Chapter 2' Complexes with metalcarbon s bonds

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Chapter 2
Complexes with metal-carbon s bonds
Metal alkyls, aryl, hydride and related s-bonded
ligands
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Outline
1. The stability of transition metal-carbon s
bonds 2. Preparation of complexes with
metal-carbon s bonds 3. Chemical properties of
complexes with metal-carbon s bonds 4. Related
s-bonded ligands complexes M-X (X SiR3,
OR, NR2, F, Cl, etc.) 5. Metal hydrides

• References and suggested readings
• 1. The Organometallic Chemistry of the Transition
  Metals, Robert H. Crabtree, 3rd Edition, 2001,
  Chapter 3
• 2. Organometallic Chemistry, G. O. Spessard, G.
  L. G. L. Miessler, Prentice-Hall New Jersey,
  1997, Chapter 6
• 3. Organotransition Metal Chemistry, Akio
  Yamamoto, 1986. Chapters 3.1, 4.2.

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1. The stability of transition metal-carbon s
bonds
Formation and breaking of metal-carbon s-bonds is
the central event in organometallic reactions and
in many organic reactions catalyzed by transition
metal complexes.
Prior to 1960s Non-transition metal alkyls are
well known, e.g.
AlMe3 ZnEt2 (1848)
Transition metal alkyls are rare. Only a few
transition metal alkyls were prepared. e.g.
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• Attempts during the 1920s through 1940s to make
  further examples of TM alkyls all failed.
• These failures led to the view that transition
  metal-carbon bonds were unusually weak.

Are transition metal-carbon bonds weak and
unstable? But today Transition metal-carbon
bonds are known for virtually all transition
metals.
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Questions

• Are transition metal carbon s bonds really
  unstable?
• What affects M-C bond strength?
• What causes the earlier failure to obtain
  transition metal alkyls

DG lt 0, thermally unstable Kinetically stable
CH4 O2 ? CO2 H2O
The thermal stability can be related to bond
dissociation energy.
M-X (g) ? M(g) X(g) DHº BDE (M-X)
If BDE (M-X) is large, M-X is thermally more
stable.
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Range of M-C bond dissociation energy (BDE).
BDE (M-C) values in MRn complexes (KJ/mol)
Non-transition metals Transition
metals M-C bond BDE Compound M-C
bond BDE
Non-transition metals BDE(M-C) 89-280 kJ/mol
Except B-C Transition metals BDE (M-alkyl)
150 - 260 BDE (M-aryl) 250 - 350 Normally, BDE
(M-alkyl) lt BDE (M-aryl) Conclusion Transition
metal-carbon bond energies are not so different
from those of main group metals.
Li-Et 209 Ti(CH2C(CH3)4 Ti-CH2R
170 Li-Bu 248 Ti(CH2Ph)4 Ti-CH2R
240 Zn-Me 176 Ti(CH2Si(CH3)4 Ti-CH2R
250 Zn-Et 145 Cp2Ti(CH3)2 Ti-CH3
250 Cd-Me 139 Cp2TiPh2 Ti-Ph
350 Hg-Me 122 (CO)5MnCH3 Mn-CH3
150 Hg-Et 101 (CO)5ReCH3 Re-CH3 220 Hg-i-Pr
89 Zr(CH2C(CH3)4 Zr-CH2R 220 Hg-Ph 136 Zr(CH
2Ph)4 Zr-CH2R 380 B-Me 363 Zr(CH2Si(CH3)4
Zr-CH2R 225 B-Et 342 CpPt(CH3)3 Pt-CH3
165 Al-Me 276 (Et3P)2PtPh2 Pt-Ph
250 Al-Et 242 Ta(CH3)5 Ta-CH3
260 Ga-Et 237 W(CH3)6 W-CH3 160
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2). Factors affecting M-C bond strength

• Affecting Factors
• M effect
• Ln effect
• R effect
• C-(sp3, sp2, sp) effect.

• a) Metal effect
• For main group metals, BDE (M-CH3), kJ/mol
• B-Me 363 Zn-Me 176
• Al-Me 276 Cd-Me 136
• Ga-Me 247 Hg-Me 122
• BDE(M-C) decreases when going down a group.

