Equilibrium can therefore be shifted in the previous reaction
scheme.
BRIDGING
35
• Triply and even quadruply bridging CO groups are
also known in metal cluster compounds.
For example, (Cp∗Co)3(μ3-CO)2
• These have CO stretching frequencies in the range
of 1600–1730 cm−1.
REACTIONS OF METAL CARBONYLS
36
All reactions of the CO ligand depend on the
polarization of the CO upon binding, and so change
in importance as the co-ligands and net charge
change.
1. Nucleophilic attack at the Carbon:
Reactions
Hydride attack at the C atom of CO here produces the unusual
formyl ligand, which is important in CO reduction to MeOH.
It is stable in this case because the final 18e complex provides no
empty site for rearrangement to a hydridocarbonyl complex (aelimination).
Reactions
38
2. Electrophilic attack at Oxygen
3. Migratory Insertion:
BRIDGING
39



There also exists the semi-bridging carbonyl in which the CO is
neither fully terminal nor fully bridging but intermediate between
the two.
This is one of the many cases in organometallic chemistry where a
stable species is intermediate in character between two bonding
types.
Below each semi-bridging CO is bending in response to the
second metal atom
LIGANDS SIMILAR TO CO
40
CS, Cse, CTe do not exist as a stable free molecule and therefore do
not provide a ready ligand source.
CN- and N2 complexes
CN- - stronger s donor than CO
- very similar to CO when it interacts with metal orbitals
- weaker p acceptor (consequence of the negative charge)
Dinitrogen is a weaker s donor and p acceptor vs. CO. However, still
of interest in reactions that might stimulate nitrogen fixation.
PROBLEM SETS
41
From Spessard and Meissler.
4-1 to 4-5, 4-8, 4-10, 4-11, 4-13
5-1, 5-3, 5-6, 5-7, 5-9, 5-13 to 5-14
6-1, 6-2, 6-3, 6-4, 6-6, 6-8
IR SPECTRA
42
No. of Bands:
Monocarbonyl complexes have single possible C-O
stretching mode – single IR band
Dicarbonyl complexes:
Linear and bent
Complexes with 3 or more carbonyls
43
Prediction of exact no. of carbonyl bands complex but can be
determined using group theory.
For convenience refer to a table.
Although can predict using a table:
- some bands may overlap
- may have very low intensity
- if isomers present, difficult to sort out
POSITIONS OF IR BANDS:
44
Increase in negative charge of the complex, causes
a reduction in the energy of the CO band.
 The bonding mode of the CO
terminal CO > double bridged (2) > triply
bridged (3)

POSITION OF IR BANDS
45

Other ligands present also affects position of IR bands. For example for the
complex Ni(CO)3L:
L
v(CO), cm-1
PF3
2111
PCl3
2097
PPh3
2069
The greater the electron density on the metal, the greater the back bonding to
CO, the lower the energy of the carbonyl stretching vibration.
MAIN GROUP vs. BINARY CARBONYL COMPLEXES
46
Electron short of filled shell
Examples of electronically equivalent species
Main Group
Metal Carbonyl
1
Cl, Br, I
Mn(CO)5, Co(CO)4
2
S
Fe(CO)4, Os(CO)4
3
P
Co(CO)3, Ir(CO)3
Cl vs. Co(CO)4
Characteristics
Cl
Co(CO)4
Ion with closed shell configuration
Cl-
[Co(CO)4]-
Nuetral dimer
Cl2
Co2(CO)8
Interhalogen compound
BrCl
ICo(CO)4
Binary Acid
HCl
HCo(CO)4
Insoluble Heavy metal Salts
AgCl
AgCo(CO)4
Disproportionation by Lewis Bases
Cl2 + Me3N Me3NCl+ + Cl-
Co2(CO)8 + C5H10NH 
[C5H10NHCo(CO)4]+ + [Co(CO)4]-
47
p BONDED Ligands
48
Alkene Complexes
Alkyne Complexes
Allyl Complexes
Diene Complexes
Cyclopentadienyl Complexes
Arene Complexes
Metallacycles
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TRANSITION METAL – ALKENE COMPLEXES
49



