1 introductory
TRANSCRIPT
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Chem 310 Lectures by: Dr. Muhammad D. Bala
Office: Block H, 3-64
Contacts: phone extn. 2616;
e-mail [email protected] texts: Shriver and Atkins: Inorg. Chem., 4th Ed.
Cotton, Wilkinson and Gaus: Basic Inorg. Chem., 3rd
Ed. David Nicholls: Complexes and 1st row transition elements
In this part of the course we have about 6 weeks to cover thefollowing topics:
1.Bonding: VBT, CFT, LFT and MOT
2.Magnetic and electronic properties of TM oxides3.Reactivities of TM complexes
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Covalent bonds by sharing pairs of electrons was firstproposed by G. N. Lewis in 1902.
It was not until 1927, however, that Walter Heitler and FritzLondon showed how the sharing of pairs of electrons holds acovalent molecule together.
The Heitler-London model of covalent bonds was the basis ofthe valence-bond theory.
The last major step in the evolution of this theory was the suggestionby Linus Pauling that atomic orbitals mix to form hybrid orbitals,such as the sp, sp2, sp3, dsp3, and d2sp3 orbitals.
The Valence-Bond Theory
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The Valence-Bond Theory
It is easy to apply the valence-bond theory to somecoordination complexes, such as the Co3+ complexes below.
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d2sp3- inner sphere complex low spin complex
sp3d2- outer sphere complex high spin complex
Note: Such a situation will not arise for for d1, d2 and d3 ion configuration.
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Assumes that all d orbitals in a complex are equal in energy.
The arbitrary use of 3d and 4d orbitals for bonding energy
differential ignored.
The theory is unable to adequately explain electronic and magnetic
properties of complexes.
VBT is widely used in organic and main group element chemistry.
In TM metal chemistry VBT is superseded by the Crystal FieldTheory (CFT).
In combination with MOT it is often referred to as the Ligand FieldTheory (LFT).
Deficiencies of VB approach to bonding
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Crystal Field Theory is based on the idea that a purely electrostaticinteraction exists between the central metal ion and the ligands.
Covalent bonding is ignored.
Crystal field theory was developed by considering two compounds:
manganese(II) oxide, MnO octahedral geometry,copper(I) chloride, CuCl tetrahedral geometry.
The Crystal-Field Theory
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Mn+-
- -
-
--
y zOctahedral complexes
Each Mn2+ ion in manganese(II) oxide issurrounded by six O2- ions arranged toward thecorners of an octahedron.
What happens to the energies of the 4sand 4porbitals on an Mn2+ ion?
Let's assume that the six O2- ions that surround each Mn2+ iondefine anXYZcoordinate system. Two of the 3dorbitals (dx2-y2 anddz2) on the Mn2+ ion point directly toward the six O2- ions. The other
three orbitals (dxy, dxz, and dyz) lie between the O2- ions.
Therefore, the five 3dorbitals on the Mn2+ ion are no longer
degenerate, i.e. no longer equal, or -of the same energy.
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Degree of e-static repulsion depends on the orientation of the d
orbitals.
d orbitals (dz2 and dx2-y2 ) with lobes directed at the ligands
experience more repulsion placed at higher energy eg
oebitals.
d orbitals (dxy, dxz and dyx) with lobes in-between the ligands
experience less repulsion placed at lower energy t2gorbitals.
Mn+-
- -
-
--
x
y zOctahedral Complexes
The ligands approach the metal along x, y, z.
e-s in d orbital are repulsed by vely charged
ligands increase in potential energy.
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Octahedral Complexes
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The s-orbital of the metal is spherically symmetric.
The three p-orbitals lie along the xyz axes, point directly towards the
ligandsTherefore they all remain degenerate.
Energy of the s- and p-orbitals is raised due to the increased repulsion
between the negative point charges representing the ligands and thenegative charge of the electrons orbital orbitals are raised in energy.
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y
x
dz2
y
y z
x
x x
zdx
2
-y2
dxy dxz dyz
Electron-density distribution in the d orbitals
egsubset
t2gsubset
E
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The Crystal-field parameter ( or 10 Dq)
The Crystal Field splittingparameter has been used tocorrelate a wide range of
properties of first-rowtransition metal complexes.
This includes structure,electronic spectra andmagnetic properties.