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• For transition metals, BDE (M-CH3), kJ/mol,
• (CO)5Mn-CH3 150 (PhCH2)3Ti-CH2Ph 240
• (CO)5Re-CH3 220 (PhCH2)3Zr-CH2Ph 380
• BDE(M-C) increases when going down a group.

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b) R effect (M-R)
Relative bond strength (M-Cl gt M-Br gt M-H or gt
M-I) M-H gt M-CF3 gt M-Ar gt M-CH3 gt
M-COCH3 M-C-R stability increases with more
electron-withdrawing R.
Bond dissociation energies DR-Mn(CO)5
Complex BDER-MnCO)5 (kJ/mol) Mn(CO)5CF3 172
Mn(CO)5C6H5 170 Mn(CO)5 CH2C6H5
87 Mn(CO)5CH3 153 Mn(CO)5COCH3 129 Mn(CO)5COC
6H5 89 Mn(CO)5COCF3 147 Mn(CO)5H 213 Mn(CO)
5I 195 Mn(CO)5Br 252 Mn(CO)5Cl 294
M-H is stronger than M-CR3 s orbital is
indirectional and more dense. M-CF3 is
stronger than M-CR3 note,F is electron
withdrawing. M-Ar is stronger than M-CR3.
back-donation for M-Ar.
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Would you expect M-CH3 bond be stronger or weaker
than
Why?
Evidence
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Stability of the following M-C bonds M-CH3 lt
M-CH2-NO2 Why ?
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• Summary
• The M-C bond in transition metal alkyl complexes
  is not as weak as was thought previously.
• There is no inherent instability of transition
  metal alkyls.
• The BDE(M-R) value increases as the atomic
  number increases among the congeners in the same
  group, in an opposite trend to non-transition
  metal alkyls.
• The D(M-R) value decrease in the order M(M-H) gt
  D(M-CF3) gt D(M-Ph) gt D(M-CH3) gt D(M-CH2CH3) gt
  D(M-CH2Ph).
• The earlier failure in obtaining TM alkyls is not
  due to thermodynamic reasons !

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Exercises Which one has the strongest stability
of the following complexes ?
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3). Origin of earlier failure in obtaining TM
alkyls (Kinetic consideration)
General questions. Q1. Why could many TM alkyl
complexes not be isolated?
Q2. What are the common decomposing pathways for
transition metal alkyls?
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Q3. How can we obtain stable M-alkyl
complexes? Stable M-alkyl complexes could be
obtained if the above decomposing pathways can be
prevented.

• b-elimination.
• The most common decomposition pathway for alkyls
  is b-H elimination, which converts a metal-alkyl
  into a hydrido-olefin complex.

hydrido-olefin complex
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• What is the condition for b-H elimination to
  occur ?
• There must be a b-H.
• B. The complex should be able to form
  four-membered co-planar transition state (see
  (A))

C. There is a vacant site cis to the alkyl (e.g.
(B). but not (C)). (need an empty orbital
to accept C-H electron.) D. The olefin formed is
stable. E. The metal has e- to go to s(C-H).
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Exercises.
(a). It is difficult for the following compounds
to undergo b-hydrogen eliminations. Suggest main
reasons.
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(b) Would you expect the following complexes to
undergo b-H elimination easily?
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(c) Which of the following complexes would you
expect to be least stable?
If an alkyl compound can undergo b-H elimination,
it is unstable. For above complexes to undergo
b-H elimination, the right steps are needed.
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For 16 electron complexes, prior ligand
dissociation is not required with the exception
of Pd(II) and particularly Pt(II), which do not
like to become 18e complexes. Thus rate of
decomposition of (Ph3P)2PtBu2 to give butane and
butene is inversely dependent upon PPh3.
The platinacyclopentane analog undergoes
decomposition 10,000 times more slowly. However
larger platinacycles undergo decomposition at a
similar rate to the dibutyl complex and by the
same mechanism.
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Further notes on b-elimination.
(i) b-H elimination on d0 (electron deficient)
metal complexes may not occur. e.g.
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(ii) Other b-elimination is also possible, e.g.
because
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Summary. Metal-alkyls that are stable to
ß-hydride elimination

• 1. Alkyls that do not have ß-hydrogens
• 2. Alkyls for which the ß-hydrogen cannot
  approach the metal center (as in cyclic alkyls or
  as a result of steric crowding)
• 3. Alkyls in which the M-C-C-H unit cannot become
  coplanar
• 4. Olefin generated is unstable
• 5. 18-electron species that cannot dissociate a
  ligand
• 6. Some d0- alkyls

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b) Reductive elimination

• Coordination number decreased by 2.
• Oxidation number decreased by 2.

