CMB004   d-BLOCK COORDINATION CHEMISTRY
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CMB004 d-BLOCK COORDINATION CHEMISTRY Prof. V. McKee (Room F2.05, email [email protected]). General Information: These lectures follow on from those given by Dr Smith in CMA002.

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CMB004 d-BLOCK COORDINATION CHEMISTRY

Prof. V. McKee

(Room F2.05, email [email protected])

General Information:

These lectures follow on from those given by Dr Smith in CMA002.

Revision material, powerpoint slides and outline lecture notes for this section will be available on LEARN.

Recommended Texts:

General inorganic textbook:

Housecroft & Sharpe “Inorganic Chemistry” Prentice Hall, 4th Edition, 2012

(mostly Chapter 20).

More specific material:

M.J. Winter, “d-Block Chemistry”, OUP, 2000

C.J. Jones, “d- and f-block Chemistry”, RSC, 2001

M. Gerloch and E.C. Constable, “Transition Metal Chemistry”, VCH,2005.

(electronic book)


He

H

Li

Be

B

C

N

O

F

Ne

Na

Mg

Al

Si

P

S

Cl

Ar

Ca

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Ga

Ge

As

Se

Br

Kr

K

Rb

Sr

Y

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Ag

Cd

In

Sn

Sb

Te

I

Xe

Cs

Ba

La

Hf

Ta

W

Re

Os

Ir

Pt

Au

Hg

Tl

Pb

Bi

Po

At

Ra

Fr

Ra

Ac

First transition series

d-block

d-Block coordination chemistry -

focus on the first transition series

  • Unique properties of transition metals – e.g. colour and magnetism – depend on the number and distribution of electrons in the d-orbitals.

  • for the first transition series these are the 3d orbitals.

  • Electronic configurations of metals generally 4s2 3dn (and +2 ions 4s0 3dn )


dxy

dxz

dyz

y

x

x

y

z

z

z

x

y

y

x

z

z

z

linear

Combination

y

x

+

=

dz2

dz2-y2

dz2-x2

5 d-orbitals:

interaxial

orbitals

Why is

this one a different

shape?

axial

orbitals

dx2-y2

dz2

Generally behave similarly


z

z

z

z

z

x

dxz

dyz

dxy

dx2-y2

dz2

You will need to know the shapes and orientation of these orbitals

In a free transition metal atom or ion the 5 d-orbitals are degenerate and the set is spherically symmetrical.

In complexes this is no longer true – the d-orbitals are split into two or more sets with different energies.

This is the basis of their unusual properties – so a key point to understand.


Which structure/bonding models can be used for transition metal complexes ?

Molecular orbital theory can be used – but it is complicated.

Hybridisation models do not work well – they do not predict the structures or properties correctly (don’t try to use them).

Crystal Field Theory (CFT)is a simple approach which gives useful results most of the time.

Assumptions of Crystal Field Theory:

1. Both the metal ion and the ligands can be treated as point electrostatic charges

2. The only interactions between the metal ion and the ligands are electrostatic

Neither of these assumptions is true for real complexes …


Imagine a free metal ion at the centre of a set of Cartesian coordinates:

z

z

y

y

+

+

x

x

E

E

Gedanken experiment (“thought experiment”):

Imagine the ion is surrounded by a sphere of negative charge:

Electrons in d-orbitals experience repulsion from the surrounding negative charge:

The 5 d-orbitals are degenerate (have the same energy):

So the energy of the d-orbitals is raised relative to the free ion – but they are still degenerate.


+ coordinates:

z

z

y

y

+

+

x

x

E

Free ion

Spherical field

What happens if all the negative charge is collected on the axes – like the ligands in an octahedral complex?

Orbitals directed along the axes will have their energy raised

Orbitals directed between the axes will have their energy lowered

z

y

x

Octahedral Complex


e coordinates:g

Do

t2g

So, in an octahedral complex, the 5 d-orbitals should split into a higher energy, doubly degenerate set and a lower energy, triply degenerate set and

both are at higher energy than in the free gaseous ion

The d-orbital configuration is usually represented in a d-orbital energy level diagram:

eg and t2g are symmetry labels:

g means the complex is centrosymmetric

e means doubly degenerate (2 orbitals with the same energy)

t means triply degenerate (3 orbitals with the same energy)

(1 or) 2 means (symmetric or) antisymmetric with respect to rotation about an axis other than the principal axis – we will not be concerned with this term.

Dis the crystal field splitting parameter (sometimes labelled 10Dq). It is the

Energy difference between the two sets of d orbitals.

The subscript o indicates octahedral geometry.