It is perhaps the single mostuseful parameter inunderstanding coordination
chemistry.
4/9
t2
e
eg
t2g
free ionoctahedralfield
tetrahedralfield
3/5
2/5
2/5
3/5
barycentre
+6Dq
-4Dq
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yellow
The Crystal Field splitting parameter variessystematically with the type of ligand.
E. g. in complexes of [CoX(NH3)5]n+ with
X = I-, Br-, Cl-, H2O and NH3 the coloursrange from purple (for X = I- ) thru to pink
(for Cl- ) to (with NH3).
The same order was observed regardless of
the metal ion.
Based on these observations RyutaroTsuchida proposed the spectrochemical seriesfor ligands.
The Crystal-field parameter ( or 10 Dq)
4/9
t2
e
eg
t2 g
free ionoctahedralfield
te trahedral
field
3/5
2/5
2/5
3/5
Th S h i l i
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From a purely ionic basis we would expect CO < H2O < C2O42-
Rh3+ >
Co3+ >
Cr3+ >
Fe3+ >
Fe2+ >
Co2+ >
Ni2+ >
Mn2+
Strong-field ions Weak-field ions
The Spectrochemical series
El fi i f TM l
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Degenerate orbitals are filled according to Hund's rules:
One electron is added to each of the degenerate orbitals in a subshellbefore a second electron is added to any orbital in the subshell lowestenergy subshell filled in first.
Electrons are added to a subshell with the same value of the spinquantum number until each orbital in the subshell has at least one
electron least electrostatic repulsion.
Order of filling d-orbitals depend both on and the pairing energy, P: If > P is large, strong field ligande-s pair up in the lower
energy subshell first, e.g. t2g for octahedral CF Low spin complex strong field inner sphere.
If
< P
is small, weak field ligand
e
-
s spread out among alld-orbitals before any pairing, e.g. t2g and eg for octahedral CF High-spin complexweak field outer sphere.
Electron configuration of TM complexes
d d T
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for d1, d2, d3 and d8, d9, d10 the ligand type has same effect on
the d-orbital electron configuration. for CFSE calculation, strong or weak field ligand will be same.
Octahedral electron configuration of octahedral TM complexes
< P and > Pd
3
d
10t2g3eg
0 t2g6eg
4
d1 d8t2g1eg
0 t2g6eg
2
d2 d9t2g
2eg0 t2g
6eg3
O h d l l fi i f h d l TM l
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Octahedral electron configuration of octahedral TM complexes
< P High spind
7 t2g5eg
2 t2g6eg
1
d4
d6
t2g3eg
1
t2g4eg
2
t2g4eg
0
t2g6eg
0
d5 t2g3eg
2 t2g5eg
0
> P Low spin
C l Fi ld S bili i E (CFSE)
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CrystalField Stabilisation Energy (CFSE)
CFT predicts stabilisation for some electron configurations in the d
orbitals.For an octahedral complex with d orbital configuration t2g
xegy, with
respect to the barycentre:
An electron in the more stable t2g subset is treated as contributing a
stabilisation of0.4o OR 4Dq.
An electron in the higher energy eg subset contributes to a
destabilisation of0.6o OR 6Dq.
Therefore, the CFSE = (0.4x 0.6y)o - P (P = pairing energy)
OR CFSE = (4x - 6y)Dq - P (P = pairing energy)
C t l Fi ld St bili ti E (CFSE)
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Q. Explain why the Co(NH3)63+ ion is a diamagnetic, low-spin
complex, whereas the CoF63- ion is a paramagnetic, high-spincomplex. Give the electron configuration and calculate the CFSE
of each complex in terms of.
Answer:Co = [Ar] 3d7 4s2 Co3+ = [Ar] 3d6
Co(NH3)63+ = strong field ligand, > P pairing of electrons t2g
6
electron configuration.
