Transition Metals

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Transition Metals

• Be familiar with the electron configurations,
  properties and common oxidation states of
  transition metals (Table 21.2), and know that
  aqueous solutions of their ions are highly
  colored (photos on p 989, 993, 997, 1002, 1015)
• Know that transition metals form complex ions,
  i.e. cations that are surrounded by neutral
  molecules with nonbonding electron pairs (called
  ligands, denoted L) (p988) know that
  coordination number is number of ligands
  surrounding the cation (Fig 21.5, Table 21.12)
  know the most common ligands (Table 21.14)
• Remember what the d orbitals look like (Fig
  21.20). Appreciate that the crystal field model
  accounts for the colors of transition metal ions
  the d orbital energies are split into two sets
  separated by an energy ? (Fig 21.21) the
  occupation of the orbitals by electrons (and the
  paramagnetism of a complex) depends on how big ?
  is (strong field or weak field, Fig 21.22) the
  ligands determine the strength of the field
  (learn about the spectrochemical serioes on p
  1013) the color of a complex ion solution also
  depends upon D (Fig 21.23, Table 21.16, Table
  21.17).
• Monday focus reading on sections 21.1, 21.3 and
  21.6. Wednesday organic chemistry, sections
  22.1-22.4. Don't worry about naming or
  reactions. Friday Natural and Artificial
  Polymers, sections 22.5 22.6. Chapter 22
• Homework problems from Ch 21 19, 21, 43, 45,
  47, 51. Ch 22 23, 39, 51, 71, 75, 83, 85, 99.

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Transition Metals
3
Electron Configurations, Properties and Common
Oxidation States of Transition Metals
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Transition Metal Ion Salts Are Often Highly
Colored
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Transition Metal Cations Form Complex Ions (also
called coordination complexes)

• Complex Ion transition metal cation surrounded
  by LIGANDS
• Ligand molecule or ions that have nonbonding
  electron pairs
• Bonding is called coordination

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Coordination Number

• Coordination number the number of ligands
  surrounding a central cation in a transition
  metal complex.
• Common coordination numbers are 2, 4 and 6
• The geometries of the ligands about the central
  atom are as shown

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Common Ligands

• Ligands have nonbonding electron pairs to donate
  to the transition metal cation
• Dentate number of electron pairs that
  coordinate to the central metal cation
• Unidentate, bidentate

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Heme
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Metalloprotein Complex Hemoglobin
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Remember the d Orbitals?
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Transition Metals Use Their Valence d Orbitals
and Orbital Hybridization for Coordination

• d2sp3 hybrids on Co2 interact with ammonia
  ligands in this complex ion

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The Ligands Create a Ligand Field

• The symmetry of the electric field around an
  isolated metal cation is spherical the same no
  matter what direction you are looking
• When you arrange ligands around the central metal
  cation, the symmetry of the electric field about
  it has the geometry determined by the arrangement
  of the ligands (e.g. Octahedral, tetrahedral,
  linear, etc.)

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Three of the d orbitals have a different spatial
relationship with crystal field than the other two
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Two Classes of d Orbitals in an octahedral ligand
field

• The lobes of two d orbitals point ALONG the axes
  of the octahedron
• The lobes of three d orbitals point BETWEEN the
  axes of the octahedron
• The environment of the orbitals along the axes is
  different from that of those that point between
  the axes
• Because the orbitals are in different
  environments, they have different energies
• The five d orbitals are split into two groups a
  group of two denoted eg and a group of three
  denoted t2g

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Crystal Field Splitting
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Where are the metal ion electrons, then?

• Depends upon how big D is!
• Consider case of d6 metal ion metal ion with 6
  d electrons (e.g. Co2)
• Big D electrons don't have enough energy to
  occupy the eg orbitals
• Note that the paramagnetism (number of unpaired
  electrons) is different in the two cases.
• High spin, low spin

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Why Are Some Transition Metal Ions Colored?
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Why are Some Transition Metal Ions
Colored?...because they absorb light in the
visible region of the electromagnetic spectrum
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Why Do Some Transition Metal Ions Absorb Light in
the Visible Region of the Electromagnetic
Spectrum?

• ...because they have electrons that are able to
  undergo transitions between the eg and t2g
  orbitals, and
• ...because D is in the visible region of the
  electromagnetic spectrum

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What Determines if Electrons Can Undergo
Transitions Within the d Shell?

• Consider the case of the silver ion, Ag
• Ag is d10 has 10 electrons in its d orbitals.
• Regardless of the value of D, the electrons can't
  undergo transitions because the d orbitals are
  fully occupied. There's nowhere to go!

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What Determines How Big D Is?

• Ligands are one very important influence!
• Ligands that interact much more strongly with the
  metal cation will increase the splitting between
  the d orbitals.
• Spectrochemical series (p 1013) determined by
  experiment
• Strong, big D Weak, small D
• CN- gt NO2- gt NH3 gt H2O gt OH- gt F- gt Cl- gt Br- gt I-

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Different Ligands Make the Same Cation Different
Colors