Download Transition metal complexes_bonding

Mysteries of polarized light
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Enantiomers have identical properties (biological
significance aside) except in one respect: the rotation
of the plane of polarization of light
Modern symbols are (+) and (-) and R- and SDays of yore d (dextrorotatory) and l (levorotatory)
Racemic mixture contains equal portions of the (+)
and (-)
Transition metal ions:
the chemistry of colour
• Colour of complex corresponds to wavelengths
of light not absorbed. Observed colour is usually
complement of colour absorbed.
• All wavelengths absorbed, complex appears
black.
• No light absorbed, complex appears white
(colourless).
• Why is colour prevalent in transition metal
compounds?
The artist’s wheel
Review of variable valence: counting
d electrons
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Transition metal ions have variable oxidation
state
Electron configurations of the elements:
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4s23dn with some exceptions
Electron configurations of the ions:
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4s electrons are removed first, followed by the d
electrons
No 1st row transition metal ions have 4s electrons
All ions have configuration 3dm
Counting d electrons
Valence bond reprise
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Valence bond theory is the simplest approach
to an orbital picture of covalent bonds
Each covalent bond is formed by an overlap of
atomic orbitals from each atom
The individual orbital identity is retained
The bond strength is proportional to the
amount of orbital overlap
Valence bond picture in complexes
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In the conventional covalent bond, each atomic
orbital brings one electron with it
In the coordination complex, the ligand provides
both, while the metal orbital is empty
Geometry and hybridization
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Original atomic orbitals are mixed into hybrid
orbitals that match directional requirements for
bonding
No of orbitals matches no of charge groups
Square planar is not seen with main group 4coordinate molecules
Coordination Geometry
number
2
Linear
4
Tetrahedral
4
Square Planar
6
Octahedral
Hybrid
orbitals
sp
sp3
dsp2
d2sp3 or sp3d2
Example
[Ag(NH3)2]+
[CoCl4]2[Ni(CN)4]2[Cr(H2O)6]3+
Electrons and magnetism
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Electrons have magnetic moments due to electron spin
Paramagnetism: substance attracted by magnetic field
Diamagnetism: substance repelled by magnetic field
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Paramagnetic effect is much greater than diamagnetic effect
Diamagnetic substances have no unpaired electrons
Paramagnetic substances have unpaired electrons
Electron configurations and
geometry
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Electronic configuration of Co2+ is [Ar]3d7
Empty 4s and 4p orbitals are used for bonding in
tetrahedral complex
3d
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4s
4p
Three unpaired d electrons mean that the Co2+ is
paramagnetic
Metal
electrons
Ligand
electrons
Square planar: How?
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VSEPR predicts 4-coordinate is tetrahedral (sp3)
Ni2+ complexes are square planar not tetrahedral
Electronic configuration of Ni2+ is 3d8
Square planar geometry is dsp2
Use of one d orbital for ligand forces pairing of the
Ni d electrons into just four 3d orbitals
Ni(CN)42- is diamagnetic (all electrons paired)
Octahedral complexes show
options even for same dn
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Two options: d2sp3 or sp3d2
Same or different?
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Low spin Co(CN)63- diamagnetic (d6) d2sp3
3d
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4s
4p
High spin CoF63- paramagnetic (d6) sp3d2
3d
4s
4p
4d
Let’s spin
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You are probably wondering: Why are some
complexes high-spin and others low spin?
Can you predict it?
Valence bond theory describes bonding in
complexes consistent with observed magnetic
properties; it cannot explain why the ligands
dictate one over the other
Enter the crystal field theory…
The crystal field theory
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Ligands are like
negative charges
Central metal ion is
positive charge
Size of repulsion
between between
ligands and d orbitals on
metal ion is the basis of
the theory
Interaction between ligands and d
orbitals determines orbital energy
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Three dxy etc interact least with the ligands
Two dx2-y2 and dz2 interact most with the ligands in an
octahedral field
Orbitals
“miss”
the
ligands
Orbitals
“hit” the
ligands
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Crystal field splitting of d
orbitals
Orbitals that interact more strongly with ligands
have
higher energy
The result is a splitting of the levels
In octahedral field the gang of two (dz2 and dx2-y2) are
higher than the gang of three (dxy, dxz and dyz)
Crystal field and the colour problem
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Electrons in partially filled d orbitals can be
excited from lower occupied to higher
unoccupied orbitals
Frequency of absorption is proportional to
crystal field splitting: Δ = hc/λ
Δ corresponds to photons in visible range
Transition metal compounds have colors
Coat of many colours
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Transition metal complexes have colours that
vary strongly with the ligand (different Δ)
Spectrochemical series orders ligands
according to the degree of crystal field
splitting achieved
Complex
Wavelength abs
Color abs
Color seen
CoF63-
700
red
Green
Co(H2O63+
600
orange
Blue
Co(NH3)5H2O
500
Blue-green
Red
Co(NH3)63+
475
blue
Yellow
Co(CN)63-
310
ultraviolet
Pale yellow
Calculating crystal field splitting
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An absorption peak of 500 nm corresponds to
a crystal field splitting of
(6.626x1034 J .s)(3.00x108 m / s)
19
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3
.
98
x
10
J
9
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500x10 m
hc
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On a molar basis
19
  (3.98x10 J / ion)(6.02x10 ion / mol )
 240kJ / mol
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23
Higher the photon energy, higher the Δ
Spectrochemical series of ligands
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Weak field
I-<Br-<Cl-<F-<H2O<NH3<en<CN-<CO
 Strong field
When the d orbitals are empty (d0) or full (d10),
the complexes are colourless – no d – d
transitions
The theory successfully accounts for observed
optical and magnetic properties
Comparison of Co(CN)63- andCoF63
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Electrons don’t want to be paired
Electrons don’t like heights
Electronic balancing act: Opposition of electronelectron repulsion (P) and lower energy of lower
lying orbitals (Δ)
High-spin complex: Δ is lower than P (electrons
unpaired, repulsion dominates)
Low-spin complex: Δ is higher than P (electrons pair,
lower energy of the lower orbitals)
Important note
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Low-spin, high-spin dichotomy only occurs for d4 –
d7.
d1 – d3 and d8 – d10 only have one configuration
Crystal field splitting in square
planar and tetrahedral complexes
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Tetrahedral is inverse of octahedral
Δ is lower than in octahedral because of fewer ligands –
all complexes high-spin
Crystal field splitting in square planar is between the
high-lying d x  y and the d xy orbital
Square planar is favoured for d8 configuration
2
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