The report in 1825 by William Zeise of crystals with
composition, KCl.PtCl2.Ethylene, prepared from KPtCl4 and
EtOH was a topic of controversy for many years due to the
nature of Zeise’s structure - only possible by the dehydration of
EtOH.
Proof of Zeise’s formulation came 13 years later when
Birnbaum isolated the complex from a solution of platinic acid,
H2PtCl6.6H20, treated with ethylene.
Zeise’s salt was the first organometallic compound to be
Overview of Organometallic Chemistry
isolated in pure form.
TRANSITION METAL – ALKENE COMPLEXES
50
• The p-acid ligand donates electron density into a metal d-orbital from a psymmetry bonding orbital between the carbon atoms.
• The metal donates electrons back from a filled d-orbital into the empty p*
antibonding orbital of the ligand (similar to dihydrogen s-complexes)
• Both of these effects tend to reduce the C-C bond order, leading to an elongated
C-C distance and a lowering its vibrational frequency.
TRANSITION METAL –ALKENE COMPLEXES
51


In the nickel compound Ni(CH2CH2)(PPh3)2 the C-C bond
distance is 143 pm (vs. 134 pm for free ethylene).
The interaction can cause carbon atoms to "rehybridize“,
for e.g in metal alkene complexes from sp2 towards sp3,
which is indicated by the bending of the hydrogen
atoms on the ethylene back away from the metal.
Molecular geometry of Zeise’s salt
(neutron diffraction)
52
Overview of Organometallic Chemistry
53






The PtCI3 moiety forms a nearly planar group with the Pt atom.
The Pt-CI bond trans to the ethylene group (2.340 A) is significantly longer
than the cis Pt-CI bonds (2.302 and 2.303 A) – trans effect !!
The C atoms are approximately equidistant from the Pt atom (2.128 and
2.135 A).
The distance from the midpoint of the C-C bond to the Pt atom is 2.022 A.
The C-C distance, 1.375 A, is slightly longer than the value found in free
ethylene (1.337 A), indicating some dp-pp* back-bonding from the
platinum atom to C2H4.
Back-bonding is also indicated by a bending of the four hydrogen atoms
away from the Pt atom.
ALKYNE COMPLEXES
54


Alkynes behave in ways broadly similar to alkenes,
but being more electronegative, they tend to
encourage back donation and bind more strong
The substituents tend to fold back away from the
metal by 30–40 in the complex, and the M−C
distances are slightly shorter than in the
corresponding alkene complexes.
Overview of Organometallic Chemistry
ALKYNES
56
• Metals can stabilize alkynes that cannot be
observed as free compounds.
ALKYNES
57
• Alkynes can also form complexes that appear to be
coordinatively unsaturated.
ALKYNES
58

Coordinatively unsaturated alkynes?
Overview of Organometallic Chemistry
ALKYNE
59
• In such cases the alkyne also donates its second
C=C π-bonding orbital, which lies at right angles to
the first.
• The alkyne is now a 4e donor.
ALKYNES
60

Four electron alkyne complexes are rare for d6
metals because of a 4e repulsion between the filled
metal dp and the second alkyne C=C π-bonding
pair.
BRIDGING METAL ALKYNE COMPLEXES
61
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ALKYNES
62

Alkynes readily bridge an M−M bond, in which case
they can act as conventional 2e donors to each
metal.
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ALKYNES
63

The alternative tetrahedrane form is the equivalent
of the metalacyclopropane picture for such a
system.
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TAUTOMERIZATION
64
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TRANSITION METAL ALLYL COMPLEXES
65




