Coordination Complexes - Summary Examples of ligand substitution reactions

[Ni(H 2 O) 6 ] 2+ (aq) + 6 NH 3 (aq)



[Ni(NH 3 ) 6 ] 2+ (aq) + 6 H 2 O(l)

Need to know: - Inert, labile, thermodynamic stability - Structural aspects of complexes (a)

cis-trans

in octahedral/square planar complexes (b) Chiral forms (enantiomers) in octahedral complexes Crystal field theory Spectrochemcial series:

Cl < H 2 O < NH 3 < CN -

-

Crystal Field Theory

-

z y

-

M

n+

x

-

Which

d

-orbitals are effected the most?

-

z M x

d

z 2 along z-axis

-

z M x

-

x

-

d

x 2 - y 2 along x-y axis These two

d

-orbitals constitute the higher energy e g set.

Less repulsion

-

z

-

M

-

d

zx orbital has lobes between z-x axis x

Likewise interactions for

d

xy and d yz

These three

d

-orbitals constitute the lower energy t 2g set.

t 2g e g Octahedral Δ o t 2 e Tetrahedral Use diagram/spectrochemical series to explain: Absorption wavelengths due to electronic excitation Paramagnetic properties (unpaired electrons) Δ t

Octahedral MO Diagram –

s

-bonded complex

L

Can be

p z

,

s

, or a

hybrid

L

z y

L L

M

n+

x

L L L

Which

d

-orbitals form s -bonds with ligands at the corners of the octahedron?

L

Octahedral MO Diagram –

s

-bonded complex

Can be

p z

,

s

, or a

hybrid

L

z z y M x

d

z 2 along z-axis

L

M x

L L

d

x 2 - y 2 along x-y axis

L

This results in the formation of 4 MO’s (e g and e g *).

Octahedral MO Diagram –

s

-bonded complex

L

NO OVERALP z

d

zx orbital has lobes between z-x axis

L

M x

L Likewise interactions for

d

xy and d yz L

These three

d

-orbitals are non-bonding

4p

The Molecular Orbital Diagram

t 1u * a 1g * 4s 3d e g * Δ o t 2g e g t 1u a 1g L AO’s

Color in coordination complexes

The colors are determined by Δ. Different ligands generate crystal fields of different strength. When the molecules absorb visible light, excited electrons jump from lower energy t

2g

to the higher energy e

g

orbital. The Δ (difference between energies of the two orbitals) is equal to the energy of the absorbed photon, and related inversely to the wavelength of the light. Weaker field ligands with smaller Δ emit light of longer λ and thus lower

v

. Similarly, stronger field ligands with larger Δ emit light of shorter λ and thus higher

v

.

λ Absorbed

400nm Violet absorbed 450nm Blue absorbed 490nm Blue-green absorbed 570nm Yellow-green absorbed 580nm Yellow absorbed 600nm Orange absorbed 650nm Red absorbed

Color observed

Green yellow observed (λ 560nm) Yellow observed (λ 600nm) Red observed (λ 620nm) Violet observed (λ 410nm) Dark blue observed (λ 430nm) Blue observed (λ 450nm) Green observed (λ 520nm)

Problems

1. (a) When water ligands in [Ti(H 2 O) 6 ] 3+ are replaced by CN ligands to give [Ti(CN) 6 ] 3 , the maximum absorption shifts from 500 nm to 450 nm. Is this shift in the expected direction? Explain. What color do you expect to observe for this ion?

t CN 2g is a stronger field ligand than H 2 O. Therefore the energy separation between the and e g levels is greater.

Ti 3+ is a

d

1 metal ion. D o =

hc

/ l . If D is larger, l is smaller.

[Ti(H 2 O) 6 ] 3+ [Ti(CN) 6 ] 3 e g e g A( l max ) = 500 nm Solution appears red D

o

A( l max ) = 450 nm Solution appears yellow D

o

t 2g t 2g

(b) The [Fe(H 2 O) 6 ] 3+ ion has a pale purple color, and the [Fe(CN) 6 ] 3 ion has a ruby red color. What are the approximate wavelengths of the maximum absorption for each ion? Is the shift of wavelength in the expected direction? Explain.

t CN 2g is a stronger field ligand than H 2 O. Therefore the energy separation between the and e g levels is greater.

Fe 3+ is a

d

5 metal ion. D o =

hc

/ l . If D is larger, l is smaller.