Principal mechanisms of ligand exchange in octahedral complexes

ML5Xk1

slowML5 + X

k2

fast

+YML5Y

r = k1 [ML5X]

ML5X + Yk1

slowML5XY

k2

fast

-XML5Y

r = k1 [ML5X][Y]

Dissociative

Associative

Dissociative pathway(5-coordinated intermediate)

Associative pathway(7-coordinated intermediate)

MOST COMMON

Experimental evidence for dissociative mechanisms

Rate is independent of the nature of L

Experimental evidence for dissociative mechanisms

Rate is dependent on the nature of L

Inert and labile complexesSome common thermodynamic and kinetic profiles

Exothermic(favored, large K)

Large Ea, slow reaction

Exothermic(favored, large K)

Large Ea, slow reactionStable intermediate

Endothermic(disfavored, small K)Small Ea, fast reaction

LM

L L

L

L

X

L

ML L

L

L

X

L

ML L

L

L

G

Ea

Labile or inert?

LFAE = LFSE(sq pyr) - LFSE(oct)

Why are some configurations inert and some are labile?

Inert !

Substitution reactions in square-planar complexesthe trans effect

T

M

L X

L T

M

L Y

L

+X, -Y

(the ability of T to labilize X)

Synthetic applicationsof the trans effect

Cl- > NH3, py

Mechanisms of ligand exchange reactions in square planar complexes

-d[ML3X]/dt = (ks + ky [Y]) [ML3X]

LM

L L

X

LM

L L

Y

LM

L L

X

LM

L L

X

LM

L L

S

LM

L L

S

S

Y

Y

+Y

+S

-X

+Y

-S

-X

Electron transfer (redox) reactions

M1(x+)Ln + M2

(y+)L’n M1(x +1)+Ln + M2

(y-1)+L’n

-1e (oxidation)

+1e (reduction)

Very fast reactions (much faster than ligand exchange)

May involve ligand exchange or not

Very important in biological processes (metalloenzymes)

Outer sphere mechanism

[Fe(CN)6]4- + [IrCl6]2- [Fe(CN)6]3- + [IrCl6]3-

[Co(NH3)5Cl]2+ + [Ru(NH3)6]2+ [Co(NH3)5Cl]+ + [Ru(NH3)6]3+

Reactions ca. 100 times fasterthan ligand exchange(coordination spheres remain the same)

r = k [A][B]

Ea

A B+

A B

A' B'+

G

"solvent cage"

Tunnelingmechanism

Inner sphere mechanism

[Co(NH3)5Cl)]2+ + [Cr(H2O)6]2+ [Co(NH3)5Cl)]2+:::[Cr(H2O)6]2+

[Co(NH3)5Cl)]2+:::[Cr(H2O)6]2+ [CoIII(NH3)5(-Cl)CrII(H2O)6]4+

[CoIII(NH3)5(-Cl)CrII(H2O)6]4+ [CoII(NH3)5(-Cl)CrIII(H2O)6]4+

[CoII(NH3)5(-Cl)CrIII(H2O)6]4+ [CoII(NH3)5(H2O)]2+ + [CrIII(H2O)5Cl]2+

[CoII(NH3)5(H2O)]2+ [Co(H2O)6]2+ + 5NH4+

Inner sphere mechanism

Reactions much faster than outer sphere electron transfer(bridging ligand often exchanged)

r = k’ [Ox-X][Red] k’ = (k1k3/k2 + k3)

Ox-X + Red Ox-X-Redk1

k2

k3

k4Ox(H2O)- + Red-X+

Ea

Ox-X Red+

Ox-X-Red

G

Ox(H2O)- + Red-X+

Tunnelingthrough bridgemechanism

Brooklyn CollegeChem 76/76.1/710G Advanced Inorganic Chemistry

(Spring 2008)

Unit 6Organometallic Chemistry

Part 1General Principles

Suggested reading:Miessler/Tarr Chapters 13 and 14

Elements of organometallic chemistry

Complexes containing M-C bonds

Complexes with -acceptor ligands

Chemistry of lower oxidation states very important

Soft-soft interactions very common

Diamagnetic complexes dominant

Catalytic applications

The d-block transition metals

Group 4 5 6 7 8 9 10 11

3d row Ti V Cr Mn Fe Co Ni Cu4d row Zr Nb Mo Tc Ru Rh Pd Ag5d row Hf Ta W Re Os Ir Pt Au

dn

0 4 5 6 7 8 9 10I 3 4 5 6 7 8 9 10II 2 3 4 5 6 7 8 9III 1 2 3 4 5 6 7 8IV 0 1 2 3 4 5 6 7V 0 1 2 3 4 5 6VI 0 1 2 3 6 5VII 0 1 2 3 4

Ligand F. C. #e (A) #e (B) # CS

X -1 2 1 1L 0 2 2 1

XL -1 4 3 2XX -2 4 2 2LL 0 4 4 2

XLL -1 6 5 3LLL 0 6 6 3

Main types of common ligands

A simple classification of the most important ligands

X

L

L2

L2X

L3

Counting electrons

Method A

Determine formal oxidation state of metalDeduce number of d electrons

Add d electrons + ligand electrons (A)

Ignore formal oxidation state of metalCount number of d electrons for M(0)

Add d electrons + ligand electrons (B)

Method B

The end result will be the same

Why is this relevant?

