Organometallic MT Complexes

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OrganometallicMT Complexes
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MT Organometallics

• Organometallic compounds of the transition
  metals have unusual structures, and practical
  applications in organic synthesis and industrial
  catalysis.

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MT Organometallics

• One of the earliest compounds, known as Zeises
  salt, was prepared in 1827. It contains an
  ethylene molecule p bonded to platinum (II).

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Zeises Salt

• The bonding orbital of ethene donates electrons
  to the metal. The filled d orbitals (dxz or dyz)
  donate electrons to the antibonding orbital of
  ethene.

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Square Planar Complexes

• The complexes of platinum(II), palladium(II),
  rhodium(I) and iridium(I) usually have
  4-coordinate square planar geometry. These
  complexes also typically contain 16 electrons,
  rather than 18.
• The stability of 16 electron complexes,
  especially with s-donor p-acceptor ligands, can
  be understood by examining a MO diagram.

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Square Planar Complexes
The electron pairs from the 4 ligands used in s
bonding occupy the bonding orbitals.
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Square Planar Complexes
The dxy, dxz, dyz and dz2 orbitals are either
weakly bonding, non-bonding, or weakly
antibonding.
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Square Planar Complexes
The dx2-y2 orbital is anti-bonding, and if
filled, will weaken the s bonds with the ligands.
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Square Planar Complexes
As a result, 16 electrons will produce a stable
complex.
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Catalysis of Square Planar Compounds

• Square planar complexes are often involved as
  catalysis for reactions. The four-coordinate
  complexes can undergo addition of organic
  molecules or hydrogen, and then be regenerated as
  the organic product is released from coordination
  to the catalyst.

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Catalysis aldehyde formation

• Pd(II) undergoes addition of an alkene which is
  subsequently converted to an alcohol. Addition
  of a hydrogen atom to the metal with subsequent
  migration to the alcohol produces an aldehyde.

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Catalysis
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Bonding of Hydrocarbons

• Hydrocarbons can bond to transition metals via
  s bonds or p bonds. Wilkinsons catalyst,
  RhCl(PPh3) is used to hydrogenate a wide
  variety of alkenes using pressures of H2 at 1 atm
  or less.
• During the hydrogenation, the alkene initially
  p bonds to the metal, and then accepts a hydrogen
  to s bond with the metal.

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Wilkinsons Catalyst
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Hydrogen Addition

• Square planar complexes are known to react with
  hydrogen, undergoing addition, and breaking the
  H-H bond.

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Hydrogen Addition

• The hydrogen bonding orbital donates electron
  density into an empty p or d orbital on the metal.

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Hydrogen Addition

• The loss of electron density in the bonding
  orbital weakens the H-H bond.

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Hydrogen Addition

• The metal can donate electron density from a
  filled d orbital (dxz or dyz)
• to the antibonding orbital on hydrogen, thus
  weakening or breaking the H-H bond.

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The Template Effect

• A metal ion can be used to assemble a group of
  organic ligands which then undergo a condensation
  reaction to form a macrocyclic ligand. Nickel
  (II) is used in the scheme below.

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MT Carbonyls

• Metal carbonyl compounds were first synthesized
  in 1868. Although many compounds were produced,
  they couldnt be fully characterized until the
  development of X-ray diffraction, and IR and NMR
  spectroscopy.

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MT Carbonyls

• Metal carbonyl compounds typically contain
  metals in the zero oxidation state. In general,
  these compounds obey the 18 electron rule.
• Although there are exceptions, this rule can be
  used to predict the structure of metal carbonyl
  cluster compounds, which contain metal-metal
  bonds.

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The 18 Electron Rule

• Many transition metal carbonyl compounds obey
  the 18-electron rule. The reason for this can be
  readily seen from the molecular orbital diagram
  of Cr(CO)6. The s donor and p acceptor nature of
  CO as a ligand results in an MO diagram with
  greatest stability at 18 electrons.