The allyl group, commonly a spectator ligand, binds
in one of two ways.
In the 1 form it is a simple X‐type ligand like Me
In the 3 form it acts as a LX -enyl ligand.
It is often useful to think in terms of the resonance
forms (2e + 1e donor)
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ALLYL
66
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ALLYL
67
• As the number of nodes on the allyl ligand increase the MOs of the free
ligand increase in energy, i.e. become less stable.
ALLYL
68
ALLYL
69
Frontier molecular orbitals of the metal allyl fragment:
 1 is occupied by 2 electrons and has appropriate symmetry,
energy and orientation to overlap with a suitable metal ds orbital.
 2 is also occupied by 2 electrons (ionic model) and has
appropriate symmetry, energy and
orientation to overlap with a suitable metal dp orbital.
 3 is unoccupied and has appropriate symmetry, energy and
orientation to overlap with a suitable metal dp orbital for back
donation.
ALLYL
70
• The plane of the allyl is slanted at an angle  with respect to the coordination
polyhedron around the metal (  is usually 5◦–10◦ ).
• The extent of orbital overlap between 2 and the dxy orbital on the metal is
improved if the allyl group moves in this way.
ALLYL
71
• The terminal CH2 groups of the allyl are twisted about the C−C vector so as to rotate
the anti hydrogens (Ha) away from the metal, and the syn hydrogens (Hs) toward
the metal.
• This allows the bonding p orbital on these carbons to point more directly toward
the metal, thus further improving the M‐L overlap.
• The 3‐allyl group often shows exchange of the syn and anti substituents. One
mechanism goes through an 1‐allyl intermediate.
• This kind of exchange can affect the appearance of the 1H NMR spectrum and also
means that an allyl complex of
a given stereochemistry may rearrange with time.
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ALLYL
72
SYNTHESIS
73
1.
From an alkene via oxidative addition:
2. Nucleophilic attack by an allyl compound
(transmetallation)
3. Electrophilic attack by an allyl compound:
Overview of Organometallic Chemistry
SYNTHESIS
74
4. From diene complexes:
TRANSITION METAL DIENE COMPLEXES
75


The diene ligand usually acts as a 4e donor in its cisoid
conformation.
Analogous to metal alkene systems the LX2 (enediyl or σ2π)
form to the metalacyclopropane extreme.
• The
L2 form is rarely seen with the LX2 form becoming more
important as the back donation increases.
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Butadiene
76
• The frontier orbitals of the butadiene, 2
(HOMO) and 3 (LUMO), are the most
important in bonding to the metal.
• Depletion of electron density in 2 by p
donation to the metal and population of
3 by back donation from the metal
lengthens the C1‐C2 bond and shortens the
C2‐C3 bond because 2 is C1‐C2
antibonding and 3 is C2‐C3 bonding.
Overview of Organometallic Chemistry
BUTADIENE
77


Binding to a metal usually depletes the ligand HOMO
and fills the ligand LUMO. This is the main reason why
binding has such a profound effect on the chemical
character of a ligand.
The structure of the bound form of a ligand is often
similar to that of the first excited state of the free ligand
because to reach this state we promote an electron from
the HOMO to the LUMO.
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CYCLOPENTADIENE - Ferrocene
78



The two cyclopentadienyl (Cp) rings of ferrocene may be orientated
in the two extremes of either an eclipsed (D5h) or staggered (D5d)
conformation.
The energy of rotation about the Fe Cp axis Fe‐is very small (~ 4
kJmol‐1) and ground state structures of ferrocene may show either of
these conformations.
There is also very little difference in electronic states between the
D5h and D5d symmetries however the D5d point group irreducible
representations are used here in the description of the electronic
structure of ferrocene as they simplify the symmetry matching of
ligand molecular orbitals and metal atomic orbitals.
FERROCENE
79



The primary orbital interactions that form the
metal‐ligand bonds in ferrocene occur between the Fe
orbitals and the p‐orbitals of the Cp ligand.
If D5d symmetry is assumed, so that there is a centre of
symmetry in the ferrocene molecule through the Fe atom
there will be centro‐symmetric (g) and antisymmetric (u)
combinations.
The five p‐orbitals on the planar Cp ring (D5h symmetry)
can be combined to produce five molecular orbitals.
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CYCLOPENTADIENYL
80
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CYCLOPENTADIENYL
81

For a bis‐cyclopentadienyl metal complex
(5‐Cp)2M , such as ferrocene, the p‐orbitals of the
two Cp ligands are combined pairwise to form the
symmetry‐adapted linear combination of molecular
orbitals (SALC’s).
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83
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CYCLOBUTADIENE
85




Most of the neutral ligands we have studied (apart from
carbenes) have been stable in the free state.
Cyclobutadienes on the other hand are highly reactive when
not complexed to a late transition metal.
The free molecule, with four p electrons, is antiaromatic and
rectangular, but the ligand is square and appears to be
aromatic.
By populating the LUMO of the free diene the ligand is
stabilized by metal back donation.
Overview of Organometallic Chemistry
CYCLOBUTADIENE
86



Thus by gaining partial control of two more p electrons
the diene attains an electronic structure resembling that
of the aromatic six p‐electron dianion.
Ligand‐to‐metal σ donation prevents the ligand from
accumulating excessive negative charge.
This again is a clear example of the free and bound
forms of the ligand being substantially different from
one another.
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CYCLOBUTADIENE
87
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METAL HYDRIDE COMPLEXES
88



