Wk6-Crystal Field Theory

8/13/2019 Wk6-Crystal Field Theory

1/48

Bondin in Coordination Com ounds Valence Bond Theory (VBT)

Cr stal Field Theor CFT

Molecular Orbital Theory (MOT)

Valence Bond Theory (VBT)

Lewis bases (ligands)

Lewis acids (metals or metal ions)

The metal uti lizes hybridization of metal s, p, and d valence orbitalsto account for the observed structures and magnetic properties ofcomplexes.


2/48


3/48

Co 3+ [Ar]3d 6

CoF 63-

sp 3d 2 hybrid orbi tals

electrons from F -, octahedral

Co 3+ [Ar]3d 6

Co(NH 3)63+

d 2sp 3 hybrid orbi tals

electrons from NH , octahedral


4/48

Bonding in coordination complexes

we treat the ligands as point negative charges and ask ourselves what the effect of repulsion between thesec arges an e -e ec rons on e me a ons w e.

Explains many of the basic physical properties oftransition metal complexes but not all of them

Molecular orbital theory is more complex than crystal

,of the observed physical properties.


5/48

Crystal Field Theory (CFT)

1929, Hans Bethe

As originally conceived, it was a model based on a purelyelectrostatic interaction between the ligands and the metal ion.

1935 modifications J.H. Van Vleck allow some covalency in the interaction

Ligand Field Theory (LFT)

1950, apply CFT to transition metal complexes

complexessymmetry consideration identical to MOT


6/48

z

x

y


7/48


8/48

Splitting of the d orbitals by an octahedral field

e g

3/5 o

2/5 o

10Dq

2g

[Ti(H2O) 6]3+ d 1 1.0

t2g 1e g 0 t2g e g 1

Purple0.5log

243 kJ/mol ( o )0.0

Frequency

20,300 cm-1


9/48

Crystal Field Stabilization Energy (CFSE)

oo

Weak field Strong field

o < P (pairing energy)

High spin

o > P (pairing energy)

Low spin


10/48

Weak field Strong field

e e

d 1 t2g 1 1 0.4 o t2g1 1 0.4 o

2g . o 2g . o

d 3 t2g 3 3 1.2 o t2g3 3 1.2 o

d 4 t 3 e 1 4 t 4 2 . .

d 5 t2g 3 e g 2 5 0.0 o t2g5 1 2.0 o

d 6 t2 4 e 2 4 0.4 o t26 0 2.4 o

d 7 t2g 5 e g 2 3 0.8 o t2g6 e g 1 1 1.8 o

d 8 t2g 6 e g 2 2 1.2 o t2g6 e g 2 2 1.2 o


11/48

Splitting of the d orbitals in a tetrahedral field

t2

2/5 t

3/5 t

t = 4/9 o(high spin)


12/48

Splitting of the d orbitals in a square planar field (d 8)

x2- y 2

x2- y 2b 1g

e g

z2

xy

xyb 2g

t2ga 1g

xz, yzxz, yz

e gNi(CN) 42- , PdCl 4

2-,

Removal of z ligands

Pt(NH 3)42+ ,PtCl 4

2-,

AuCl 4-


13/48


14/48


15/48

Cr stal Field Theor

Consider the ligands are point negative

charges or as dipoles. How do these chargesinteract with the electrons in the d-orbitals?


16/48

Bonding in Complex Ions:Crystal Field Theory

Consider bonding in a complex to be an

positively charged nucleus and the.

Electrons on metal atom repel electrons on.

Focus particularly on the d -electrons on the


17/48

Octahedral complexesTwo of the d-orbitals point towards the ligands

Repulsion between the ligand electrons andelectrons in these two d-orbitals destabilizes them

is referred to as the li and field s littin for theoctahedral complex


18/48

Octahedral Com lex andd

-Orbital Energies


19/48


20/48


21/48

En r Eff in10

m


22/48

Ligand field splitting

The ligand field splitting depends upon theme a , e ox a on s a e o e me a , an eligand type

Mn2+ < Ni 2+ < Co 2+ < Fe 2+ < V 2+ < Fe 3+ < Co 3+

< Mn 4+ < Mo 3+ < Rh 3+ < Ru 3+ < Pd 4+ < Ir 3+ en > py H3 > EDTA 4- > SC - > H 2O >

- > 2- > - > - > - > - > - > -

Small

Weak field ligands


24/48

Ligand field splitting parameters


25/48


26/48


27/48

Tetrahedral Crystal Field


28/48


29/48


30/48


31/48


32/48

High spin-low spin equilibria

Some coordinationcomplexes show a phasetransition from high to lowspin as a function of

This has been used as a

means of informationstorage for prototype smartcards! occurs because is slightlytem erature de endent inthe solid state


33/48

Experimental methods for determiningthe spin state

absorption spectrum and the magnetic

Absorption spectra are usually harder to

interpret than magnetic dataBoth the orbital and spin angular momenta ofthe electrons influence the magnetic properties

o a samp e- But in coordination complexes the orbital


34/48


35/48

Magnetic Properties of CoordinationCompounds and Crystal Field Theory .

Paramagnetism illustrated:


36/48

Interpreting magnetic measurementse magne c suscep y s measure as a unc on

of temperature A ma netic moment for each metal ion can be

obtained from the magnetic susceptibility- We can think of the unpaired electrons on each atomas e av ng e a e ar magne . e magne cmoment of an atom or ion is a measure of how strong

Theoretically, for spin only systems = 2 [S(S+1)] 1/2 B- B (Bohr magneton) are the units in which the

magne c momen s g ven- S is the total spin angular momentum. It tells you how

S = 0.5n, where n is the number of parallel spinunpaired electrons in the ion


37/48


38/48


39/48


40/48

Effect of Ligands on the Colorso oor nat on ompoun s


41/48


42/48

arises as a consequence of the ligand field splitting


43/48

Electronic spectra

rans t on meta comp exes w t more t an oned electron often show absorption bands at

not just one transition corresponding to

he a earance of multi le bands is due to electron-electron repulsion

may have more than one state for a given

electron configuration


44/48


45/48


46/48


47/48


48/48

Lattice energies of 3d oxides

Double humped trend due to CSFE and high

Wk6-Crystal Field Theory

Documents