The process is the second most common
decomposition pathway for metal alkyls. e.g.
More detailed discussion will be given later.
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c) Other pathways
a-H elimination
For early transition metal alkyls, a-hydride
elimination can be an important
decomposition pathway.
Example
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Another example,
Which intermediate is more likely?
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g-H elimination. e.g.
Summary Common pathways of decomposing
M-alkyls b-H elimination
Reductive elimination Others.
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Exercises. 1) For each pair of complexes listed
below, which one is less stable?
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2) Which one is least stable ?
3) which one is most stable ?
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2. Preparation of complexes with metal-carbon s
bonds.
A. Synthesis by alkyl transfer reactions. R-
M-X ---------gt M-R X- B. Synthesis from
anionic transition metal complexes. M- R-X
---------gt M-R X- C. Synthesis by oxidative
addition reactions. M R-X -----------gt
X-M-R D. Synthesis involving insertion Reactions.
M-X A -----gt M-A-X E. Synthesis involving
elimination reactions. M-A-R ------gt M-R A F.
Synthesis by attack on coordinated ligands
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A. Synthesis by alkyl transfer reactions
R- M-X ---------gt M-R X- Typical R-
reagents RLi, RMgX, AlR3, AlR2X, ZnR2, etc.
Examples Preparation of homoletic alkyl
complexes
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Note different alkylating agents may have
different reactivity, e.g.
ZnR2 is also a weak alkylating reagents. e.g.
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Example. Preparation of alkyl complexes R-
LnMXy ---------gt LnMRy X-
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B. Synthesis from anionic transition metal
complexes. M- R-X ---------gt M-R
X- Examples
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Generation of LnM-
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Exercise. Suggest reagents for the following
preparations.
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C. Synthesis by oxidative addition reactions.
Typical oxidative addition reactions
Examples of preparation of complexes.
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Examples of preparation of complexes.
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Cyclometalation, a closely related reaction.
Synthesis by oxidative addition of C-H bonds.
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D. Synthesis involving insertion Reactions.
M-X A -----gt M-A-X Examples
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E. Synthesis involving elimination reactions.
M-A-R ------gt M-R A (A should be very
stable) A CO, CO2, SO2, N2, e.g.
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F. Synthesis by attack on coordinated ligands
We will go to more details later.
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3. Chemical properties of complexes with
metal-carbon s bonds
(1) b-hydrogen elimination
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(2) Reductive elimination
e.g.
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(3) a, g-elimination, e.g.
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(4) Migratory insertion
The insertion of CO into an M-C bond results in
formation of an acyl ligand. This is one of the
industrially important reactions of metal alkyls.
Mechanistically, it involves intramolecular
nucleophilic attack by the alkyl on the carbon of
CO. ?More discussion later.
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Carbon dioxide can give mono- and bidentate
insertion products
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Insertion of alkenes.Polymerization of olefins,
one of the most important catalytic industrial
processes, relies on the facile insertion of
alkenes into metal-alkyl bonds
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(5) Electrophilic attack on alkyls
d-

• As the metal electronegativity increases, the
  nucleophilic reactivity of the M-C bond
  decreases.
• Nucleophilic reactivity decreases with more
  stable carbanions (i.e. -CH3 lt -C6H5 lt -CCR).
• Early transition metal alkyls are very sensitive
  to water and oxygen.
• Late transition metal alkyls are more stable to
  water and oxygen.