Main group metal hydrides play an important role as
reducing agents (e.g. LiH, NaH,LiAlH4, LiBH4).
The transition metal M-H bond can undergo insertion with a
wide variety of unsaturated compounds to give stable
species or reaction intermediates containing M-C bonds
They are not only synthetically useful but are extremely
important intermediates in a number of catalytic cycles.
89
Overview of Organometallic Chemistry
METAL HYDRIDE PREPARATION
90
1.
Protonation (requires an electron rich basic metal
center)
2. From Hydride donors (main group metal hydrides)
Overview of Organometallic Chemistry
HYDRIDE
91
3. From H2 (via oxidative addition – requires a
coordinatively unsaturated metal center)
4. From a ligand (-elimination)
Overview of Organometallic Chemistry
HYDRIDE
92



A metal hydride my have acidic or basic character depending on the
electronic nature of the metal involved (and of course its ligand set).
Early transition metal hydrides tend to carry significant negative
charge on the H atom whereas later more electronegative transition
metals favour a more positive charge on the H atom (the term
Hydride should therefore not be taken literally).
Reactivity can also depend upon the substrate, e.g. CpW(CO)3H is a
H+ donor to simple bases, a H• donor to toward styrene and a H−
donor to carbonium ions.
Overview of Organometallic Chemistry
HYDRIDES
93


HCo(CO)4 is a strong acid due to the electron
withdrawing effect of the s-donating, p-accepting
CO ligands on the Co(I) center.
With s-donating, p-donating ligands the hydride
can become quite basic and reactive towards H−
transfer.
Overview of Organometallic Chemistry
HYDRIDES
95

Applied in catalysis
to promote ketone
reduction e.g.
acetone to
isopropanol. The
formate ion (HCO2−
) is used as a source
of H− with
liberation of CO2.
Overview of Organometallic Chemistry
HYDRIDES
96

The same catalyst can
be used for reduction
of CO2 under the
appropriate
conditions.
Overview of Organometallic Chemistry
DIHYDROGEN COMPLEXES
97

An electrophile E+ can react with an X-H bond to
give a s complex 1, in which the X-H bond acts as a
2e donor. (not to be confused with s-bonding
Hydrides)
Overview of Organometallic Chemistry
DIHYDROGEN COMPLEXES
98

1 a and 1 b show two common ways of representing
1. Coordination to E+ alters the chemical properties
of the X-H bond and can activate it either for
nucleophilic attack at X or deprotonation.
Overview of Organometallic Chemistry
DIHYDROGEN COMPLEXES
99


The X-H bond is always coordinated side-on to E+,
as in 1.
s complexes bind by donation of the X-H sbonding electrons in a 2e 3 center bond to the
metal.
Overview of Organometallic Chemistry
100
• X = H, Si, Sn, B, or P…..at least one H must always be
present.
• The H atom has a small atomic radius and carries no
lone pairs or other substituent's, allowing the hydrogen
end of the X-H bond to approach close to the metal and
so allow the filled M dp orbital to back-bond relatively
strongly onto the lobe of the X-H s* orbital that is
located on the H atom.
Overview of Organometallic Chemistry
DIHYDROGEN COMPLEXES
101
The bonding picture for a s complex.
a) Only the 1 orbital which bonds
over all three centers, is occupied.
Occupation of 2 would lead to
the opening of one edge of the
triangle (nodal plane marked as a
dotted line).
b) In an M-(H-X) complex the electrons
of the X-H s bond are donated to
an empty metal d-orbital. This is
analogous to the binding of the lone
pair on NH3 to a metal atom.
c) Electron density from the M(dp) orbital is
donated to the X-H s* orbital (back-donation).
This resembles M(dp) + CO(p*) back-donation
and is unique to transition metal
s complexes.
Overview of Organometallic Chemistry
102

An isolable complex must have some backbonding,
but strong back-donation leads to cleavage of the
X-H bond by oxidative addition to give an X-M-H
complex.
Overview of Organometallic Chemistry
103



