Chapter 12 Coordination Chemistry IV

Reactions and Mechanisms

• •

Coordination Compound Reactions

Goal is to understand reaction mechanisms Primarily substitution reactions, most are rapid Cu(H 2 O) 6 2+ + 4 NH 3  [Cu(NH 3 ) 4 (H 2 O) 2 ] 2+ + 4 H 2 O but some are slow [Co(NH 3 ) 6 ] 3+ + 6 H 3 O +  [Co(H 2 O) 6 ] 3+ + 6 NH 4 +

• • • •

Coordination Compound Reactions

Labile compounds - rapid ligand exchange (reaction half-life of 1 min or less) Inert compounds - slower reactions Labile/inert labels do not imply stability/instability (inert compounds can be thermodynamically unstable) these are kinetic effects In general: – Inert: octahedral d 3 , low spin d 4 - d 6 , strong field d 8 planar square – Intermediate: weak field d 8 – Labile: d 1 , d 2 , high spin d 4 - d 6 , d 7 , d 9 , d 10

• • • • •

Substitution Mechanisms

Two extremes: Dissociative (D, low coordination number intermediate) Associative (A, high coordination number intermediate) S N 1 or S N 2 at the extreme limit Interchange - incoming ligand participates in the reaction, but no detectable intermediate – Can have associative (I a ) or dissociative (I d ) characteristics Reactions typically run under conditions of excess incoming ligand We’ll look briefly at rate laws (details in text), consider primarily octahedral complexes

Substitution Mechanisms

Pictures:

Substitution Mechanisms

Substitution Mechanisms

Determining mechanisms

What things would you do to determine the mechanism?

• • • •

Dissociation (D) Mechanism

ML 5 ML 5 X  ML 5 + Y  + X ML 5 Y

k

1 ,

k

-1

k

2 1 st step is ligand dissociation. Steady-state hypothesis assumes small [ML 5 ], intermediate is consumed as fast as it is formed

d

[ML 5 Y] =

k

2

k

1 [ML 5 X][Y]

dt k

Ğ1 [X] +

k

2 [Y] Rate law suggests intermediate must be observable no examples known where it can be detected and measured Thus, dissociation mechanisms are rare - reactions are more likely to follow an interchange-dissociative mechanism

Interchange Mechanism

• ML 5 X + Y  ML 5 X .

Y k 1 , k –1 ML 5 X .

Y  ML 5 Y + X k 2 RDS • 1 st reaction is a rapid equilibrium between ligand and complex to form ion pair or loosely bonded complex (not a high coordination number). The second step is slow.

d

[ML 5 Y]

dt

=

k

2

K

1 [M] 0 [Y] 1 +

K

1 [Y] 0 + (

k

2 0 /

k

Ğ1 ) 

k

2

K

1 [M] 0 [Y] 0 1 +

K

1 [Y] 0 Reactions typically run under conditions where [Y] >> [ML 5 X]

Interchange Mechanism

• Reactions typically run under conditions where [Y] >> [ML 5 X] [M] 0 = [ML 5 X] + [ML 5 X .

Y] [Y] 0  [Y] • • Both D and I have similar rate laws: If [Y] is small, both mechanisms are 2 nd is inversely related to [X]) order (rate of D If [Y] is large, both are 1 st order in [M] 0 , 0-order in [Y]

d

[ML 5 Y]

dt

=

k

2

K

1 [M] 0 [Y] 1 +

K

1 [Y] 0 + (

k

2 0 /

k

Ğ1 ) 

k

2

K

1 [M] 0 [Y] 0 1 +

K

1 [Y] 0

d

[ML 5 Y] =

dt k

2

k

1 [ML 5 X][Y]

k

Ğ1 [X] +

k

2 [Y]

Interchange Mechanism

D and I mechanisms have similar rate laws: Dissociation ML 5 X  ML 5 ML 5 + Y  + X ML 5 Y

k k

1 2 ,

k

-1 Interchange ML 5 X + Y  ML 5 X .

Y  ML ML 5 5 X .

Y Y + X k 1 , k –1 k 2 RDS • • If [Y] is small, both mechanisms are 2 nd order (and rate of D mechanism is inversely related to [X]) If [Y] is large, both are 1 st order in [M] 0 , 0-order in [Y]

Association (A) Mechanism

ML 5 X + Y  ML 5 XY k 1 , k -1 ML 5 XY  ML 5 Y + X k 2 • 1 st reaction results in an increased coordination number. 2nd reaction is faster

d

[ML 5 Y] =

dt k

1

k

2 [ML 5 X][Y]

k

Ğ1 +

k

2 

k

[ML 5 X][Y] • • Rate law is always 2nd order, regardless of [Y] Very few examples known with detectable intermediate

Factors affecting rate

• Most octahedral reactions have dissociative character, square pyramid intermediate • Oxidation state of the metal: High oxidation state results in slow ligand exchange [Na(H 2 O) 6 ] + > [Mg(H 2 O) 6 ] 2+ > [Al(H 2 O) 6 ] 3+ • Metal Ionic radius: Small ionic radius results in slow ligand exchange (for hard metal ions) [Sr(H 2 O) 6 ] 2+ > [Ca(H 2 O) 6 ] 2+ > [Mg(H 2 O) 6 ] 2+ • For transition metals, Rates decrease down a group Fe 2+ > Ru 2+ > Os 2+ due to stronger M-L bonding

Dissociation Mechanism

Evidence: Stabilization Energy and rate of H 2 O exchange.

