Transcript Ch 24 Part 2 PowerPoint

Chap 24 Part 2
Color and Magnetism
Color
Color of a complex depends on; (i) the metal, (ii)
its oxidation state & (iii) ligands (i.e., everything)
For example, pale blue [Cu(H2O)6]2+
versus dark blue [Cu(NH3)6]2+.
Partially filled d orbitals usually give rise to
colored complexes because they can absorb
light from the visible region of the spectrum.
 The color of the complex is the sum of the
light not absorbed (reflected) by the
complex.
Color and Magnetism
Color
Color
 A plot of absorption intensity of light versus
wavelength is called an absorption spectrum
for the complex or compound.
Color and Magnetism
Since the spectrum for [Ti(H2O)6]3+ has a maximum
absorption at 510 nm (green & yellow), & transmits
all other wavelengths, the complex is purple.
Color and Magnetism
Magnetism
Transition metal complexes that are
paramagnetic have unpaired e-’s & those
that are diamagnetic have no unpaired e-’s.


Consider the d6 Co metal ion:
[Co(NH3)6]3+ has no unpaired electrons, but
[CoF6]3- has four unpaired electrons per ion.
(note, s e-’s are lost first before d e-’s in a metal
cation)
We need to develop a bonding theory to account
for both color and magnetism in transition metal
complexes.
Crystal-Field Theory
Crystal field theory (CFT) describes bonding &
can account for many of the color and magnetic
properties in transition metal complexes.
 Lewis A/B model assumes bonding results from
ligand e-’s donated into hybridized d metal orbital.
Complex has lower E
 CFT assumes that the interaction between ligand &
metal is electrostatic (pos. nuclei & neg. e-’s).
Crystal-Field Theory
An octehedral array of
negative ligands shown
as small (blue) dots
approaching the five
different d orbitals of a
metal ion.
Crystal-Field Theory
Although there is an overall reduction in
E, the negative ligands repel d e-’s giving
rise to a slight increase in E.
The E gap is called D or the CF
splitting E.
Two of the five d orbitals are higher
in E.
Crystal-Field Theory
[Ti(H2O)6]3+
Crystal-Field Theory
A Spectrochemical series is a listing of
ligands in order of their ability to increase D:
Cl- < F- < H2O < NH3 < en < NO2- (N-bonded) < CN-
Weak field ligands (Cl- & F-) lie on the low
end of the spectrochemical series.
Strong field ligands (CN-) lie on the high end
of the spectrochemical series.
Crystal-Field Theory
2
As Cr3+ goes from complexes with weak field ligands
to strong field ligands, D increases and the color of
the complex changes from green to yellow.
Crystal-Field Theory
Electron Configurations in
Octahedral Complexes
Recall that the s e-’s are lost first for the metal ion. So, Ti3+ is
d1, V3+ is a d?? and Cr3+ is a d?? ion.
 We apply Hund’s rule to the 2 sets of 5 d-orbitals.
 The first three e-’s go into different d orbitals with their
spins parallel.
 We have a choice for the placement of the fourth
electron:
 if it goes into a higher energy orbital, then there is an
energy cost associated with promotion (D);
 if it goes into a lower energy orbital, then there is an
energy cost associated with e- spin pairing.
Crystal-Field Theory
Weak-field ligands
(which have a small
D) tend to favor
adding electrons to
the higher-energy
orbitals (high-spin
complexes) because
D is less than the
spin-pairing energy.
Strong-field ligands (which have a large D)
tend to favor adding electrons to lower-energy
orbitals (low-spin complexes) because D is
greater than the spin-pairing energy.
Crystal-Field Theory
Tetrahedral & Square-Planar
In a tetrahedral field the dxy, dyz, & dxz orbitals are
of higher E than the dx2-y2 and the dz2 orbitals.
 Because there are only 4 ligands, D for
a tetrahedral field is smaller than D for
an octahedral field.
 This causes all tetrahedral complexes
to be high spin (unless told otherwise).
Crystal-Field Theory
Tetrahedral & Square-Planar
Square planar complexes can be thought
of as octahedral complexes with the two
ligands along the z-axis removed.
 As a consequence the four planar ligands are
drawn in closer towards the metal.
 Relative to the octahedral field, the dz2 orbital
is greatly lowered in energy, the dyz, and dxz
orbitals lowered in energy, the dxy, and dx2-y2
orbitals are raised in energy.
Crystal-Field Theory
Most d8 metal ions
form square planar
complexes.
The majority of
complexes are low
spin (i.e.
diamagnetic).
Examples: Pd2+,
Pt2+, Ir+, and Au3+.
End of Chapter 24
Chemistry of Coordination
Compounds
Homework:
7, 13, 14, 17, 19-21, 23-26, 28, 31, 38, 30,
42-44, 47