coordination chemistry ii (1)

8/22/2019 Coordination Chemistry II (1)

1/77

Coordination

Chemistry IIBonding, including crystal field theory

and ligand field theory


2/77

Basis for Bonding Theories

Models for the bonding in transition metal

complexes must be consistent with observed

behavior. Specific data used include stability (or

formation) constants, magnetic susceptibility,and the electronic (UV/Vis) spectra of the

complexes.


3/77

Bonding Approaches

Valence Bond theory provides the

hybridization for octahedral complexes. For the

first row transition metals, the hybridization can

be: d2sp3 (using the 3d, 4s and 4p orbitals), orsp3d2 (using the 4s, 4p and 4d orbitals).

The valence bond approach isnt used

because it fails to explain the electronic spectraand magnetic moments of most complexes.


4/77


5/77

Crystal Field Theory

In crystal field theory, the electron pairs on

the ligands are viewed as point negative charges

that interact with the dorbitals on the central

metal. The nature of the ligand and thetendency toward covalent bonding is ignored.


6/77

d Orbitals


7/77


Ligands, viewed as point charges, at the

corners of an octahedron affect the various d

orbitals differently.


8/77



9/77


The repulsion

between ligand lone

pairs and the d

orbitals on the metalresults in a splitting of

the energy of the d

orbitals.


10/77

d Orbital Splitting

__ __ __ __ __

Spherical field

__ __

dz2 dx2-y2

__ __ __

dxy dxz dyz

o0.6o

0.4o

Octahedral field

eg

t2g


11/77

d Orbital Splitting

In some texts and articles, the gap in the d

orbitals is assigned a value of 10Dq. The upper

(eg) set goes up by 6Dq, and the lower set (t2g)

goes down by 4Dq.

The actual size of the gap varies with the

metal and the ligands.


12/77

d Orbital Splitting

The colors exhibited by most transition

metal complexes arises from the splitting of the

dorbitals. As electrons transition from the

lower t2gset to the egset, light in the visiblerange is absorbed.


13/77

d Orbital Splitting

The splitting dueto the nature of theligand can beobserved and

measured using aspectrophotometer.Smaller values of oresult in colors in the

green range. Largergaps shift the color toyellow.


14/77

The Spectrochemical Series

Based on measurements for a given metal

ion, the following series has been developed:

I-


15/77

The Spectrochemical Series

The complexes of

cobalt (III) show the

shift in color due to the

ligand.(a) CN, (b) NO2

, (c)

phen, (d) en, (e) NH3, (f)

gly, (g) H2O, (h) ox2, (i)

CO3 2.


16/77

Ligand Field Strength Observations

1. o increases with increasing oxidation number

on the metal.

Mn+2


17/77

Ligand Field Theory

Crystal Field Theory completely ignores the

nature of the ligand. As a result, it cannot

explain the spectrochemical series.

Ligand Field Theory uses a molecular orbital

approach. Initially, the ligands can be viewed as

having a hybrid orbital or ap orbital pointing

toward the metal to make bonds.


18/77


19/77

Octahedral Symmetry

http://www.iumsc.indiana.edu/morphology/sym

metry/octahedral.html
http://www.iumsc.indiana.edu/morphology/symmetry/octahedral.htmlhttp://www.iumsc.indiana.edu/morphology/symmetry/octahedral.htmlhttp://www.iumsc.indiana.edu/morphology/symmetry/octahedral.htmlhttp://www.iumsc.indiana.edu/morphology/symmetry/octahedral.html


20/77

Ligand Field Theory

Oh E 8C3 6C2 6C4

3C2

(=C42) i 6S4 8S6 3h 6d

6 0 0 2 2 0 0 0 4 2

This reduces to A1g+ Eg+ T1u

Consider the sigma bonds to all six ligandsin octahedral geometry.


21/77

Ligand Field Theory

The A1ggroup

orbitals have the same

symmetry as an s

orbital on the centralmetal.


22/77

Ligand Field Theory

The T1u group


symmetry as thep

orbitals on the centralmetal.

