1 Introductory

8/3/2019 1 Introductory

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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In TM metal chemistry VBT is superseded by the Crystal FieldTheory (CFT).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.

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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)

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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.


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.)


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



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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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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.



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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.



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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.



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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



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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.



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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+)



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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-

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1 Introductory