D is an important quantity.


r coordinates:

1

z1z2

4pe0

r2

constant

square of distance

CFT assumes the only interactions between metal and ligands are electrostatic

Electrostatic energy depends only on the size of the charges and the distance between them.

Electrostatic

Energy F =

product of the

charges

So, the energies of the “spherical field” and the “octahedral complex” are equal - since the total charges have not changed and the distance between them has not changed


The total stabilisation of the t coordinates:2g orbitals

(relative to the spherical distribution)

The total destabilisation of the eg orbitals

(relative to the spherical distribution)

E

eg

D - x

x

spherically

symmetric

Do

t2g

Centre of Gravity Rule: the total energy of the system did not change in going from the spherical arrangement to octahedral geometry, so:

2x = 3(D – x)

2x =3D – 3x

3/5D

5x =3D

2/5D

x =3/5D

D – x =2/5D

the interaxial t2g orbitals are eachstabilised by- 2/5D

the axial eg orbitals are each destabilised by3/5D

- relative to a spherically symmetric arrangement.

(Note the minus sign indicates lower energy – ie more stable)


You should now be able to do Tutorial Question 1 (handout): coordinates:

Tutorial Question 1:

Sketch labelled diagrams showing the shapes and orientations of the dxy and dx2-y2orbitals.

Using Crystal Field Theory, explain clearly why the energies of the

dxy and dx2-y2 orbitals are different in an octahedral complex ion.


d coordinates:x2-y2

square planar complex

dxy

z

y

L

dz2

L

L

M

x

L

}

dxz

dyz

ligands lie along x and y

z

z

z

z

z

x

dxz

dyz

dxy

dx2-y2

dz2

Crystal Field Theory for Other Common Geometries

CFT Key Idea: To predict d-orbital splittings it is only necessary to consider which d-orbitals are most closely approached by ligands in that geometry.

Which orbital is at highest energy?

Next highest energy?

Next?

Lowest?


L coordinates:

L

dx2-y2

L

L

dxy

L

L

L

L

M

L

M

L

L

M

L

L

dz2

L

L

tetragonal

square planar

octahedral

}

dxz

dyz

L

dx2-y2 dz2

dxz dyz dxy

Alternative – consider effect of gradually removing two ligands from an octahedral complex to form a square plane – progressive tetragonal distortion

Note: many Cu(II) and Mn(III) complexes have tetragonal geometry


y coordinates:

L

L

x

M

z

L

L

conventional

orientation

y

interaxial

y

axial

x

x

z

d-orbital splitting for tetrahedral

Tetrahedral geometry

Consider dx2—y2 :

Consider dxy:

Not directed towards ligands – but closer

Doesn’t point directly at any ligands


e coordinates:g

E

t2

Do

2/5Dt

spherically

symmetric

Dt

3/5Dt

e

t2g

tetrahedral

octahedral

2/5Do

3/5Do

The most common geometries are octahedral and tetrahedral (know these)

– and their d-orbital energy level diagrams are the inverse of each other:

The subscript “g” is not used in the tetrahedral case – not centrosymmetric

The centre of gravity rule applies in both cases

Values of D are less for tetrahedral complexes than for octahedral ones as:

1) The distinction between weaker/stronger interaction with ligands is less marked

2) There is less electrostatic repulsion with 4 ligands than with 6 (similar) ligands

In theory Dt= 4/9Do in practice Dt≈ 1/2Do


e coordinates:g

E

t2

Do

2/5Dt

spherically

symmetric

Dt

3/5Dt

e

t2g

tetrahedral

octahedral

3/5Do

2/5Do

The most common geometries are octahedral and tetrahedral (know these)

– and their d-orbital energy level diagrams are the inverse of each other:

In theory Dt= 4/9Do in practice Dt≈ 1/2Do


L coordinates:

L

L

M

L

L

L

L

L

M

L

trigonal bipyramid

Note: compressed tetragonal

L

L

We will concentrate on octahedral and tetrahedral geometries because they are the most common (and the simplest).

However, the same arguments can be applied to any geometry.

Tutorial question 2:

Deduce the crystal field splitting patterns for the following geometries:

i) Compressed tetragonal

ii) Trigonal bipyramidal


  • At this point you should be able to: coordinates:

  • Remember the orientations of the 5 d-orbitals

  • State the assumptions underlying Crystal Field Theory

  • Explain the origin of d-orbital splitting.

  • Derive d-orbital energy level diagrams for any geometry

  • Define the crystal field splitting parameter, D

  • Apply the Centre of Gravity Rule

  • Use the orbital symmetry labels and conventions.

Note: it is electrons in orbitals that interact with ligands, not the orbitals themselves.


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