In complexes with weak back-bonding, the length of X-H
bond is similar to that in free X-H.
The acidity and electrophilicity of X-H can be strongly
enhanced, however, because s bonding reduces the
electron density in the X-H unit.
Stronger back-donation can lead to s complexes with
elongated X-H bonds and reduced electrophilicity of
the X-H group.
Overview of Organometallic Chemistry
104
Overview of Organometallic Chemistry
105
“We propose the term “agostic” which will be used to
discuss the various manifestations of covalent interactions
between carbon-hydrogen groups and transition metal
centres in organometallic compounds. The word agostic
will be used to refer specifically to situations in which a
hydrogen atom is covalently bonded simultaneously to
both a carbon atom and to a transition metal atom.”
Overview of Organometallic Chemistry
AGOSTIC BONDS
106



The β C−H bond is bound to the metal in a way that
suggests that the alkyl is beginning the approach to the
transition state for β elimination.
These agostic alkyls can be detected by X-ray or
neutron crystal structural work and by the high-field shift
of the agostic H in the proton NMR.
The lowering of the J(C,H) and ν(CH) in the NMR and IR
spectra, respectively, on binding is symptomatic of the
reduced C−H bond order in the agostic system.
Overview of Organometallic Chemistry
107
• The reason that β elimination does not occur is that the d0 Ti has no electron density
to back donate into the σ∗ orbital of the C−H bond.
• This back donation breaks the C−H bond in the β-elimination reaction, much as
happens in oxidative addition.
Overview of Organometallic Chemistry
108


Agostic binding of C−H bonds also provides a way
to stabilize coordinatively unsaturated species.
They are also found in transition states for reactions
such as alkene insertion/β elimination either by
experiment or in theoretical work.
Overview of Organometallic Chemistry
109
Overview of Organometallic Chemistry
M-H and M-C s-bonds
110
M
H
Hydride

M
C
Alkyl

C
M
C
M
C
M
Vinyl (alkenyl)
C
Acetylide (alkynyl)
Aryl
Transition-metal Organometallics

Synthesis of metal alkyls
111

Metathesis
TiCl4 + 4 BzMgCl
TiBz4 + 4 MgCl2
(Bz = benzyl, C6H5CH2)

Electrophilic attack on metal
Mn(CO)5

MeI
Insertion
Ar
MeMn(CO)5
Ar
N
N
N
Co H
C2H4
N
N
Co Et
N
Ar
Transition-metal Organometallics
Ar
Synthesis of metal alkyls
112

Oxidative addition
 often
starts with electrophilic attack
O
O
O
Rh
L
L
Me
O
Rh
L
L
L = P(OPh)3
I
L
MeI
O
I
Rh
O
Me
L
I
Transition-metal Organometallics
L = PPh3
Decomposition of metal alkyls
113
Dominant: -hydrogen elimination
M
H
M
M
H
H
M
Alternatives:
H

homolysis

a/g/-eliminations

reductive elimination (especially with H or another alkyl)

ligand metallation
Transition-metal Organometallics
How to prevent -hydrogen elimination ?
114

No -hydrogen
CH3, CH2CMe3, CH2SiMe3, CH2Ph

No empty site cis to alkyl
Et
H
N
Co
N
O

H
O
N
N
O
OH2
Product of elimination unstable
Transition-metal Organometallics
?
How to prevent -hydrogen elimination ?
115

Planar transition state inaccessible
H
L2Pt
H
L2Pt H
L2Pt
???
even for 5-membered metallacycles -elimination is difficult !
(basis of selective ethene trimerization)
Transition-metal Organometallics
Reactions of metal alkyls
116

Insertion, of both polar and non-polar C=X bonds:
 olefins,
acetylenes, allenes, dienes
 (ketones etc)
 CO, isocyanides

Reductive elimination
Transition-metal Organometallics
CARBENE COMPLEXES

117



The concept of a double bond between transition metals and carbon
constitutes one of the most important elements in the field of
organometallic chemistry
The notion of a metal–carbon double bond was first brought forward
by Fischer and Maasbol in 1964 with the synthesis of
(CO)5W=C(Ph)(OMe)
Soon after the discovery of Fischer type complexes their chemistry
was systematically explored and they have been since well
established as valuable species in organic synthesis as well as in
catalytic processes
Schrock later prepared a number of tantalum complexes including
(Np) Ta=CH(CMe ) and (5-Cp)2MeTa=CH2
CARBENES
118