(T representations

are triply degenerate.)


23/77

Ligand Field Theory

The Eggroup


symmetry as the dz2

and dx2-y2 orbitals onthe central metal.

(E representations are

doubly degenerate.)


24/77

Ligand Field Theory

Since the ligands

dont have a

combination with t2g

symmetry, the dxy, dyzand dxyorbitals on the

metal will be non-

bonding whenconsidering bonding.


25/77

Ligand Field Theory

The molecular

orbital diagram is

consistent with the

crystal fieldapproach.

Note that the

t2gset of orbitals is

non-bonding, andthe egset of orbitals

is antibonding.


26/77

Ligand Field Theory

The electrons

from the ligands

(12 electrons

from 6 ligands inoctahedral

complexes) will

fill the lowerbonding orbitals.{


27/77

Ligand Field Theory

The electrons

from the 4s and

3d orbitals of the

metal (in the firsttransition row)

will occupy the

middle portion ofthe diagram.

{


28/77

Experimental Evidence for Splitting

Several tools are used to confirm the

splitting of the t2gand egmolecular orbitals.

The broad range in colors of transition metal

complexes arises from electronic transitions as

seen in the UV/visible spectra of complexes.

Additional information is gained from

measuring the magnetic moments of thecomplexes.


29/77

Experimental Evidence for Splitting

Magnetic susceptibilitymeasurements can be

used to calculate the

number of unpaired

electrons in a compound.

Paramagnetic

substances are attracted

to a magnetic field.


30/77

Magnetic Moments

A magnetic balance can be used to

determine the magnetic moment of a substance.

If a substance has unpaired electrons, it is

paramagnetic, and attracted to a magnetic field.

For the upper transition metals, the spin-

only magnetic moment, s, can be used to

determine the number of unpaired electrons.

s = [n(n+2)]1/2


31/77

Magnetic Moments

The magnetic moment of a substance, in

Bohr magnetons, can be related to the number

of unpaired electrons in the compound.

s = [n(n+2)]1/2

Where n is the number of unpaired electrons


32/77

Magnetic Moments

Complexes with 4-7 electrons in the d

orbitals have two possibilities for the

distribution of electrons. The complexes can be

low spin, in which the electrons occupy the lowert2gset and pair up, or they can be high spin. In

these complexes, the electrons will fill the upper

egset before pairing.


33/77

High and Low Spin Complexes

If the gap between

the dorbitals is large,

electrons will pair up and

fill the lower (t2g) set oforbitals before

occupying the egset of

orbitals. The complexesare called low spin.


34/77


In low spin

complexes, the size

of o is greater than

the pairing energy ofthe electrons.


35/77


If the gap between

the dorbitals is small,

electrons will occupy the

egset of orbitals beforethey pair up and fill the

lower (t2g) set of orbitals

before. The complexesare called high spin.


36/77


In high spin

complexes, the size

of o is less than the

pairing energy of theelectrons.


37/77

Ligand Field Stabilization Energy

The first row transition metals in water are

all weak field, high spin cases.

do d1 d2 d3 d4 d5 d6 d7 d8 d9 d10

LFSE 0 .4o .8 1.2 .6 0 .4 .8 1.2 .6 0


38/77

Experimental Evidence for LFSE

The hydration energies of the first row

transition metals should increase across the period

as the size of the metal ion gets smaller.

M2+ + 6 H2O(l)M(H2O)62+


39/77

Experimental Evidence for LFSE

The heats of

hydration show two

humps consistent

with the expected LFSEfor the metal ions. The

values for d5 and d10 are

the same as expectedwith a LFSE equal to 0.


40/77

Experimental Evidence of LFSE

do d1 d2 d3 d4 d5 d6 d7 d8 d9 d10

LFSE 0 .4o .8 1.2 .6 0 .4 .8 1.2 .6 0


41/77

High Spin vs. Low Spin

3d metals are generally high spin complexes except

with very strong ligands. CN- forms low spin

complexes, especially with M3+ ions.

