Coordination Compounds: Reactions and Mechanism

Part III

TYPES OF REACTIONS

I. Substitution ReactionMechanism:

Ligand Substitution:

ML4 + L’ ML3 L’ + L (Td)

ML6 + L’ ML5 L’ + L (Oh)

Note: No change in Coordination Number(CN) andOxidation State (OS) of the central metal atom

Example:

[PtCl4 ]2- + NH3 [PtCl3 NH3 ]- + Cl-

[Cr(CO)6 ]2+ + py [Cr(CO)5 py]2+

A. Kinetic Terms

• Inert

– Exchange ligand in a very slow rate

– t1/2 > one minute at room temp

• Labile

– Exchange ligand in a very fast rate

– t1/2 < one minute at room temp

Examples:

[Co(NH3)6]3+ + 6[H3O]+ [Co(H2O)6 ]3+ + 6NH4

+

- Thermodynamically unstable and kineticallyinert

[Ni(CN)4]2- + 4 CN-1 [Ni(CN)4]2- + 4 CN-1

- Thermodynamically stable and kinetically labile

Notes:

• metal ions with electron in antibonding eg

orbital, ligand substitution is not easy due to M-L bond.

• Metal ions with electron in the bonding orbital, ligand substitution should not be difficult due to the absence of electrostatic repulsion (rapid)

B. Octahedral Complexes

1. SN1 or Dissociative (D) mechanism

- normally form 5 membered complex where large (+) Ssystem (one molecule breaking into more molecules)

- the rate is dependent on one species

Example:

ML6 ML5 + L

ML5 + L’ ML5L’

2. SN2 or Associative (A) Mechanism

- Bimolecular Nucleophilic Substitution that form 7 membered complex

Example:

ML6 + L’ ML6L’

ML6L’ ML5L’ + L

Net: ML6 + L’ ML5L’ + L

Rate = k [ML6+*L’+

3. Mixed type

trans-[Co(en)2Cl2]+ + H2O trans-[Co(en)2Cl(H2O)]2++ Cl-

Rate = kobs [trans-[Co(en)2Cl2]+

Where kobs = k1 + k2 [H2O]

Propose the mechanism involved in this reaction.

4. Isomerization Reaction

– Chemical reaction that involves bond breaking and bond forming

Example:

Trans-[Co(en)2Cl2]+ cis- [Co(en)2 Cl2]+

- Intramolecular twist without bond breaking ndbond forming

a. Bailar twist and

b. Ray-Dutt twist

C. Square Planar Complexes

– For Pt(II), sometimes for Rh(I), Ir(I), Pd(II), Au(III), d8 or d9 -like configuration

Example:[Pt(NH3 )4 ]

2+ + Cl- [Pt(NH3 )3 Cl]+ + NH3

Rate = kobs [Pt(NH3 )4 ]2+ where

kobs = k1 + k2 [Cl-]

Trans Effect

– Labilization of ligand trans to a substituent

Order of decreasing trans-directing ability of ligands:

CO~CN- ~ C2H4 > PR3 (R = allyl, aryl) > H- >Me-

>Ph- ~ NO2- ~ I- SCN- > Br- > Cl- >py > NH3 > F-

~ OH- > H2O

Trans Effect Theory

1. Polarization Theory

- the more polarizable the substituent, the greater the trans

directing ability ( with a substituent that is more polarizable, the ligand directly opposite will be removed)

Polarizability: H- > Me-

I- > Br- > Cl- > F-

2. - Activation Theory

- pi bonding capable of trans directing thru pi electrons from

central metal and therefore weakens the M-L bond trans to it

- because of pi overlap, the electron density opposite to pi overlap because lower and the position trans to the ligand will have less electron density than in the pi bonding orbitals

D. Tetrahedral Complexes

– Fast substitution

– Involve associative mechanism

– Intermediate: 5-membered complex

Example:

[Co(CO)3(NO)] + L [Co(CO)2(NO)L] + CO

II. Redox (Oxidation-Reduction) Reaction

– Involves electron transfer

Example:

[V(H2 O)6 ]2+ + 2H+ 2 [V(H2 O)6 ]3+ + H2

What is the oxidation reaction for this?

What is the reduction reaction for this?

Ligand Transfer:

[Co(NH3 )5Cl] + [Cr(H2O)6]2+ + 3H2 OCN = 6 OS = 3 CN = 6 OS = 2

[Co(H2O)6]2+ + [Cr(H2O)5 Cl]2+ + 5NH4+

III. Addition / Dissociation

- there is a change in coordination number but no change in oxidation state

- it involves electron defficient center with vacant orbitals readily forming complexes through the addition of extra ligand.

Examples:

• Addition Reaction

BF3 + F- [BF4 ]

-

B(III) B(III)

CN : 3 CN : 4

• Dissociation Reaction

[Cr(CO)5 (Cs)] [Cr(CO)4 (Cs)] + CO

CN : 6 CN : 5

IV. Oxidative Addition / Reductive Elimination

– There is a change both in C.N and O.S of the central metal atom

Example:

Oxidative Addition

2[Co(CN)5 ]3- + H2 2[Co(CN)5 H]3-

Co (II) H (0) Co(III) H(-1)

CN : 5 CN : 6

Reductive Elimination

[PdCl6 ]2- [PdCl4 ]

2- + Cl2

Pd(IV) Cl(-1) Pd(II) Cl(0)

V. Free Radical

– Reaction involving paramagnetic species and sometimes photochemically activated species

VI. Insertion Reaction

- placement of a group between central metal atom and another ligand bonded to it without a change in O.S and C.N

Example:

CH3 Mn(CO)5 + PPh3 CH3 COMn(CO)4 (PPh3 )

Note: CO is inserted between methyl group and Mn