Transition Metals and Coordination Chemistry

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Chapter 19

• Transition Metals and Coordination Chemistry

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The Differences between Main Group Metals and
Transition Metals

• Transition metals are more electronegative than
  the main group metals.
• The main group metals tend to form salts. The
  transition metals form similar compounds, but
  they are more likely than main group metals to
  form complexes.
• NaCl(s)? Na(aq)Cl-(aq)
• CrCl3(s) 6 NH3(l) ?CrCl3 6 NH3(s)

Violet Yellow
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Electron Configurations

• Sc Ar4s23d1
• Ti Ar4s23d2
• V Ar4s23d3
• Cr Ar4s13d5
• Mn Ar4s23d5

• FeAr4s23d6
• CoAr4s23d7
• Ni Ar4s23d8
• Cu Ar4s13d10
• ZnAr4s23d10

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Half Filled Set of 3d Orbitals

• Cr Ar4s13d5 Cu Ar4s13d10
• The orbital energies are not constant for a
  given atom but depend on the way that the other
  orbitals in the atom are occupied. Because the 4s
  and 3d orbitals have similar energies, the 4s23dn
  and 4s13dn1. configurations have similar
  energies.
• For most elements, 4s23dn is lower in energy, but
  for Cr and for Cu the 4s13dn1 is more stable.

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Oxidation States

• CoAr4s23d7 Co2 Ar3d7 Co3Ar3d6
• The discussion of the relative energies of the
  atomic orbitals suggests that the 4s orbital has
  a lower energy than the 3d orbitals. Thus, we
  might expect cobalt to lose electrons from the
  higher energy 3d orbitals, but this is not what
  is observed.
• In general, electrons are removed from the
  valence-shell s orbitals before they are removed
  from valence d orbitals when transition metals
  are ionized.

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The 4d and 5d Transition Series
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Lanthanide Contraction

• Since the 4f orbitals are buried in the interior
  of these atoms, the additional electrons do not
  add to the atomic size.
• The increasing nuclear charge causes the radii of
  lanthanide elements (Z58-71) to decrease
  significantly going from left to right.

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Coordination Number
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Ligands

• A ligands is a neutral molecule or ion having a
  lone pair that can be used to form a bond to a
  metal ion.
• Because a ligand donates an electron pair to an
  empty orbital on a metal ion, the formation of a
  metal-ligand bond (coordinate covalent bond) can
  be described as the interaction between a Lewis
  base (the ligand) and a Lewis acid (the metal
  ion).

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Isomerism
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Structural IsomerismCoordination Isomers

• Isomers involving exchanges of ligands between
  complex cation and complex anion of the same
  compound.
• Co(NH3)6Cr(CN)6 Co(CN)6Cr(NH3)6
• Ni(C2H4)3Co(SCN)4 Ni(SCN)4Co(C2H4)3
• Cr(NH3)5SO4Br Cr(NH3)5BrSO4

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Structural IsomerismLinkage Isomers

• Isomers in which a particular ligand bonds to a
  metal ion through different donor atoms.
• Co(NH3)5ONOCl2Co(NH3)5NO2Cl2

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Co(NH3)5NO2Cl2 Co(NH3)4ONOCl2
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Co(NH3)5NO22
Co(NH3)5ONO2
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Stereo-isomerismGeometric Isomers/cis-trans
Isomers

• Stereoisomers Molecules have the same molecular
  formula and the same connectivity of atoms, but
  differ only in the three-dimensional arrangement
  of those atoms in space.
• Geometric Isomers Atoms or groups of atoms can
  assume different positions around a rigid ring or
  bond.

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Stereo-isomerismOptical Isomer

• Optical isomerism is a form of isomerism whereby
  the different 2 isomers are the same in every way
  except being non-superimposable mirror images()
  of each other.

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The two structures are nonsuperimposable mirror
images. They are like a right hand and a left
hand.
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• Simple substances which show optical isomerism
  exist as two isomers known as enantiomers.
• A molecule which has no plane of symmetry is
  described as chiral. The carbon atom with the
  four different groups attached which causes this
  lack of symmetry is described as a chiral center.

chiral center
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• One enantiomer will rotate the light a set number
  of degrees to the right. This is called the
  Dextrorotator (from the Latin dexter, "right"??)
  isomer or () isomer.
• The other enantiomer will rotate the plane
  polarized light the same number of set degrees in
  the opposite left direction. This isomer is said
  to be a Levorotatory (from the Latin laevus,
  "left ??) isomer or (-) isomer.