Two different patterns of reactivity emerged during
the development of these systems resulting in their
classification as Fischer and Schrock type carbenes.
Each represents a different formulation of the
bonding of the -CR2 group to the metal and real
cases fall somewhere between the two.
Overview of Organometallic Chemistry
CARBENES
120
Overview of Organometallic Chemistry
CARBENES
121
• Free carbene CH2 has two distinct spin isomers: singlet and triplet
– not resonance forms (sinlget ↔ triplet resonance forbidden)
• Singlet and triplet forms have different H-C-H angles
• In the singlet state 2e- are paired up in the sp2 orbital leaving
the pz orbital unoccupied
• In the triplet state both the sp2 and p orbitals are singly occupied
Overview of Organometallic Chemistry
CARBENES
122
• (a) Singlet and triplet forms of a carbene
• (b) In the Fischer case, direct CM donation predominates and the
carbon tends to be positively charged.
• (c) In the Schrock case, two covalent bonds are formed, each
polarized toward the carbon
giving it a negative charge.
Overview of Organometallic Chemistry
CARBENES
123




Taylor and Hall used ab-initio calculations to differentiate between the
electronic structures of Fischer and Schrock type carbene complexes.
Calculations on a variety of free carbenes indicated that: heteroatom and
phenyl substituents preferentially stabilize a singlet ground state.
alkyl and hydride substituents stabilized a triplet ground state at the
carbene carbon.
Carbenes are both thermodynamically and kinetically unstable therefore
forming very strong metal-carbene bonds disfavoring dissociation e.g. just as
pramagnetic triplet :CH2 can dimerize to form diamagnetic H2C=CH2, it also
binds to a triplet LnM fragment to give a diamagnetic LnM=CH2 complex.
FISCHER CARBENES
124
Overview of Organometallic Chemistry
SHROCK CARBENES
125
• A Schrock carbene forms two covalent bonds via unpaired
electrons.
• Each M-C bond is polarized towards the carbene carbon because
C is more electronegative than M, leading to a nucleophillic carbene
carbon.
126
Overview of Organometallic Chemistry
SCHROCK CARBENE SYNTHESIS
127


High valent metal alkyls of the early transition metals can undergo proton
abstraction at the a carbon to give nucleophillic Schrock carbenes
• This reaction is believed to involve an a-proton abstraction (possibly
agostic) by a neighbouring Np ligand liberating tBuMe.
128
• One requirement of this a-abstraction reaction is that the molecule
must be sterically crowded.
• For example, simple substitution of Cl in Np2TaCl3 with the bulky Cp
or PMe3 ligands induces a-abstraction and tBuMe elimination
producing the corresponding Schrock carbene complex.
129
• On replacing the Np ligand with the benzyl ligand a more
sterically demanding ligand set is required to induce a-proton
abstraction liberating toluene to produce corresponding Schrock
carbene complex.
• Typically 2 Cp rings can be used or even
pentamethylcyclopentadiene (Cp*)
Overview of Organometallic Chemistry
REACTIONS
130
• Their nucleophillic character allows them to form
adducts with Lewis acids.
• They react with ketones in a similar fashion as
Wittig (Ph3P=CH2) reagents
Overview of Organometallic Chemistry
REACTIONS
131

Similar to Fischer carbenes Schrock type complexes
also react with alkenes and alkynes to form
metalacycles
Overview of Organometallic Chemistry
132

Schrock type complexes react with alkynes to form
metalacyclobutenes which can rearrange to form the
p-extended carbene-ene systems
METAL CARBYNES
133


Have similar bonding formulations as per Fischer
and Schrock carbenes
The free carbyne can be of doublet (Fischer) or
quartet (Schrock) multiplicity
Overview of Organometallic Chemistry
METAL CARBYNES
134




The carbyne ligand is linear
Carbyne carbon is sp hybridized
The MC bond is very short (1.65 – 1.90 Å)
Characteristic low-field 13CNMR resonance in the
range +250 to +400 ppm
Overview of Organometallic Chemistry
FISCHER CARBYNES
135
• A doublet (Fischer) carbyne is sp hybridized
• Contains one filled sp orbital capable of donating 2e- to a metal
centre
• Contains one singly occupied p orbital capable forming an
additional p bond
• The remaining empty p orbital is capable of MC p back
donation
• 3e- donor covalent model / 4e- donor ionic model
Overview of Organometallic Chemistry
SCHROCK CARBYNE
136
•A
quartet (Schrock) carbyne is also sp hybridized
• Contains three singly occupied orbitals (one sp and two p)
capable of forming three covalent
M-C bonds (one s and two p bonds)
• This class of ligand is X3-type
• 3e- ligand in covalent model (or 6e- ionic model)
Overview of Organometallic Chemistry
SYNTHESIS
137

