Transcript 11/17 lecture notes

Chem. 1B – 11/17 Lecture
Announcements I
• Lab
– Last Quiz Monday/Tues on Exp 10, 14 and
Chapter 24
– No Lab next Wednesday
– Experiment 10 report due
• Exam 3
– Two weeks (and three lectures) from today
– On electrochemistry and Chapter 24
– Last year’s exam did not cover last parts of
Ch. 24
Announcements II
• Mastering
– Ch. 24 assignment due 11/26
• Today’s Lecture
– Transition Elements (Ch. 24)
• Bonding in Coordination Complexes - Theory
Chapter 24 Transition Metals
• Optical Isomer Demonstration
– Show with models of MX2YZ and MWXYZ
Chapter 24 Transition Metals
• Coordination Complex – Bonding Theory
– cont.
– To understand how electrons in the d shells
influence bonding, we must understand the
shapes of d orbitals
– Two different classes of d orbitals occurs
•
Off axes orbitals
dyz
z
z
dxy – lies
in xy
plane
dxz
z
y
y
x
y
x
x
Chapter 24 Transition Metals
• Coordination Complex – Bonding Theory
– cont.
– Two different classes of d orbitals occurs
•
On axes orbitals
dz^2
dx^2 – y^2
z
z
y
y
x
x
Chapter 24 Transition Metals
• Coordination Complex – Bonding Theory –
cont.
–
–
In octahedral binding, because the ligands bring the
electrons, lower energy results when the binding axes
orbitals (dz2 and dx2-y2) are UNFILLED
Or alternatively, the ligands cause a split in energy
levels of d shell orbitals
Metal in octahedral complex
Free atom
E
D
On axis
Off axis
Chapter 24 Transition Metals
• Coordination Complex – Bonding Theory
– cont.
– How does d orbital splitting affect
coordination complexes?
– Electrons go to low energy states first
– Example: [Cr(CN)6]3- has 4 – 1 = 3 d shell
electrons – they should occupy the three offaxes orbitals
On axis
Off axis
Chapter 24 Transition Metals
• Coordination Complex – Bonding Theory
– cont.
– When we add more than 3 electrons (e.g. 4
electrons), there are two possibilities:
•
•
fill bottom orbitals first
or go to top orbitals
– Filling depends on D gap (larger leads to
“low spin” states – first shown, while smaller
leads to “high spin” states – second shown)
Chapter 24 Transition Metals
• Coordination Complex – Bonding Theory
– Role of Ligands
– Particular metals, such as Fe, can form
complexes with different properties (e.g.
colors or magnetic properties) depending on
ligands
– Ligands affect size of D gap
– “Strong” ligands result in large D gap, while
“weak” ligand results in smaller D gap (with
the idea that more tightly held electrons will
overlap more with d shell electrons)
Chapter 24 Transition Metals
• Coordination Complex – Bonding Theory
– Role of Ligands and Metal
– Ligand Strength (see text for full range)
CNstrongest
NH3
H2O
Cl-
I-
weakest
Weak Field Ligands – tend to give high spin states
– Metal Ion Strength (greater charge, Fe3+ vs.
Fe2+, increases D)
Chapter 24 Transition Metals
• Coordination Complex – Magnetic and
Light Absorbing Properties
– Magnetic Properties:
•
•
Compounds or atoms with unpaired electrons
are magnetic (since half filled shells will have
electrons with the same spin)
Example: Fe [Kr]4s23d6 will have 4 unpaired
electrons and is magnetic
3d
•
4s
Other metals, e.g. Zn (d10), are not magnetic
E
Chapter 24 Transition Metals
• Coordination Complex – Magnetic
Properties – cont.
– Octahedral Complexes will have d electrons
split into to energy states by ligand field
– Large D gap complexes give rise to “low
spin” states that are less magnetic vs. “high
spin” states
– Examples: [Fe(CN)6]4- vs. [Fe(Br)6]4large D
small D
Chapter 24 Transition Metals
• Coordination Complex – Light Absorbing
Properties
– Gap between on- and off-axes d orbitals
can also lead to transitions between two
states
– Example: [Cr(CN)6]3•
Absorption of light causes electronic transition
from low energy to high energy state:
Chapter 24 Transition Metals
• Coordination Complex – Light Absorbing
Properties – cont.
– Many coordination complexes absorb visible
light (lgreen light ~ 525 nm or E = hc/l = 3.8
x 10-19 J)
– The larger the D gap, the greater the E,
and the smaller the l value energy
– Visible colors go ROYGBIV (red, orange,
yellow, green, blue, indigo, violet – from
longer to shorter wavelength)
Chapter 24 Transition Metals
• Coordination Complex – Light Absorbing
Properties – cont.
–
–
–
–
–
–
Example: [Co(H2O)6]2+ (used for the Drierite color
demonstration)
Color is pink/purple (but pink is red + white =
seen color because complex absorbs other colors)
Using color wheel (text) expected absorbance is in
green (measured in Chem 31 as 510 nm)
Color wheel used because we see reflected light
ED = ?
If we switched to NH3 as a ligand (stronger), what
shift would be expected?
Chapter 24 Transition Metals
• Coordination Complex – Other Geometries
–
–
Besides octahedral geometries, tetrahedral and
square planar geometries have different overlaps
with d orbitals resulting in different d orbital splitting
In tetrahedral complexes, the complex can be
positioned (see Fig. 24.17) where ligand bonds
interact with “off-axis” d orbitals (dxy, dxz, and dyz)
making these orbitals higher in energy and on-axis d
orbitals lower in energy (however with small D values
and high spin states)
Metal in tetrahedral complex
D
Off axis
On axis
Chapter 24 Transition Metals
• Coordination Complex – Other
Geometries
–
–
In square planar geometry, overlap is most with dx^2
– y^2 (but is more complex as shown below)
Square planar geometry is common for d8 ions in
which dx2 – y2 orbitals are unoccupied (low spin)
Metal in square planar
complex
dx2 – y2
dxy
dZ2
dxz
dyz
on axis and off
axis in xy plane
Chapter 24 Transition Metals
• Questions
1. Which two d orbitals do octahedral complexes
overlap with the most?
2. Which d orbital is there the greatest overlap in
square planar complexes?
3. Give the number of unpaired electrons for the
following metals in octahedral complexes for low
spin states/high spin states
a) Fe3+ - octahedral
b) Co2+ – octahedral
c) Cu2+ - tetrahedral
d) Mn3+ - octahedral
Chapter 24 Transition Metals
• Questions – cont.
4. Ti3+ is purple while Ti4+ is uncolored. Explain.
5. For which of the following metals in octahedral
complexes does the ligand NOT play a role in the
number of unpaired electrons?
a) Mn2+ b) Fe3+
c) Co2+
d) Ni2+
6. [Fe(en)3]3+ undergoes a ligand replacement
reaction and forms [FeX6]3-. The new complex
absorbs at shorter wavelengths. What do we know
about the strength of X as a ligand?