Stable mononuclear diamagnetic complexesgenerally contain 18 or 16 electrons

The reactions of such complexesgenerally proceed through 18- or 16-electron intermediates

Although many exceptions can be found, these are very useful practical rulesfor predicting structural and reactivity properties

C. A. Tollman, Chem. Soc. Rev. 1972, 1, 337.

Why 18 electrons?

antibonding

Organometallic complexes

18-e most stable

16-e stable (preferred for Rh(I), Ir(I), Pt(II), Pd(II))

<16-e OK but usually very reactive

> 18-e possible but raregenerally unstable

A closer look at some important ligands

Typical -donor ligands

Hydride: M-H (terminal) is a -1, 2e (1e) ligand (-bridging) is a -1, 2e (1e) ligandH

M M

Alkyl: M-CH3 (terminal) is a -1, 2e (1e) ligand (-bridging) is a -1, 2e (1e) ligand

H3C

M M

Alkoxide: M-OR (terminal) is a -1, 2e (1e) ligand (-bridging) is a -1, 4e (3e) ligand

RO

M M

Thiolate: M-SR (terminal) is a -1, 2e (1e) ligand (-bridging) is a -1, 4e (3e) ligand

RS

M M

Halide: M-Cl (terminal) is a -1, 2e (1e) ligand (-bridging) is a -1, 4e (3e) ligandCl

M M

Amide: M-NR (terminal) is a -1, 2e (1e) ligand (-bridging) is a -1, 4e (3e) ligand

RN

M M

Phosphide: M-PR2 (terminal) is a -1, 2e (1e) ligand (-bridging) is a -1, 4e (3e) ligand

R2P

M M

Other important C-donor ligands

M M M

terminal, 1-aryl, alkenyl, alkynyl, -1, 2e (1e)

M M

M' M'

bridging, 2-alkenyl, alkynyl, -1, 4e (3e)

M M

or 1-allyl -1, 2e (1e) or 3-allyl -1, 4e (3e)

Other important ligands

M

4-diene, 4e

M

M

M

2- (2e) 4- (4e) 6- (6e) arene

M

M

1-Cp -1, 2e (1e) 5- Cp -1, 6e (5e)

M MO

CM O CM

N

CM N CM

2-alkene or alkyne, 2e 2- / side-bonded and 1- / end-bondedaldehyde/ketone, 2e imine, 2e

Other important ligands

M N NM C O M N O M N

O

carbonyl, 2e dinitrogen, 2e linear nitrosyl+1, 2e (3e)

bent nitrosyl-1, 2e (1e)

M CR2

Fischer carbene, 2e (2e)Schrock carbene, -2, 4e(2e)

M CR

Fischer carbyne, 4e (3e)Schrock carbyne, -3, 6e(3e)

M O

Oxo, -2, 4e (2e)

M NR

imido, -2, 4e (2e)

M N

nitrido, -3, 6e (3e)

M NR3

amine, 2e

M PR3

phosphine, 2e

M AsR3

arsine, 2e

M SbR3

stibine, 2e

The M-L-X game

Group 4 5 6 7 8 9 10

3d row Ti V Cr Mn Fe Co Ni

4d row Zr Nb Mo Tc Ru Rh Pd5d row Hf Ta W Re Os Ir Pt

Neutral stable compounds

0 ML7 ML6 ML5 ML4

I MXL6 MXL5 MXL3 (16e)II MX2L6 MX2L5 MX2L4 MX2L2 (16e)III MX3L4 (16e) MX3L4 MX3L3

IV MX4L4 (16e) MX4L3 (16e) MX4L3 MX4L2

V MX5L2 (16e)

Each X will increase the oxidation number of metal by +1.

Each L and X will supply 2 electrons to the electron count.

Group 4 5 6 7 8 9 10

3d row Ti V Cr Mn Fe Co Ni

4d row Zr Nb Mo Tc Ru Rh Pd5d row Hf Ta W Re Os Ir Pt

Stable monocationic compounds

0IIIIIIIVV

Group 4 5 6 7 8 9 10

3d row Ti V Cr Mn Fe Co Ni

4d row Zr Nb Mo Tc Ru Rh Pd5d row Hf Ta W Re Os Ir Pt

Stable monocationic compounds

0 [M(NO)L6]+ [M(NO)L5]

+ [M(NO)L4]+ ML4

I [ML6]+ (16e) [ML6]

+ [ML4]+

(16e)

II [MXL7]+ [MXL6]

+ [MXL5]+ MX2L2 (16e)

III [MX2L5]+

(16e) [MX2L5]+ [MX2L4]

+

IV [MX3L5,6]+ [MX3L4]

+ (16e) [MX3L4]+ MX4L2

V [MX4L3]+

(16e)

Now looking at compounds having a charge of +1 to obey 18 e rule.