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The eg orbitals are destabilizing to the
complex. Since the 12 bonding orbitals are
filled with electrons from the CO molecules, 6
electrons from the metal will produce a stable
complex.
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MT Carbonyls

• The CO stretching frequency is often used to
  determine the structure of these compounds. The
  carbon monoxide molecule can be terminal, or
  bridge between 2 or 3 metal atoms.
• The CO stretching frequency decreases with
  increased bonding to metals. As the p orbital
  on CO receives electrons from the metal, the CO
  bond weakens and the ? decreases.

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MT Carbonyls

• As the p orbital on CO receives electrons from
  the metal, the CO bond weakens and the ?
  decreases.

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MT Carbonyls

• Mn2(CO)10

Fe2(CO)9
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MT Carbonyls

• Co4(CO)12

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MT Carbonyls
? for free CO 2143 cm-1
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MT Carbonyls
? for free CO 2143 cm-1
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MT Carbonyls

• The CO stretching frequency will also be
  affected by the charge of the metal.
• Compound ? (cm-1)
• Fe(CO)62 2204
• Mn(CO)6) 2143
• Cr(CO)6 2090
• V(CO)6- 1860
• Ti(CO)62- 1750

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MT Carbonyls

• The IR spectra of transition metal carbonyl
  compounds are consistent with the predictions
  based on the symmetry of the molecule and group
  theory.
• The more symmetrical the structure, the fewer
  CO stretches are observed in the IR spectra.

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MT Carbonyls

• If there is a center of symmetry, with CO
  ligands trans to each other, a symmetrical
  stretch will not involve a change in dipole
  moment, so it will be IR inactive. An asymmetric
  stretch will be seen in the IR spectrum. As a
  result, trans carbonyls give one peak in the IR
  spectrum.

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MT Carbonyls

• If CO ligands are cis to each other, both the
  symmetric stretch and the asymmetric stretch will
  involve a change in dipole moment, and hence two
  peaks will be seen in the IR spectrum.

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MT Carbonyls

• Metal carbonyls with a center of symmetry
  typically show only 1 C-O stretch in their IR
  spectra, since the symmetric stretch doesnt
  change the dipole moment of the compound.
  Combined with the Raman spectrum, the structure
  of these compounds can be determined.

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Nomenclature for Ligands

• The hapticity of the ligand is the number of
  atoms of the ligand which directly interact with
  the metal atom or ion. It is indicated using the
  greek letter ? (eta) with the superscript
  indicating the number of atoms bonded.

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Cyclopentadienyl Compounds

• The ligand C5H5 can bond to metals via a s bond
  (contributing 1 electron), or as a p bonding
  ligand. As a p bonding ligand, it can donate 3,
  or more commonly 5 electrons to the metal.

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Cyclopentadienyl Compounds

• W(?3-C5H5)(?5-C5H5)(CO)2 has two p bonded
  cyclopentadienyl rings. One donates 3 electrons,
  and the other donates 5.

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Counting Electrons

• There are two common methods for determining
  the number of electrons in an organometallic
  compound.
• One method views the cylcopentadienyl ring as
  C5H5-, a 6 electron donor. CO and halides such
  as Cl- are viewed as 2 electron donors. The
  oxidation state of the metal must be determined
  to complete the total electron count of the
  complex.

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Counting Electrons

• The other method treats all ligands as neutral
  in charge. ?5-C5H5 is viewed as a 5 electron
  donor, Cl is viewed as a chlorine atom and a 1
  electron donor, and CO is a 2 electron donor.
  The metal is viewed as having an oxidation state
  of zero in this method.

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Counting Electrons

• In either method, a metal-metal single bond is
  counted as one electron per metal. Metal-metal
  double bonds count as two electrons per metal,
  etc.

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Ferrocene

• Fe(?5-C5H5)2 , ferrocene, is known as a
  sandwich compound. In the solid at low
  temperature, the rings are staggered.
• The rotational barrier is very small, with free
  rotation of the rings.

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Ferrocene

• The cyclopentadienyl rings behave as an
  aromatic electron donor. They are viewed as
  C5H5- ions donating 6 electrons to the metal.
  The iron atom is considered to be Fe(II).