Entering Group Effects Small incoming ligand effect = D or I d mechanism

Entering Group Effects Not close = I a mechanism Close = I d mechanism

Activation Parameters

Ru II vs. Ru III substitution

Conjugate Base Mechanism Conjugate base mechanism: complexes with NH 3 -like or H 2 O ligands, lose H + , ligand trans to deprotonated ligand is more likely to be lost.

[Co(NH 3 ) 5 X] 2+ + OH ↔ [Co(NH 3 ) 4 (NH 2 )X] + + H 2 O (equil) [Co(NH 3 ) 4 (NH 2 )X] +  [Co(NH 3 ) 4 (NH 2 )] 2+ + X (slow) [Co(NH 3 ) 4 (NH 2 )] 2+ + H 2 O  [Co(NH 3 ) 5 H 2 O] 2+ (fast)

Conjugate Base Mechanism Conjugate base mechanism: complexes with NR 3 or H 2 O ligands, lose H + , ligand trans to deprotonated ligand is more likely to be lost.

Reaction Modeling using Excel Programming

Square planar reactions

• Associative or I a mechanisms, square pyramid intermediate • Pt 2+ is a soft acid. For the substitution reaction

trans

-PtL 2 Cl 2 + Y →

trans

-PtL ligand will affect reaction rate: 2 ClY + Cl – in CH 3 OH PR 3 >CN – >SCN – >I – >Br – >N 3 – >NO 2 – >py>NH 3 ~Cl – >CH 3 OH • Leaving group (X) also has effect on rate: hard ligands are lost easily (NO 3 – , Cl – ) soft ligands with  electron density are not (CN – , NO 2 – )

Trans effect

• • • • • • • In square planar Pt(II) compounds, ligands

trans

to Cl are more easily replaced than others such as ammonia Cl has a stronger

trans effect

a more labile ligand than NH 3 ) than ammonia (but Cl – is CN – ~ CO > PH 3 > NO 2 – > I – > Br – > Cl – > NH 3 > OH – > H 2 O Pt(NH 3 ) 4 2+ + 2 Cl –  PtCl 4 2 – + 2 NH 3 Sigma bonding - if Pt-T is strong, Pt-X is weaker (ligands share metal d-orbitals in sigma bonds) Pi bonding - strong pi-acceptor ligands weaken P-X bond Predictions not exact

Trans Effect:

Trans Effect: First steps random loss of py or NH 3

Trans Effect:

Electron Transfer Reactions Inner vs. Outer Sphere Electron Transfer

Outer Sphere Electron Transfer Reactions Rates Vary Greatly Despite Same Mechanism

Nature of Outer Sphere Activation Barrier

Nature of Outer Sphere Activation Barrier

Inner Sphere Electron Transfer Co(NH 3 ) 5 Cl 2+ + Cr(H 2 O) 6 2+  (NH 3 ) 5 Co-Cl-Cr(H 2 O) 5 4+ + H 2 O Co(III) Cr(II) Co(III) Cr(II) (NH 3 ) 5 Co-Cl-Cr(H 2 O) 5 4+  (NH 3 ) 5 Co-Cl-Cr(H 2 O) 5 4+ Co(III) Cr(II) Co(II) Cr(III) H 2 O + (NH 3 ) 5 Co-Cl-Cr(H 2 O) 5 4+  (NH 3 ) 5 Co(H 2 O) 2+ + (Cl)Cr(H 2 O) 5 2+

Inner Sphere Electron Transfer Co(NH 3 ) 5 Cl 2+ + Cr(H 2 O) 6 2+  (NH 3 ) 5 Co-Cl-Cr(H 2 O) 5 4+ + H 2 O Co(III) Cr(II) Co(III) Cr(II) (NH 3 ) 5 Co-Cl-Cr(H 2 O) 5 4+  (NH 3 ) 5 Co-Cl-Cr(H 2 O) 5 4+ Co(III) Cr(II) Co(II) Cr(III) H 2 O + (NH 3 ) 5 Co-Cl-Cr(H 2 O) 5 4+  (NH 3 ) 5 Co(H 2 O) 2+ + (Cl)Cr(H 2 O) 5 2+ Nature of Activation Energy: Key Evidence for Inner Sphere Mechanism:

Example [Co II (CN) 5 ] 3 + Co III (NH 3 ) 5 X 2+  Products Those with bridging ligands give product [Co(CN) 5 X] 2+ .