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Octahedral Complexes
eg
t2g
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Strong Field and Low Spin

• The splitting of d orbital energies explains the
  color and magnetism of complex ions.
• If the splitting produced by the ligands is very
  large, a situation called strong field case, the
  electrons will pair in the low energy t2g
  orbitals.
• The strong field case is also called low spin
  case.
• This gives a diamagnetic complex in which all
  electrons are pairs.
• ?0gtP

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Weak Field and High Spin

• If the splitting produced by the ligands is
  small, the electrons will occupy all five
  orbitals before pairing occurs called weak field
  case.
• The weak field case is also called high spin
  case.
• In this case, the complex is paramagnetic.
• ?0ltP

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Octahedral transition-metal ions with d1, d2, or
d3 configurations
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Octahedral transition-metal ions with d4, d5,
d6, and d7 configurations
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For octahedral d8, d9, and d10 complexes , there
is only one way to write satisfactory
configurations.
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weak field case strong
field case with paramagnetic with
diamagnetic
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The Color of Complexes

• Very commonly for the first transition series,
  the energy corresponds to that of visible light,
  so that d-d transitions are the cause of the
  delicate colours of so many of the complexes.

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Charge Effect of Metal Ions

• As the metal ion charge increases, the ligands
  are drawn closer to the metal ion because of its
  increased charge density.
• As the ligands move closer, they cause greater
  splitting of the d orbitals, thereby producing a
  larger ? value.
• The magnitude of ? for a given ligand increases
  as the charge on the metal ion increases.
• NH3-Co2 (weak field) NH3-Co3 (strong field)

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Spectrochemical Series

• I- lt Br- lt SCN- Cl- lt F- lt OH- ONO- lt C2O42- lt
  H2Olt NCS- lt EDTA4- lt NH3 pyr en lt phen lt CN-
  CO
• Mn2 lt Ni2 lt Co2 lt Fe2 lt V 2 lt Fe3 lt Co3 lt
  Mn3 lt Mo3 lt Rh3 lt Ru3 lt Pd4 lt Ir3 lt Pt4

pyr pyridine phen phenol
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Tetrahedral complex
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Energy Splitting of Tetrahedral Complex

• Because a tetrahedral complex has fewer ligands,
  the magnitude of the splitting is smaller.
• The difference between the energies of the t2g
  and eg orbitals in a tetrahedral complex (?t) is
  slightly less than half as large as the splitting
  in analogous octahedral complexes (?o).
• ?t 4/9?o

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Square Planar and Linear Complex
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Ligand Field Theory

• Ligand field theory can be considered an
  extension of crystal field theory such that all
  levels of covalent interactions can be
  incorporated into the model. Treatment of the
  bonding in LFT is generally done using molecular
  orbital theory.

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Molecular Orbital Model
eg
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Molecular Orbital of Complex

• The dz2, dx2-y2, 4s, 4px, 4py and 4pz orbitals
  will be involved in the MOs in the s complex
  ions.
• The dxz, dyz and dxy orbitals (the t2g set) of
  the metal ion do not overlap with ligand
  orbitals. They are called nonbonding orbitals.
• The eg orbitals is relatively little
  contribution from ligand orbitals. This lack of
  mixing is caused by the large energy difference
  between the ligand orbitals and the metals ion 3d
  orbitals.

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The Effect of Weak Field Ligands

• A ligand with a electronegative donor atom will
  have lone pair orbitals of very low energy (the
  electrons are very firmly bound to the ligand)
  these orbitals do not mix very thoroughly with
  the metal ion orbitals. This will result in a
  small difference between the t2g and eg
  orbitals.

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The Effect of Strong Field Ligands

• The strong field ligands produce larger degree of
  mixing between the orbitals of ligands and metal
  ions
• This gives a relatively large amount of d-orbital
  splitting, and low spin case results.

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Biological Importance of Coordination
Complexes-Hemoglobin

• The principal electron transfer molecules in the
  respiratory chain are iron-containing species
  called cytochromes, consisting of two main part
  an iron complex called heme and a protein.
• (cytochromes heme protein)
• A metal ion coordinated to a rather complicated
  planar ligand is called a porphyrin.
• The various porphyrin molecules act as
  tetradentate ligands for many metal ions,
  including iron, cobalt and magnesium

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Chlorophyll
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Myoglobin

• Iron plays a principal role in the transport and
  storage of oxygen in mammalian blood and tissues.
• Oxygen is stored using a molecule called
  myoglobin, which contains a heme complex and a
  protein.
• In myoglobin, the Fe2 ion is coordinated to four
  nitrogen atoms of porphyrin ring and to one
  nitrigen atom of the protein chain.
• Since Fe2 ion is normally six-coordinate, this
  leaves one position open for attachment of an O2
  molecule.

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Heme?Myoglobin???
Hb 4O2 ltgt Hb(O2)4 Hemoglobin
Oxyhemoglobin
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Myoglobin molecule
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Hemoglobin

• The transport of O2
• in the blood is
• carried out by
• hemoglobin, a
• molecule consisting
• of four myoglobin
• molecules units.

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Conformation change when heme is oxidized

• ?????????????,?????????????????
• ??????????
• ???????(????????????)
• ?????????,??????Hemoglobin????,?????????Heme??????
  ??????

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Normal red blood cell (right) and a sickle cell,
both magnified 18,000 times.