4d & 4d metals generally have a larger value of o

than for 3d metals. As a result, complexes are

typically low spin.


42/77

Nature of the Ligands

Crystal field theory and ligand field theory

differ in that LFT considers the nature of the

ligands. Thus far, we have only viewed the

ligands as electron pairs used for makingbonds with the metal. Many ligands can also

form bonds with the metal. Group theory

greatly simplifies the construction of molecularorbital diagrams.


43/77

Considering Bonding

To obtain red for bonding, a set of

cartesian coordinates is established for each of

the ligands. The direction of the bonds is

arbitrarily set as theyaxis (or the pyorbitals).The px and pz orbitals are used in bonding.


44/77

Considering

Bonding

y y

y

y y

y

x

x

x

x

x

x

zz

z

z

zz

Oh E 8C3 6C2 6C4

3C2(=C42) i 6S4 8S6 3h 6d

12 0 0 0 -4 0 0 0 0 0

Consider only the px and

pz orbitals on each of

the ligands to obtain .


45/77

Considering Bonding

This reduces to T1g+ T2g+ T1u + T2u. The T2g

set has the same symmetry as the dxy, dyz and dxzorbitals on the metal. The T1u set has the same

symmetry as the px, pyand pz orbitals on the metal.

Oh E 8C3 6C2 6C43C2

(=C42)

i 6S4 8S6 3h 6d

12 0 0 0 -4 0 0 0 0 0


46/77

Considering Bonding

reduces to: T1g+ T2g+ T1u + T2u.

The T1gand T2ugroup orbitals for the ligands dont matchthe symmetry of any of the metal orbitals.

The T1u set has the same symmetry as the px, pyand pzorbitals on the metal. These orbitals are used primarily tomake the bonds to the ligands.

The T2gset has the same symmetry as the dxy, dyz and dxzorbitals on the metal.


47/77

Bonding

The main source of bonding is between

the dxy, dyz and dxz orbitals on the metal and the

d, p or * orbitals on the ligand.


48/77

Bonding

The ligand may have empty d or * orbitals

and serve as a acceptorligand, or full p or d

orbitals and serve as a donorligand.


49/77

Bonding

The empty antibonding orbital on CO can

accept electron density from a filled dorbital on

the metal. CO is api acceptorligand.

empty*

orbitalfilled d

orbital


50/77

Donor Ligands (LM)All ligands are donors. Ligands with filled

p or dorbitals may also serve as pi donor ligands.

Examples of donor ligands are I-, Cl-, and S2-.

The filled p or d orbitals on these ions interactwith the t2gset of orbitals (dxy, dyz and dxz) on

the metal to form bonding and antibonding

molecular orbitals.


51/77

Donor Ligands (LM)The bonding orbitals,

which are lower in energy,

are primarily filled with

electrons from the ligand,the and antibonding

molecular orbitals are

primarily occupied byelectrons from the metal.


52/77

Donor Ligands (LM)The size of o

decreases, since it is now

between an antibonding t2g

orbital and the eg* orbital.This is confirmed by

the spectrochemical series.

Weak field ligands are alsopi donor ligands.


53/77

Acceptor Ligands (ML)Ligands such as CN,

N2 and CO have empty

antibonding orbitals

of the proper symmetryand energy to interact

with filled dorbitals on

the metal.


54/77

Acceptor Ligands (ML)The metal uses the

t2gset of orbitals (dxy,

dyz and dxz) to engage in

pi bonding with theligand. The * orbitals

on the ligand are usually

higher in energy thanthe d orbitals on the

metal.


55/77

Acceptor Ligands (ML)The metal uses the

t2gset of orbitals (dxy,

dyz and dxz) to engage in

pi bonding with theligand. The * orbitals

on the ligand are usually

higher in energy thanthe d orbitals on the

metal.


56/77

Acceptor Ligands (ML)The interaction

causes the energy of the

t2gbonding orbitals to

drop slightly, thusincreasing the size of

o.


57/77


58/77

Summary

1. All ligands are donors. In general, ligand that

engage solely in bonding are in the middle of

the spectrochemical series. Some very strong

donors, such as CH3- and H- are found high inthe series.