Fischer first prepared metal carbyne complexes by
the electrophilic abstraction of methoxy from a
methoxy methyl Fischer carbene.
SYNTHESIS
138


In a more general approach, Schrock carbynes can
be prepared by deprotonation of an a-CH
Intramolecular oxidative addition of a bound
Schrock carbene (a elimination)
Overview of Organometallic Chemistry
SYNTHESIS
139

Metathesis of tertiary butoxide (tBuO) complexes
(triple bi-nuclear oxidative addition, i.e. +III change
in oxidation state)
REACTIONS
140


Fischer carbynes are electrophillic and thus prone
to nucleophillic attack.
Nucleophiles such as PMe3, pyridine, alkyl lithiums,
and isonitriles react with Fischer carbynes to give the
corresponding Fischer carbene complex.
Overview of Organometallic Chemistry
REACTIONS
141
• Alternatively the nucleophile may attack the metal
centre producing a ketenyl complex
Overview of Organometallic Chemistry
REACTIONS
142
• In contrast, Schrock carbynes are nucleophillic and
prone to attack by electrophiles
Overview of Organometallic Chemistry
PHOSPHINES
143



Tertiary phosphines, PR3, are important because they constitute one
of the few series of ligands in which electronic and steric properties
can be altered in a systematic and predictable way over a very
wide range by varying R.
They also stabilize an exceptionally wide variety of ligands of
interest to the organometallic chemist as their phosphine complexes
(R3P)nM−L.
Phosphines are more commonly spectator than actor ligands.
Overview of Organometallic Chemistry
PHOSPHINES
144
Overview of Organometallic Chemistry
PHOSPHINES
145



Like NR3, phosphines have a lone pair on the central atom
that can be donated to a metal.
Unlike NR3, they are also p-acids, to an extent that depends
on the nature of the R groups present on the PR3 ligand.
For alkyl phosphines, the p acidity is weak; aryl,
dialkylamino, and alkoxy groups are successively more
effective in promoting p acidity.
PHOSPHINES
146

In the extreme case of PF3, the p acidity becomes as
great as that found for CO!
In the case of CO the p* orbital accepts electrons from
the metal.
The σ* orbitals of the P−R bonds play the role of
acceptor in PR3.
PHOSPHINES
147




Whenever the R group becomes more electronegative, the orbital
that the R fragment uses to bond to phosphorus becomes more stable
(lower in energy).
This implies that the σ* orbital of the P−R bond also becomes more
stable.
At the same time, the phosphorus contribution to σ* orbital increases,
and so the size of the σ* lobe that points toward the metal increases
Both of these factors make the empty σ* more accessible for back
donation.
PHOSPHINES
148

The final order of increasing p-acid character is:
PMe3 ≈ P(NR2)3 < PAr3 < P(OMe)3 < P(OAr)3 < PCl3
< CO ≈ PF3
PHOSPHINES
149
• The empty P−R s* orbital plays the role of acceptor in metal complexes of PR3.
• As the atom attached to the P atom becomes more electronegative, the empty
P−X s* orbital becomes more stable (lower in energy) making it a better acceptor of
electron density from the metal center.
PHOSPHINES
150



Occupation of the P−R σ* orbital by back donation from the metal
also implies that the P−R bonds should lengthen slightly on binding.
In practice, this is masked by a simultaneous shortening of the P−R
bond due to donation of the P lone pair to the metal, and the
consequent decrease in P(lone pair)–R(bonding pair) repulsions.
Once again, as in the case of CO, the M−L p bond is made at the
expense of a bond in the ligand, but this time it is a σ, not a p, bond
PHOSPHINES
151
Overview of Organometallic Chemistry
PHOSPHINES – TOLMAN ELECTRONIC PARAMETER
152