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Examples.
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Exercises. Predict the products of the following
reactions
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(6) Special properties of some M-R complexes
Small metallocylcles show interesting
rearrangement.
Larger metallacylcles show Normal properties of
alkyls
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4. Related s-bonded ligand complexes
M-SiR3, M-OR, M-NR2, M-F, M-Cl, M-BR2
A. M-SiR3 M-Si bond are longer than M-C
BDE(M-SiR3) may be stronger than BDE(M-CR3)
Reason a) some ?-interaction (Si has
empty d orbitals)
b) M-SiR3 is less sterically congested.
Can LnM-SiCH2R undergo b-H elimination?
In general, M-SiR3 has properties similar to
M-CR3.
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• M-OR, M-NR2, M-F, M-Cl, etc.
• (1) Difference between M-CR3 and M-X
• M-CR3 M-X
• no dp-pp interaction possible dp-pp
  interaction

M-X bond is stabilized for 16e or less species
(early TM)
M-X bond is destabilized for 18e species
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Consequences
M-X can stabilize complexes with less than 18e.
e.g.
Why?
p bonding can make these complexes to formally
have 18e.
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With high valent transition metals, p-donation
becomes important. M-O-C angles approaching 120
or even 180 are common.
An angle near 120 suggests that the O is sp2
hybridized and OR- acting as a 4 electron donor
. An angle near 180 suggests that the O is sp
hybridized and OR- acting as a 6 electron donor.
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Linear alkoxide have sometime be considered to be
isoelectronic with the Cp ligand.
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(2) Similarity between M-CR3 and M-OR
Both can undergo b-H elimination.
Use of the reaction.
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C. M-BR2 complexes
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5. Metal hydrides, M-H
History 1931 H2Fe(CO)4 prepared by Hieber,
although the structure was not accepted
initially. 1955-1964 Cp2ReH, (PR3)2PtHCl, and
K2ReH9 prepared showing that M-H bonds do
exist. 1984 Dihydrogen complexes discovered
(M-H2).
A. Structural types of metal hydrides.
(1) Terminal hydride or classical hydride, e.g.
(2) Non-classical hydride or molecular hydrogen
complexes (1984)
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Q1. Would you expect the H2 molecule in the
following complex can rotate freely? why?
Consider the bonding between M and H2.
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Q2. LnMH2 can have two forms, depending on M and
L. Rationalize the structural difference in the
following pairs.
In terms of bonding
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(3) "Hydrogen-bonding" type interaction
-------- H-X s complexes
Often the interaction can be represented as
For example,
agostic
agostic
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Additional examples
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(4) Bridging hydrides
Examples
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For electron counting purpose, we can use the
resonance structures like
Exercise. 18e or not?
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(5) BH4-, BH3L, and AlH4- complexes. A variety
structures are possible. e.g. for BH4-
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For electron counting purpose Ionic model
M(BH4) ? M BH4-
Covalent model M(BH4) ? MH BH3
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B. Synthesis of metal hydride complexes.
(1) By protonation
(require a basic metal complex)
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(2) From hydride donors
M-X H- donor ----------gt M-H X- Typical H-
donors NaH, LiAlH4, NaBH4, LiBHEt3 ...
Examples.
WCl6 LiBHEt3 PR3 ------gt WH6(PR3)3
In some cases, complexes with BH4 or AlH4
entities may be obtained when NaBH4 or NaBH4 are
used.
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(3) From H2
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What are the likely intermediates or transition
states?
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(4) From ligands
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C. Chemical properties of M-H complexes
(1) Acidic and hydridic LnM-H
What about a transition metal hydride
complexes? LnM-H can be acidic or hydridic !
Examples
hydridic M-H !
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Acidic M-H !
(ii) Factors influencing hydridic and acidic
properties of M-H
M-H
M-H-
Properties of M-H are dependent on the e- density
on M. In general, increase in electron-richness
of metal center, will ----gt decrease acidic
characters -----gtincrease hydridic characters
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Exercises. For each of the following pairs,
which one is more acidic, which one is more
hydridic? (a) H-Co(CO)4 vs H-Co(CO)3(PPh3)
(b) H-Co(CO)3(PMe3) vs H-Co(CO)3(PPh3)
(c) CpRu(CO)2H vs CpOs(CO)2H
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General trends - pKa generally decreases as you
move down a column - pKa generally increases as
you move to the right across the transition
series - electron donating substituents decrease
pKa values
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(2) Chemical reactions
a. Deprotonation reactions with nucleophiles
(bases)
e.g.
WH6(PMe3)3 NaH ------gt NaWH5(PMe3)3
H2 OsH4(PR3)4 NaH ---------gt NaOsH3(PR3)4
H2
b. Reactions with electrophiles.
e.g. WH6(PMe3)3 HBF4 3 MeCN
-----gt WH2(MeCN)3(PR3)32 H2
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C. Insertion reactions
e.g.
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Summary of reactivity of M-H