1117 Lecture Announcements I Lab Last Quiz MondayTues 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 ID: 551438
Download Presentation The PPT/PDF document "Chem. 1B –" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
Chem. 1B – 11/17 LectureSlide2
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. 24Slide3
Announcements II
Mastering
Ch
. 24 assignment due
11/26
Today’s Lecture
Transition Elements (Ch. 24)
Bonding in Coordination Complexes - TheorySlide4
Chapter 24 Transition Metals
Optical Isomer Demonstration
Show with models of MX
2
YZ and MWXYZSlide5
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
x
y
z
x
y
z
d
xy
– lies in xy plane
x
y
z
d
xz
d
yz
Slide6
Chapter 24 Transition Metals
Coordination Complex – Bonding Theory – cont.
Two different classes of d orbitals occurs
On axes orbitals
x
y
z
x
y
z
d
x^2 – y^2
d
z^2
Slide7
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 (d
z2
and d
x2-y2
) are UNFILLED
Or alternatively, the ligands cause a split in energy levels of d shell orbitals
E
Free atom
Metal in octahedral complex
On axis
Off axis
DSlide8
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 off-axes orbitals
On axis
Off axisSlide9
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)Slide10
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)Slide11
Chapter 24 Transition Metals
Coordination Complex – Bonding Theory – Role of
Ligands
and Metal
Ligand Strength (see text for full range)
Metal Ion Strength (greater charge, Fe
3+
vs. Fe
2+
, increases
D
)
strongest
CN
-
weakest
NH
3
Cl
-
I
-
H
2
O
Weak Field Ligands – tend to give high spin statesSlide12
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]4s
2
3d
6
will have 4 unpaired electrons and is magnetic
Other metals, e.g. Zn (d
10
), are not magnetic
E
4s
3dSlide13
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
]4-
large
D
small
DSlide14
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:Slide15
Chapter 24 Transition Metals
Coordination Complex – Light Absorbing Properties – cont.
Many coordination complexes absorb visible light (
l
green 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)Slide16
Chapter 24 Transition Metals
Coordination Complex – Light Absorbing Properties – cont.
Example: [Co(H
2
O)
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 lightED = ?If we switched to NH3 as a ligand (stronger), what shift would be expected?Slide17
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 (d
xy
, d
xz
, and d
yz
) 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
On axis
Off axis
DSlide18
Chapter 24 Transition Metals
Coordination Complex – Other Geometries
In square planar geometry, overlap is most with d
x^2 – y^2
(but is more complex as shown below)
Square planar geometry is common for d
8
ions in which d
x2 – y2
orbitals are unoccupied (low spin)
Metal in square planar complex
d
x2 – y2
d
xy
on axis and off axis in xy plane
d
Z2
d
xz
d
yzSlide19
Chapter 24 Transition Metals
Questions
Which two d orbitals do octahedral complexes overlap with the most?
Which d orbital is there the greatest overlap in square planar complexes?
Give the number of unpaired electrons for the following metals in octahedral complexes for low spin states/high spin states
a) Fe
3+
- octahedral b) Co
2+
– octahedral
c) Cu
2+
- tetrahedral d) Mn
3+ - octahedralSlide20
Chapter 24 Transition Metals
Questions – cont.
Ti
3+
is purple while Ti
4+
is uncolored. Explain.
For which of the following metals in octahedral complexes does the ligand NOT play a role in the number of unpaired electrons?
a) Mn
2+
b) Fe
3+
c) Co
2+ d) Ni
2+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?