CoF63- = weak field ligand, < P spreading of electrons t2g
4 eg2
CFSE = (0.4x 0.6y)o
1. Co(NH3)63+ x = 6 and y=0 CFSE = 2.4 .
2. CoF63- x = 4 and y = 2 CFSE = 0.4.
CrystalField Stabilisation Energy (CFSE)
Cr t l Fi ld St bili ti n En r (CFSE)
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Q. Determine which of the following are more likely to be high spincomplexes:
(1) [Fe(CN)6]3-
(2) [FeF6]3-
(3) [Co(H2O)6]+3
(4) [Co(CN)6]-3
(5) [Co(NH3)6]+3
(6) [Co(en)3]+3
Solution: Compare the ligands on the spectrochemical series. Sincewe want a high spin complex, we want weak field ligands. The weakerfield ligands in the above are H2O and F
-, so complexes 2 and 3 are
more likely to be high spin. (The cyanide complexes are least likely.)
CrystalField Stabilisation Energy (CFSE)
Crystal Field Stabilisation Energy (CFSE)
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Q1. Determine the CFSE if Dq = 2100 cm-1 for the following
configurations:
a) d4 high spin.
b) d4 low spin, assume the pairing energy P = 28,000 cm-1.
Q2. Given a Dq value of 1040 and 3140 cm-1 for high spin and lowspin d6 ion respectively, determine the CFSE in the followingconfigurations if the pairing energy P = 17,600 cm-1:
a ) weak field
b) strong field
CrystalField Stabilisation Energy (CFSE)
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Q1. a) d4 high spin.
Configuration = t2g3; eg1.Therefore from CFSE = (-4x +6y)Dq + PCFSE = -4 x 3 + 6 = -6Dq = -12,600cm-1 ; remember for high spin
up to d5 configuration P = 0
b) d4 low spin, P = 28,000 cm-1.
Configuration = t2g4 CFSE = -4 x 4 = -16Dq + P = -5600 cm-1
Q2. a ) d6 in a weak fieldConfiguration = t2g
4; eg2.Therefore from CFSE = (-4x +6y)Dq +dP
CFSE = -4 x 4 + 6 x 2 + P = -4Dq + P = 13,440cm-1
b) d6 in a strong fieldConfiguration = t2g
6.Therefore from CFSE = (-4x)Dq + 3P
CFSE = -4 x 6 = -24Dq + 3P = -22,560cm-1
Tetrahedral complexes
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Tetrahedral complexes
In the tetrahedral case the electrons go to the lower energy doublydegenerate e orbital first. The upper triply degenerate t2 orbital is filled in afterwards. The subscripts g (gerund, german for even) and u (ungerund,german for uneven) are missing for the tetrahedral geometry.
g and u refer to inversion about a centre of symmetry which isabsent for the tetrahedron.
Tetrahedral complexes
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Tetrahedral complexes
Note: For tetrahedral complexes, those orbitals which point towards the
edges (dxy, dyz and dxz) e subset are raised to energy higher thanthose which point towards the faces t2 subset.
That is, the exact opposite of the octahedral crystal field.
A tetrahedral complex has fewer ligands, only 4 to be exact.
The orbitals point in-between ligands rather than at the ligands lower repulsion.
Due to the reasons above, the magnitude of the tetrahedral CF splitting
is smaller t =4/9o
Because t < o it is energetically favourable to spread out electronsall tetrahedral complexes are high-spin.
Tetrahedral complexes
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p
Also for low spin complexes to exist t > P, hence low spin tetrahedral
complexes of any TM configuration are very rare.
Some factors that favour formation of tetrahedral complexes:
Bulky ligands: inter-ligand repulsion exacerbated in an octahedral
arrangement Steric factors
Weak field ligands: Many TM halides are tetrahedral
Electronic configuration of metal ion favours = zero (d0, d5 and d10)
or low values of OSPE d1, d6 and to a lesser extent d2, d7
configurations.
Weak field ions: Central metal in low oxidation state is low.
Octahedral Vs TetrahedralSummary: Field preference
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Summary: Field preference
tet = 4/9 oct.
Octahedral d3 ; d8 thend4 and d9
Tetrahedral (?) d1; d6 andd2; d7
No influence d0; d5 (highspin); and d10
Octahedral Site PreferenceEnergies, is defined as:
OSPE = CFSE (oct) -CFSE (tet)
0 1 2 3 4 5 6 7 8 9 10
0.2
0.4
0.6
0.8
1.0
1.2
Number of d electrons
CF
SE
( un
its of
)o
octahedral
tetrahedral
OSPE OSPE
Tetragonally distorted complexes: the Jahn-Teller Effect
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g y p J
The Jahn-Teller Theorem was published in 1937 and states:
Any non-linear molecular system in a degenerate electronic state will beunstable and will undergo distortion to form a system of lower symmetryand lower energy thereby removing the degeneracy
In simple terms it means that no nonlinear molecule can be stable in adegenerate electronic state. The molecules must become distorted toremove the degeneracy
In an octahedral crystal field, the t2gorbitals occur at lower energy thanthe egorbitals.