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Bonding of Ferrocene

• Group theory is used to simplify the analysis
  of the bonding. First, consider just a single
  C5H5 ring. Determine ?p by considering only the
  pz orbitals which are perpendicular to the
  5-membered ring.

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Bonding of Ferrocene
D5h E 2C5 2C52 5C2 sh 2S5 2S53 5 sv
?p
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Bonding of Ferrocene
D5h E 2C5 2C52 5C2 sh 2S5 2S53 5 sv
?p 5
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Bonding of Ferrocene
D5h E 2C5 2C52 5C2 sh 2S5 2S53 5 sv
?p 5 0
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Bonding of Ferrocene
D5h E 2C5 2C52 5C2 sh 2S5 2S53 5 sv
?p 5 0 0
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Bonding of Ferrocene
D5h E 2C5 2C52 5C2 sh 2S5 2S53 5 sv
?p 5 0 0 -1
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Bonding of Ferrocene
D5h E 2C5 2C52 5C2 sh 2S5 2S53 5 sv
?p 5 0 0 -1 -5
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Bonding of Ferrocene
D5h E 2C5 2C52 5C2 sh 2S5 2S53 5 sv
?p 5 0 0 -1 -5 0
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Bonding of Ferrocene
D5h E 2C5 2C52 5C2 sh 2S5 2S53 5 sv
?p 5 0 0 -1 -5 0 0
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Bonding of Ferrocene
D5h E 2C5 2C52 5C2 sh 2S5 2S53 5 sv
?p 5 0 0 -1 -5 0 0 1
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Bonding of Ferrocene
D5h E 2C5 2C52 5C2 sh 2S5 2S53 5 sv
?p 5 0 0 -1 -5 0 0 1

• ?p reduces to A'1, E'1 and E'2
• Group theory can be used to generate drawings of
  the p molecular orbitals.

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Bonding of Ferrocene?p reduces to A'1, E'1 and
E'2

E'2 E'1 A'1
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Bonding of Ferrocene

• The totally bonding orbital (A'1) has no nodes,
  and is lowest in energy.

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Bonding of Ferrocene

• The middle set of orbitals (E'1) are
  degenerate, with a single node. These orbitals
  are primarily bonding orbitals.

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Bonding of Ferrocene

• The upper set of orbitals (E'2) are degenerate,
  with two nodes. These orbitals are primarily
  anti-bonding orbitals.

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Bonding of Ferrocene

• Once the molecular orbitals of the
  cyclopentadienyl ring has been determined, two
  rings are combined, and matched with symmetry
  appropriate orbitals on iron.

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Bonding of Ferrocene

• The A'1 orbitals on the two cyclopentadienyl
  rings have the same symmetry as the dz2 orbital
  on iron.
• Since the metal orbital is located in the
  center of the C5H5 rings, this is essentially a
  non-bonding orbital.

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Bonding of Ferrocene

• The E'1 orbitals on the rings have the same
  symmetry as the dxz and dyz orbitals of the iron.

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Bonding of Ferrocene

• The E'2 orbitals on the rings have the same
  symmetry as the dxy and dx2-y2 orbitals of the
  iron.

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Bonding of Ferrocene

• These are the bonding orbitals of ferrocene.
  If the upper cyclopentadienyl ring is flipped
  over, a set of antibonding orbitals results.

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MO Diagram

• The frontier orbitals are neither strongly
  bonding nor strongly antibonding. As a result,
  metallo-cene compounds often diverge from the 18
  electron rule.

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MO Diagram

• If the complex has more than 18 electrons, the
  e1u orbitals, which are slightly antibonding (the
  dxzand dyz), become occupied. This lengthens the
  M-C distance.

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Electron Count and Stability

• (?5-Cp)2M e- count M-C(pm) ?Hdissoc.
• Fe 18 206.4 1470 kJ/mol
• Co 19 211.9 1400
• Ni 20 219.6 1320
• ?Hdissoc refers to the complex dissociating to
  M2 and 2C5H5-