NO+ is isoelectronic to CO

X increases O N by 1

Elec count: 4 (M) +2 (NO) +12 (L6) = 18

Elec Count: 4 (M) + 4 (L2) + 10 (L5)

Actors and spectators

Actor ligands are those that dissociate or undergo a chemical transformation

(where the chemistry takes place!)

Spectator ligands remain unchanged during chemical transformations

They provide solubility, stability, electronic and steric influence(where ligand design is !)

Organometallic Chemistry1.2 Fundamental Reactions

Reaction (FOS) (CN) (NVE)

Association-Dissociation of Lewis acids 0 ±1 0

Association-Dissociation of Lewis bases 0 ±1 ±2

Oxidative addition-Reductive elimination ±2 ±2 ±2

Insertion-deinsertion 0 0 0

Fundamental reaction of organo-transition metal complexes

FOS: Formal Oxidation State;

CN: Coordination Number

NVE: Number of valence electrons

(FOS) = 0; (CN) = ± 1; (NVE) = 0

Lewis acids are electron acceptors, e.g. BF3, AlX3, ZnX2

W:H

H+ BF3 W

H

HBF3

This shows that a metal complex may act as a Lewis base

The resulting bonds are weak and these complexes are called adducts

Association-Dissociation of Lewis acids

(FOS) = 0; (CN) = ± 1; (NVE) = ±2

Association-Dissociation of Lewis bases

A Lewis base is a neutral, 2e ligand “L” (CO, PR3, H2O, NH3, C2H4,…)in this case the metal is the Lewis acid

HCo(CO)4 HCo(CO)3 + CO

Crucial step in many ligand exchange reactionsFor 18-e complexes, only dissociation is possible

For <18-e complexes both dissociation and association are possiblebut the more unsaturated a complex is, the less it will tend to dissociate a ligand

(FOS) = ±2; (CN) = ± 2; (NVE) = ±2

Oxidative addition-reductive elimination

Very important in activation of hydrogen

Cl PPh3

COIrI

Ph3P+ H2

Cl PPh3

HIrIII

Ph3P

H

COVaska’s compound

Mn+ +M(n+2)+

X YX-Y

Oxidative addition-reductive elimination

Cl PPh3

COIrI

Ph3P+ H2

Cl PPh3

HIrIII

Ph3P

H

COVaska’s compound

H

H

M

Concerted reaction

via

Cl PPh3

COIrI

Ph3P+ CH3I

Cl PPh3

COIrIII

Ph3P

CH3

Cl PPh3

COIrIII

Ph3P

CH3

I

+

I-

SN2 displacement

cis addition

trans addition

Also radical mechanisms possible

Ir: Group 9

H becomes H-

CH3+ has become CH3

-

Oxidative addition-reductive elimination

Mn+ +M(n+2)+

X YX-Y

Not always reversible

Mn+ +M(n+2)+

X RR-X

Mn+ +M(n+2)+

H RR-H

(FOS) = 0; (CN) = 0; (NVE) = 0

Insertion-deinsertion

M-X + L M-L-X

(CO)5Mn-CH3 + CO (CO)5Mn-C-CH3

O

Very important in catalytic C-C bond forming reactions(polymerization, hydroformylation)

Also known as migratory insertion for mechanistic reasons

Mn: Group 7

Migratory Insertion

MnOC

OC CO

CO

CH3

CO

+ COMn

OC

OC C

CO

CO

CO

O

CH3

Mn

OC

OC C

CO

CO

O

CH3

k1 k2

+ CO

Also promoted by including bulky ligands in initial complex

Insertion-deinsertionThe special case of 1,2-addition/-H elimination

LnM H

R2C CR'2

LnM

R2C

CR'2

H

A key step in catalytic isomerization & hydrogenation of alkenesor in decomposition of metal-alkyls

Also an initiation step in polymerization

Attack on coordinated ligands

M L

Nu-

E+

Favored for electron-poor complexes(cationic, e-withdrawing ligands)

Favored for electron-rich complexes(anionic, low O.S., good donor ligands)

Very important in catalytic applications and organic synthesis

Some examples of attack on coordinated ligands

Nucleophilic addition Electrophilic addition

Nucleophilic abstraction Electrophilic abstraction

PtCl

Cl py pyPt

Cl

Cl py

N+

-

FeCp

OCOC OH

OH-

FeCp

OCOC OH2

+

FeCp

OCOC

-H2O

Ta

Cp

Cp

CH3

CH3

+ Me3PCH2 Ta

Cp

Cp

CH2

CH3

+ Me4P+

O

Fe(CO)3

O

Fe(CO)3

Et

+

Et3O+