Only important for odd number occupancy of the eg level eg1 or eg
3.
The effect of Jahn-Teller distortions is best documented for Cu2+
complexes (with 3 electrons in the eg level) where the result is that mostcom lexes are found to have elon ation alon the z-axis.
Tetragonally distorted complexes: the Jahn-Teller Effect
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Tetragonally distorted complexes: the Jahn-Teller Effect
Consider the octahedral Cu2+ ion d9 t2g6 eg
3
The eg levels are degenerate 3 electrons dx2-y21 dz2
2 or dx2-y22 dz2
1
Hence the degeneracy of the eg levels is lifted ligands along the axesexperience different shielding effects 2 e-s vs. 1 e-. Resulting in z -axis contraction (dx2-y2
2 dz21) or elongation (dx2-y2
1 dz22).
As the z axis elongation increases the energy of the dz2 orbital dropslower than the dxyorbital which along with dx2-y2 orbital rises in energy.
Cu2+
X'
X'
X
X X
X
Cu---X Cu---X X = Cl 2.30 2.95X = Br 2.40 3.18
X = F 1.93 2.27
Tetragonally distorted complexes: the Jahn-Teller Effect
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Tetragonally distorted complexes: the Jahn Teller Effect
Veryrare
Also arises due to liganddissimilarity, e.g. in MA4B2
(dx2-y21 dz2
2) due to the2 electrons on the z axis
the dz2 orbital is moreshielded from the Cu2+
centre elongation
along this axes alsocalled tetragonal
distortion lower inenergy.
Tetragonally distorted complexes: the Jahn-Teller Effect
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The distortion has resulted in lower energy for the systemstabilisation.
Therefore many Cu2+ complexes are tetragonal even with six identicalligands, e.g. [Cu(H2O)6]
2+.
The main reason for tetragonal distortion is to remove the instability
brought about by the non-linearity and achieve stability in TM complexes.
No distortion for t2g3, t2g
3 eg2 , t2g
6, t2g6 eg
2 , t2g6 eg
4 .
Important cases of distortion are: t2g3 eg
1 (high spin Cr2+ and Mn3+) andt2g
6 eg1 (low spin Co2+ and Mn3+)
Tetragonally distorted complexes: the Jahn-Teller Effect
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Tetragonally distorted complexes: the Jahn Teller Effect
eg
t2g
(a) Octahedralfield
(b) Small tetragonaldistortion
(c) Large tetragonaldistortion
dxy
dxz
dyz
dx2-y
2
dz2
dz2
dz2
dx2-y
2
dx2-y
2
dyz
dyz
dxzdxz
dxydxy
Square planar complexes
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Recall Jahn-Teller tetragonal distortion discussed above.
Mostly formed by metals with d8 ions with strong field ligands, e.g.[NiII(CN)4]
2-. Note that [NiIIX4]2- forms tetrahedral complexes, why?
All complexes of Pt(II) and Au(III) are square-planar, including thosewith weak field ligands such as [PtCl4]
2-. Also true for most complexes
of Rh(I), Ir(I) and Pd(II) 4dand 5dcomplexes.
y
x
A square planar complex is
obtained when it is
energetically favourable to
have the configurationdyz
2dxz2dxy
2dx2-y22 for 4d8 and
5d8 ions.
dx2
-y2
dz2
dxy
} dxz; dyzFull dz2 and empty dx2-y2
Th l i h f h CFT k i i bl
Molecular Orbital Theory (MOT) and LigandField Theory (LFT)
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The purely e-static approach of the CFT makes it unsuitable toadequately explain metal-ligand bonding. TM compounds such as
Ni0(CO)4 with Ni in zero oxidation state must be purely covalent noM-L e-static attraction.
We have already seen that on purely e-static grounds, the order of thespectrochemical series is unviable, e.g. we expect CO < H2O < C2O4
2-