Chapter 19 Coordination Complexes - Columbia University Lecture20... · Chapter 19 Coordination...
Transcript of Chapter 19 Coordination Complexes - Columbia University Lecture20... · Chapter 19 Coordination...
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Chapter 19 Coordination Complexes
19.1 The Formation of Coordination Complexes19.2 Structures of Coordination Complexes19.3 Crystal-Field Theory and Magnetic Properties19.4 The Colors of Coordination Complexes19.5 Coordination Complexes in Biology
C1403 Lecture 20 Tuesday, November 16, 2005
Infrared spectroscopy (IR tutor)
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H2O
Ni
H2O H2O
H2O
H2O
H2O
H3N
Ni
H3N NH3
NH3
NH3
NH3
en
Ni
en en
en
en
en
dmgl
Ni
dmgl dmgl
dmgl
dmgl
dmgl
NC
Ni
NC CN
CN
CN
CN
NH3 enen
dmgldmgl
KCN
Green Blue
PinkPurple
Yellow
Ammonia EthyleneDiamine
Dimethylglyoxime
PotassiumCyanide
Ligand substitutions and color changes: Ni2+
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Enantiomers: non superimposable mirror images
A structure is termed chiral if it is not superimposable on its mirrorimage (we use idealized geometric structures with ligands considered
as points)
Two chiral structures: non superimposable mirror images
StructureMirror imageOf structure
How do we decide if two structures are superimposable?
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Plane of symmetry Achiral (one structure)
Chirality: the absence of a plane of symmetry:Enantiomers are possible
A molecule possessing a plane of symmetry is achiral anda superimposible on its mirror image
Enantiomers are NOT possible
No plane of symmetryChiral (two enantiomer)
NH3
CoH2O
H2OCl
Cl
NH3
NH3
CoCl
ClH2O
H3N
H2O
NH3
CoNH3
H2OCl
Cl
H2O
Are the following chiral or achiral structures?
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Which are enantiomers (non-superimposable mirror images)and which are identical (superimposable mirror images)?
Look for a plane of symmetry in idealized geometric structures: If thereis one the mirror images are the same structure. If there is not one,
the mirror images are enantiomers (optical isomers).
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19.3 Crystal Field Theory: Splitting of the 5 dorbitals
Consider the response of the energyof the d orbitals to the approach of 6negatively charged ligands (a “crystalfield”) along the x, y and z axes of the
metal
The two d orbitals (dx2-y2 and dz2) thatare directed along the x, y and z axes
are affected more than the otherthree d orbitals (dxy, dxz and dyz)
The result is that the dx2-y2 anddz2 orbital increase in energyrelative to the dxy, dxz and dyz
orbitals (D0 is called the “crystalfield energy splitting” t2g orbitals
eg orbitals
crystalfieldenergysplitting
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Crystal field splitting of the 5 d orbitals by the “crystal field” of 6ligands
t2g orbitals
eg orbitals
Crystal field splitting
Orbitals “on axis”: “energy increases”
Orbitals “off axis”: “energy decreases”
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Electronic configurations of dn complexes fromparamagnetism and diamagnetism
Magnet offMagnet on:Paramagnetic
Magnet on:diamagnetic
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Building of weak field, high spin electron configurationsfor d2, d3, d4, and d5 systems (Hund’s rule applies)
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Crystal Field Splitting of d orbitals: high spin and low spin situationsfor a d6 metal
Net unpairedspins = 0:Diamagnetic
Net unpairedspins = 4:Stronglyparamagnetic
Low spinElectrons spinpair
High spinElectrons doNot spin pair
Ligand strength: (Weak) I- < F- < H2O < NH3 < CN- (Strong)
Large splitting
Small splitting
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The dn electron configurations of M(II) cations of theatoms of transition metals
Element Neutral atom 2+ Ion 3+ Ion
Sc [Ar]4s23d1 [Ar]3d1 [Ar]
Ti [Ar]4s23d2 [Ar]3d2 [Ar]3d1
V [Ar]4s23d3 [Ar]3d3 [Ar]3d2
Cr [Ar]4s13d5 [Ar]3d4 [Ar]3d3
Mn [Ar]4s23d5 [Ar]3d5 [Ar]3d4
Fe [Ar]4s23d6 [Ar]3d6 [Ar]3d5
Co [Ar]4s23d7 [Ar]3d7 [Ar]3d6
Ni [Ar]4s23d8 [Ar]3d8 [Ar]3d7
Cu [Ar]4s13d10 [Ar]3d9 [Ar]3d8
Zn [Ar]4s23d10 [Ar]3d10 [Ar]3d9
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Large splittingLow spin
Small splittingHigh spin
Energy gap largerthan advantage dueto Hund’s rule
Energy gap small;Hund’s rule applies
How many unpaired spins in Fe(CN)64- and in Fe(H2O)6
2+?
