Mekanisme Reaksi Anorganik

I. PengantarA. Parallel dengan Kimia Organik

1) Beberapa istilah dan konsep sama dengan mekanisme organik

2) Geometri kompleks lebih banyak terdapat dalam kompleks anorganik

a) Kemungkinan penataan ulang lebih banyak

b) Kemungkinan isomer lebih banyak

3) Tidak semua ion logam bereaksi seperti yang terjadi pada atom karbon

B. Sejarah dan Goal atau tujuan

1) Werner dan Jorgenson menemukan beberapa reaksi dasar

2) Experimen beberapa tahun yang lalu oleh peneliti menghasilkan beberapa usulan mekanisme

3) Mechanism tidak dapat dibuktikan karena :

a) Kita tidak dapat mengamati reaksi molekul secara langsung

b) Bukti atau pendukung mekanisme yang dapat diajuakn untuk aturan mekanismenya

4) Goal: sintesis dan perkiraan produk melalui pemilihan kondisi reaksi yang sesuai..

C. Tipe Reaksi : Substitusi, Oksidasi/Reduksi tions: Substitution, Oxidation/Reduction, Ligand Reactions

II. Substitution ReactionsA. Inert and Labile Complexes

1) Labile Complexes = those undergoing substitution with t½ < 1 minute

a) Many analytically useful reactions are labile substitutions

[Cu(H2O)6]2+ + NH3 [Cu(NH3)4(H2O)2]2+ + H2O

[Fe(H2O)6]3+ + SCN- [Fe(H2O)5(SCN)]2+ + H2O

b) Lability is generally a function of the metal ion, not the ligands

[Fe(H2O)5(OH)]2+ + H+ [Fe(H2O)6]3+

[Fe(H2O)6]3+ + Cl- [Fe(H2O)5Cl]2+

[Fe(H2O)5Cl]2+ + PO43- Fe(H2O)5PO4

Fe(H2O)5PO4 + SCN- [Fe(H2O)5(SCN)]2+

[Fe(H2O)5(SCN)]2+ + F- [Fe(H2O)5F]2+

c) Often, the reaction happens at the diffusion limit = as soon as the reactants are mixed, they are done reacting. Diffusion = 1011 s-1

d) Labile Metal ions = those with small or zero LFSE

a) d1, d2, d7, d9, d10

b) High spin d4-d6

2) Inert Complexes = those undergoing substitution with t½ > 1 minute

a) Inert does’t mean unreactive; Inert doesn’t mean thermodynamically stable

b) Inert does mean slow to react (also known as Robust)

[Fe((H2O)5F]2+ = labile, but it is very thermodynamically stable

[Co(NH3)6]3+ = inert, but thermodynamically unstable

c) Inert complexes react slowly, so their products can be isolated and studied

d) Inert metal ions = those with large LFSE

i. d3, low spin d4-d6

ii. Low spin d8

e) High spin (weak field) d8 metals are intermediate in lability

B. The Dissociative (D) Substitution Mechanism

1) The mechanism is essentially the same as SN1 in organic chemistry

a) Dissociation of one ligand results in an isolatable intermediate

b) The new ligand binds at the open coordination site

2) The kinetic analysis

a) The Steady State Hypothesis

i. Intermediates are high energy species (5-coord metal ion)

ii. They react almost as soon as they are formed

iii. Their concentrations are small and constant over most of the course of a reaction

b) The D rate Law

C. The Interchange (I) Substitution Mechanism

1) The mechanism involves the incoming ligand (Y) in the rate determining step

a) Y can weakly assist the leaving ligand (X) = Dissociative Interchange (ID)

b) Y can strongly begin bond formation before X leaves = Associative Interchange (IA)

2) The kinetic analysis

a) Assumptions to simplify the analysis

i. [Y] = very large, [Y]0 = [Y]

ii. k2 << k-1 which means reaction #1 is in equilibrium with K1 = k1/k-1

iii. [M]0 = [ML5X] + [ML5X•Y]

b) Steady State Equation

D. Comparison of D and I Rate Laws

1) We can rewrite the I and D rate laws for comparison:

2) If [Y] is small, both become second order expressions

a) For D, we get

b) For I, we get[X]

k[M][Y] Rate

00[Y]k[M] Rate

3) If [Y] is large, both become first order expressions

For both I and D, we get

4) These similarities make it difficult to ever tell these two mechanisms apart based on experimental data

a) Usually, we vary the concentration of [M], [Y], or [X] to find the order

b) Isolation of ML5 is proof of the D mechanism

E. The Associative (A) Substitution Mechanism

1) The mechanism is essentially the SN2 mechanism from Organic chemistry

a) Y and X are both partially bonded to M at the transition state

b) No Intermediate is usually isolated

0]M[k'

k Rate

F. Experimental Evidence for the Octahedral Substitution Mechanism1) The usual mechanism for Octahedral complexes is Dissociation

2) Even if a reaction is thermodynamically downhill, a large Ea will make it slow

3) Thermodynamically uphill reactions won’t occur even if Ea is small4) Ligand Field Activation Energy = LFAE = difference in ligand field stabilization

energy between the octahedral complex and the 5-coordinate intermediate supports the assignment of labile and inert metal ions

3) Oxidation State: higher charge = slower reaction due to greater ligand attraction4) Ionic Radius: smaller ionic radius = slower reaction due to greater ligand

attraction5) Other Evidence for Dissociative Mechanism

a) Incoming ligand identity has no effect on rateb) Bulky X increases the rate

c) DVa = volume of activation is positive for octahedral substitutions because one molecule splits into two at the intermediate

G. The Associative Substitution Mechanism and Octahedral Complexes

1) This mechanism is sometimes observed, but is rare

2) If the identity of Y influences the rate, that suggests Association

3) If DSa is negative (molecules coming together), that suggests Association

H. The Conjugate Base Mechanism (SN1CB) of Substitution

1) This mechanism requires a deprotonatable ligand on the complex (NH3, H2O)

2) It also requires presence of hydroxide OH- in aqueous solutions

3) Mechanism:

4) Deprotonation lowers the charge on the complex ion, so X- leaving is easier

5) Evidence

a) H exchange on ammonia ligands is well known

b) RNH2 is faster than NH3, because steric crowding favors dissociation

c) R3N ligands completely stop the reaction (no ionizable protons)

6) The position trans to X is usually the one deprotonated. The trigonal bipyramidal intermediate is more easily achieved.