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C

H

A

P 8

T

E

R

PART B: 1st Row Transition Metals & Coordination Compounds

Outlines2

� Crystal Field Theory (CFT)

�Metal-Ligand Bond in Complexes

� CFT for Octahedral Complexes

� CFT for Tetrahedral Complexes

� CFT for Square Planar Complexes

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Crystal Field Theory (CFT)

CFT is a theory that accounts for the colours and the magnetic and other properties of transition metal complexes in terms of the splitting of the energies of metal ion d orbitals by the electrostatic interaction with the ligands.

Many of the colours and magnetic properties of transition metal complexes are related to the d electrons in metal orbitals.

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Metal-Ligand Bond in Complexes

� Ligand donates a pair of electrons into a suitable empty orbital on the metal.

� The ligands are attracted strongly toward the metal center.

� Because of the electrostatic attraction between the positive metal ion and the electrons of the ligands, the assembly of the metal ion and the ligands is lower in energy than the fully separated charges.

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CFT for Octahedral Complexes

� Although the positive metal ion is attracted to the electrons in the ligands, the d electrons on the metal ion feel repulsion from the ligands.

� An octahedral array of negative charges approaching a metal ion.

� The ligands to be negative points of charge that repel the electrons in the d orbitals.

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� In octahedral complexes, the ligands approaching the metal ion along the x, y and z axes.

The orientations of the d orbital relative to the charges

The lobes point toward the charges The lobes point between the charges

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� Because the d orbitals on the metal ion have different shapes, they do not all have the same energy under the influence of the crystal field.

� The dz2 and dx

2-y2 orbitals experience the same amount of repulsion from the crystal

field, so they stay at the same energy.

� The dxy, dxz and dyz orbitals experience exactly the same repulsion, so they stay at the same energy.

� Because their lobes point right at the negative charges, dz2 and dx

2-y2 orbitals

experience stronger repulsion than those in the dxy, dxz and dyz orbitals.

� Therefore, an energy separation or splitting, occurs between the three lower-energy d orbitals and the two higher-energy d orbitals.

� This energy gap is labeled ∆, a quantity that is often called the crystal field splitting energy.

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� This diagram shows the effects of the negative points of charge on the energies of the d orbitals.

� When negative charges are brought up to the metal ion, the average energy of the d orbitalsincreases (center).

� Because the repulsion felt by the dz2 and dx

2-y2 is greater than that felt by the dxy, dxz and dyz

orbitals, the five d orbitals split into a lower-energy set of three and a higher-energy set of two.

e set

t2 set

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Isolatedtransition metal

atom Crystal field splitting diagramfor an octahedral complex

e set

t2 set

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e set

t2 set

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Ligandsapproach metal

d-orbitals not pointing directly at axis are least affected (stabilized) by electrostatic interaction

d-orbitals pointing directly at axis are affected most by electrostatic interaction

e set

t2 set

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CFT for Tetrahedral Complexes

� When there are only four ligands about the metal, the geometry is generally tetrahedral, except for the special case of metal ions with a d8 electrons configuration (square planar).

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� The crystal-field splitting of the metal d orbitals in the tetrahedral complexes differs from that in octahedral complexes.

� The splitting of the metal d orbitals in the tetrahedral crystal is just the opposite of that for the octahedral case.

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� The three metal d orbitals in the t2 set are raised in energy, and the two orbitals in the e set are lowered.

� Because of there are only four ligands instead of six, as in octahedral case, the crystal-field splitting in much smaller for tetrahedral complexes.

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CFT for Square Planar Complexes

� Square planar complexes, in which four ligands are arranged about the metal ion in a plane, can be envisioned as formed by removing the two ligands from along the vertical z-axis of the octahedral complex.

� Square planar complexes are characteristics of metal ions with a d8 electron configuration.

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� The changes that occur in the energy levels of the d orbitals are illustrated as follows.

� The dz2 orbital is now considerably lower in energy than the dx

2-y

2 orbital because the ligands along the vertical z-axis have been removed.

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dx2-y

2

dxy

dyz dxz

eg

eg

t2g

t2g

∆tet

tetrahedral isolated before accounting octahedral square planarfield metal ion for electron field field

repulsion

Energy of attraction of point charges

∆oct∆sqp

dz2

∆∆∆∆tet = 4/9 ∆∆∆∆oct

COMPARISON OF CFT IN THREE DIFFERENT GEOMETRIES

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