What is the charge of Fe in Fe(CN)64- and in Fe(H2O)6
2+?
Fe2+ in both cases Fe = [Ar]3d64s2; Fe2+ = [Ar]3d6
What kind of ligands are CN-
and H2O?CN- is a strong field ligand andH2O is a weak field ligand
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Ti(H2O)63+ 3d1 1 (t2g)
1 (!)1
Cr(H2O)63+ 3d3 3 (t2g)
3 (!!!)3
Fe(H2O)63+ 3d5 5 (t2g)
3(eg)2 (!!! )3(!!)2
Fe(CN)63- 3d5 1 (t2g)
5 (!"!"!)5
Fe(H2O)62+ 3d6 4 (t2g)
4(eg)2 (!"!!)4(!!)2
Fe(CN)62- 3d6 0 (t2g)
6 (!"!"!")6
Ni(H2O)62+ 3d8 2 (t2g)
6(eg)2 (!"!"!")6(!!)2
Cu(H2O)62+ 3d9 1 (t2g)
6(eg)3 (!"!"!")6(!"!)3
Zn(H2O)62+ 3d10 0 (t2g)
6(eg)4 (!"!"!")6(!"!")4
Complex Valence
electrons
Unpaired
electrons
Electron
configuration
Spin
configuration
Electronic configuration of some n+ dn metal cations inoctahedral complexes
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19.4 Crystal Field Theory: The Color of CoordinationCompounds
h# =
The energy gap betweenthe eg and t2g orbitals, $0,(the crystal field splitting)equals the energy of aphoton: $0 = h# = $E
As $0, varies, h# will alsovary and the color of thecompound will changeAbsorption of a photon causes a
jump from a t2g to an eg orbital
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H2O
Ni
H2O H2O
H2O
H2O
H2O
H3N
Ni
H3N NH3
NH3
NH3
NH3
en
Ni
en en
en
en
en
dmgl
Ni
dmgl dmgl
dmgl
dmgl
dmgl
NC
Ni
NC CN
CN
CN
CN
NH3 enen
dmgldmgl
KCN
Green Blue
PinkPurple
Yellow
Ammonia EthyleneDiamine
Dimethylglyoxime
PotassiumCyanide
Ligand substitutions and color changes: Ni2+
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Strong field Ligands (violet, low spin)
Weak fieldLigands (red, high
spin)
The spectrochemical series of color and magnetic properties:weak field (red, high spin), strong field (violet, low spin)
A d5 electron metal ionSpectrochemical series
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The color that we see is the color thatis not absorbed, but is transmitted.The transmitted light is thecomplement of the absorbed light.
Numbers are nm
So if red light is mainly absorbed thecolor is green; if green light is mainlyabsorbed, the color is red.
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Color of complexes depend on the value of $0 = h# = $E
$0 = h#
“red absorption”
“violetabsorption”
“looks green” “looks yellow
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Crystal Field Splitting of d orbitals: high spin and low spin situationsfor a d5 metal (why are some complexes colorless?)
ColoredColorless or veryweakly colored Why?
Color corresponds to theabsorption of light antransitions between dorbitals for metals.
The transition for (b) is“spin forbidden” becausean electron would need to“flip” its spin in beingexcited from a t orbitalto a e orbital
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In real systems there areregions of different lightabsorptions leading to a widerange of colors
Visualization of color as transmitted light which is notabsorbed and the brain’s perception of mixed colored
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19.5 Coordination Complexes in Living Systems
Porphines, hemes, hemoglobin
Photosynthesis: electron transfer
Vitamin B12
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23Porphine (C20H14N4))
heme (C34H32N4O4Fe))
Porphines and hemes: important molecules in living systems
These planar molecules have a “hole” in the center which to which ametal can coordinate
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Why do we need to eat d metals?
Some critical enzymes in our cells are metalloproteins,giant biolmolecules which contain a metal atom
These metalloproteins control key life processes suchas respiration and protect cells against disease
Hemoglobin is a metalloprotein which contains an ironatom and transports O2 through out living systems
Vitamin B12, which prevents pernicious anemia, containsa Co atom which gives the vitamin a red color
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Reversible addition of O2 to hemoglobin
The mechanism by which oxygen is carried throughoutthe body
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26Vitamin B12 (Co[C62H88N13O14P])CN
Involved in many important biological processes,including the production of red blood cells
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27Chlorophyll (C55H72N4O5Mg)
A very important porphine that converts solarphotons into food energy: chlorophyll