The electronic effect of various PR3 ligands can be adjusted by changing the
R group as, quantified by Tolman, who compared the ν(CO) frequencies of a
series of complexes of the type LNi(CO)3, containing different PR3 ligands.
The increase in electron density at the nickel from PR3 σ-donation is
dispersed through the M-L p system via p-backbonding. Much of the electron
density is passed onto the CO p* and is reflected in decreased v(CO)
stretching frequencies which corresponds to weaker CO bonds.
PHOSPHINE
153
TOLMAN – CONE ANGLES
154



The second important feature of PR3 as a ligand is the variable
steric size, which can be adjusted by changing R.
CO is so small that as many can bind as are needed to achieve 18e.
In contrast, the same is rarely true for phosphines, where only a
certain number of phosphines can fit around the metal.
This can be a great advantage in that by using bulky PR3 ligands, we
can favor forming low-coordinate metals or we can leave room for
small but weakly binding ligands,
Overview of Organometallic Chemistry
PHOSPHINE
155
The usual maximum number of phosphines that can bind
to a single metal is




two for PCy3 or P(i-Pr)3
three or four for PPh3
four for PMe2Ph
five or six for PMe3
Overview of Organometallic Chemistry
PHOSPHINE
156
Coordination Number (CN) – the number of bonding
groups at metal center.
Low CN favored by:
1. Low oxidation state (e- rich)
metals.
Although Pd(P(tBu)2Ph)2 is
coordinatively unsaturated
2. Large, bulky ligands.
electronically, the steric bulk
of both P(tBu)2Ph ligands prevents
additional ligands from coordinating
to
Overview of Organometallic Chemistry
the metal.
TOLMAN CONE ANGLE
157


The cone angle is obtained by taking a space-filling model of
the M(PR3) group, folding back the R substituents as far as they
will go, and measuring the angle of the cone that will just
contain all of the ligand, when the apex of the cone is at the
metal.
Although the procedure may look rather approximate, the
angles obtained have been very successful in rationalizing the
behavior of a wide variety of complexes.
159

160


An important part of organometallic chemistry consists in varying the
steric and electronic nature of the ligand environment of a complex to
promote whatever properties are desired: activity or selectivity in
homogeneous catalysis, reversible binding of a ligand, facile
decomposition, or high stability.
Using the Tolman plot we can relatively easily change electronic
effects without changing steric effects
e.g., by moving from PBu3 to P(OiPr)3]
Also, we can relatively easily change steric effects without changing
electronic effects
e.g., by moving from PMe3 to P(o-tolyl)3
1H
and 13C NMR
161
13C:
- for ligands that do not contain hydrogen (CO)
- decoupled spectra shows singlets for each atom

Saturated Carbon appear between 0-100 ppm with
electronegative substituents increasing the shifts.

CH3-X : directly related to the electronegativity of X.

The effects are non-additive: CH2XY cannot be easily predicted
162
Shifts for aromatic compounds appear between 110-170 ppm
p-bonded metal alkene may be shifted up to 100 ppm: shift
depends on the mode of coordination
Metal carbonyls are found between 170-290 ppm. (very long
relaxation time make their detection very difficult)
Metal carbene have resonances between 250-370 ppm
Overview of Organometallic Chemistry
163
Overview of Organometallic Chemistry
164
1HNMR
Overview of Organometallic Chemistry
Review Questions
165
For each of the following pairs of complexes, which
will have the lowest average CO infrared stretching
frequency? briefly explain your reasoning.
Cp2Y(CH3)(CO) vs. Cp2V(CH3)(CO)
CpFe(CO)2(PF3) vs. CpOs(CO)(PMe3)2
Overview of Organometallic Chemistry
166

The most electron-rich metal with d-electrons will
p-backbond the most with the CO ligands and
have the lowest CO stretching frequency
Overview of Organometallic Chemistry
167
Cr(CO)4(PEt3)2
PtCl4(CO)2
vs. W(CO)4(PPh3)2
vs.
NiBr2(CO)2
Overview of Organometallic Chemistry
168
Sketch out a neutral 18-electron structure showing the geometry about
the metal center as accurately as you can at this point in the course for
the following metals and ligands. Use at least one metal and each type
of ligand shown. Try to keep your structure as simple as possible .Show
your electron counting.
A)
B)
C)
W, μ-PR2, CO, H
Pt, CH, Cl, PMe3
Nb, O, CH3, Cp, PMe3
Overview of Organometallic Chemistry

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