Bonding in Coordination Compounds

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Bonding in Coordination Compounds

• Valence-Bond Theory
• Explain structure and magnetic properties of
  complex in terms of hybrid orbitals
• Ex Co(NH3)63 is diamagnetic (no unpaired e)
• Co Ar4s23d7
• Co3 Ar3d6
• Principles a) Pairs of ligand electrons
  donated into vacant metal orbitals

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• b) Metal d-electrons arranged to agree with
  observed magnetic properties
• Recall that Hunds rule says to leave es
  unpaired as long as possible within a subshell.
  Now we will sometimes violate Hunds rule.
• 
  
• 3d 4s
  4p
• Orbitals available for ligand electron pairs are
  d2sp3 set.
• This hybridization leads to octahedral geometry.

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Hybridization and Geometry (Review)

• sp linear
• sp2 trigonal planar
• sp3 tetrahedral
• dsp2 square planar
• sp3d or dsp3 trigonal bipyramidal
• sp3d2 or d2sp3 octahedral

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Figure 20.18Hybrid Orbitals on Co3
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• Ex CoF63 has 4 unpaired electrons (d6 ion).
• 
  
• 3d 4s
  4p
• __ __ __
• 4d
• Ligands use sp3d2 hybrid orbital set.
• Geometry is octahedral.

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• Ex NiCl42 has 2 unpaired electrons
• Ni Ar4s23d8
• Ni2 Ar3d8
• 
  
• 3d 4s
  4p
• 4 ligand pairs use sp3 hybrid orbitals.
• Geometry is tetrahedral.

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• Ex PtCl42 is diamagnetic.
• Pt2 is d8 ion like Ni2.
• 
  __
• 5d 6s
  6p
• Ligand electron pairs use dsp2 hybrid orbitals.
• Geometry is square planar.
• Use dx2y2, px, and py orbitals to direct
  electron density along the x- and y-axes.

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• Ex Ag(NH3)2 is diamagnetic
• Ag Kr5s14d10
• Ag Kr4d10
• 
  __ __
• 4d 5s
  5p
• Ligand electron pairs use sp hybrid orbitals.
• Geometry is linear.

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Figure 20.19The Hybrid Orbitals Required for
Tetrahedral, Square Planar, and Linear Complex
Ions
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Crystal-Field Theory

• Accounts for magnetic spectral properties in
  terms of splitting of the d-orbital energies
• Ex Consider an octahedral complex, ML6n
• We place the ligands along the x-, y-, and
  z-axes.
• Now, which d-orbitals on the metal will be
  repelled most strongly by the negative charge of
  the ligands (which are closest to the ligands)?

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Figure 20.20 An Octahedral Arrangement of
Point-Charge Ligands and the Orientation of the
3d Orbitals
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• The dx2y2 and dz2 orbitals point directly along
  the x-, y-, and z-axes, right at the ligands.
• Thus, these two orbitals experience a great deal
  of repulsion and are raised in energy.
• These two orbitals are called the t2g set.
• The other three d-orbitals point in between the
  ligands (little repulsion).
• Since the total energy of the d-orbitals must be
  conserved, these three orbitals are lowered in
  energy.
• These three orbitals are called the eg set.

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Figure 20.21 The Energies of the 3d Orbitals for
a Metal Ion in an Octahedral Complex
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• Strong-field and Weak-field Cases
• The splitting ?o between the t2g and eg orbitals
  depends on metal size, metal charge, and the
  nature of the ligand.
• Strong-field Case ? is large, electrons prefer
  to populate the lower-energy set (t2g) as long as
  possible.
• Weak-field Case ? is small, electrons prefer to
  stay unpaired in the t2g and eg sets as long as
  possible (as in Hunds rule)

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• Predicting Strong- or Weak-Field Cases
• 2nd and 3rd transition series tend to be
  strong-field cases (4d and 5d metals)
• Metal charge 3 and higher tends to be
  strong-field. Charge 2 and lower, weak-field.
• Effect of ligands the spectrochemical series.
• CN gt NO2 gt en gt NH3 gt H2O gt OH gt F gt Cl gt
  Br gt I

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• Magnetic Properties
• High-spin complex has the maximum number of
  unpaired d-electrons (weak-field case)
• Low-spin complex has the minimum number of
  unpaired d-electrons (strong-field case)

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• Ex Co(NH3)63 d6 ion
• 3 charge and relatively strong ligand suggest
  strong-field case. Therefore, low-spin.
• __ __ eg
• t2g
• So the complex is diamagnetic.

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• Ex CoF63 d6 ion
• 3 charge suggests strong-field, but very weak
  ligand and 1st-row metal suggest weak-field.
  Therefore, high-spin.
• eg
• t2g
• Therefore, 4 unpaired electrons

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Color Spectroscopy

• Figure 20.23 The Visible Spectrum
• Observed color is the complement of absorbed
  color.
• Ex A red substance absorbs short wavelengths
  (v, b, g).
• A violet substance absorbs long wavelengths (r,
  o, y).

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• Figure 21.24 (a) When white light shines on a
  filter that absorbs in the yellow-green region,
  the emerging light is violet. (b) Because the
  complex ion Ti(H2O)63 absorbs yellow-green
  light, a solution of it is violet.

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• Strong-field Cases
• ? is large, short-wavelength light is absorbed.
• Colors tend toward the red end of the spectrum.
• Weak-field Cases
• ? is small, long-wavelength light is absorbed.
• Colors tend toward the blue end of the spectrum.
• Ex Co(NH3)63 yellow (absorb
  violet)
• Co(NH3)4Cl2 green (absorb red)

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Other Geometries

• d-orbitals closer to ligands will be higher in
  energy
• d-orbitals farther from ligands will be lower.
• Tetrahedral Complexes
• Metal is at center of a cube, ligands are at the
  alternate corners. Axes come out of the center
  of the faces of the cube.
• Which d-orbitals are closest to the ligands?

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• Figure 21.26 (a) Tetrahedral and octahedral
  arrangements of ligands shown inscribed in
  cubes.(b) The orientations of the 3d orbitals
  relative to the tetrahedral set of point charges.

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• In this case, the dxy, dxz, and dyz orbitals are
  closest to the ligands. However, they do not
  point directly at the ligands. These are called
  the t2 orbitals.
• The dz2 and dx2y2 orbitals are farther from the
  ligands. These are called the e orbitals.
• The energy-level splitting pattern will be
• Inverted from the octahedral pattern
• Have a smaller splitting ?t 4/9 ?o

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Figure 21.27 The crystal field diagrams for
octahedral and tetrahedral complexes.
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• Ex. What is the d-electron configuration for
  CoCl42-?
• We have Co2, a d7 ion.
• Note that tetrahedral cases are always high-spin
  (weak-field) since ?t is so much smaller than ?o.
• t2
• e 3
  unpaired electrons