2. Ligands with filledp or dorbitals can also serve

as donors. This results in a smaller value ofo.


59/77

Summary

3. Ligands with emptyp, d or * orbitals can also

serve as acceptors. This results in a larger

value of o.

I-


60/77

4 Coordinate Complexes

Square planar and tetrahedral complexes are

quite common for certain transition metals. The

splitting patterns of the dorbitals on the metal

will differ depending on the geometry of thecomplex.


61/77

Tetrahedral Complexes

The dz2 and dx2-y2 orbitals

point directly between the

ligands in a tetrahedral

arrangement. As a result, thesetwo orbitals, designated as ein

the point group Td, are lower in

energy.


62/77


The t2set of orbitals,consisting of the dxy, dyz, and

dxz orbitals, are directed more

in the direction of the ligands.

These orbitals will be

higher in energy in a

tetrahedral field due to

repulsion with the electrons

on the ligands.

T


63/77


The size of the splitting,T, is considerable smaller

than with comparable

octahedral complexes. This is

because only 4 bonds are

formed, and the metal orbitals

used in bonding dont point

right at the ligands as they doin octahedral complexes.

T h d l C l


64/77


In general, T 4/9o. Since the splitting

is smaller, all tetrahedral

complexes are weak-

field, high-spin cases.


65/77

Tetragonal Complexes

Six coordinate complexes, notably those of

Cu2+, distort from octahedral geometry. One

such distortion is called tetragonal distortion, in

which the bonds along one axis elongate, withcompression of the bond distances along the

other two axes.


66/77


The elongationalong the zaxis causes

the dorbitals with

density along the axis todrop in energy. As a

result, the dxz and dyz

orbitals lower in energy.


67/77


The compressionalong the xandyaxis

causes orbitals with

density along these axesto increase in energy.

.


68/77


For complexes with1-3 electrons in the egset

of orbitals, this type of

tetragonal distortion maylower the energy of the

complex.


69/77

Square Planar Complexes

For complexes with 2electrons in the egset of

orbitals, a d8 configuration,

a severe distortion mayoccur, resulting in a 4-

coordinate square planar

shape, with the ligands

along the zaxis no longer

bonded to the metal.


70/77


Square planarcomplexes are quitecommon for the d8 metalsin the 4th and 5th periods:

Rh(I), IR(I), Pt(II), Pd(II)and Au(III). The lowertransition metals have large

ligand field stabalizationenergies, favoring four-coordinate complexes.


71/77


Square planarcomplexes are rare for the

3rd period metals. Ni(II)

generally forms tetrahedralcomplexes. Only with very

strong ligands such as CN-,

is square planar geometry

seen with Ni(II).


72/77


The value of sp for agiven metal, ligands and

bond length is

approximately 1.3(o).


73/77

The Jahn-Teller Effect

If the ground electronic configuration of a non-linear

complex is orbitally degenerate, the complex will distort

so as to remove the degeneracy and achieve a lower energy.


74/77


The Jahn-Teller effect predicts whichstructures will distort. It does not predict the

nature or extent of the distortion. The effect is

most often seen when the orbital degneracy is inthe orbitals that point directly towards the

ligands.


75/77


In octahedral complexes, the effect is mostpronounced in high spin d4, low spin d7 and d9

configurations, as the degeneracy occurs in the

egset of orbitals.

d4 d7 d9

eg

t2g


76/77


The strength of the Jahn-Teller effect istabulated below: (w=weak, s=strong)

# e- 1 2 3 4 5 6 7 8 9 10

High

spin* * * s - w w * * *

Low

spin w w - w w - s - s -

*There is only 1 possible ground state configuration.

- No Jahn-Teller distortion is expected.


77/77

Experimental Evidence of LFSE

do

d1

d2

d3

d4

d5

d6

d7

d8

d9

d10

LFSE 0 .4o .8 1.2 .6 0 .4 .8 1.2 .6 0

coordination chemistry ii (1)

Documents