chem 522 chapter 04 carbonyl, phosphine complexes and ligand substitution reaction

CHEM 522Chapter 04Carbonyl, Phosphine

complexes and Ligand Substitution Reaction

Bonding

• σ Donation

• π Back bonding

• From IR it is possible to tell how good is the metal as a π base

Preparation of CO Complexes

• Direct reaction of metal with CO

• CO replace weakly bonded ligands

Preparation of CO Complexes

• From CO and a reducing agent (like Na, S2O4

2- and CO)

Preparation of CO Complexes

• From a reactive carbonyl compound followed by desertion

Metal Carbonyls Reactions

• Nucleophilic attack at carbon• Reaction wit Me- give carbenes• Reaction with Me3NO give a free bonding site for metal

Metal Carbonyls Reactions

• Nucleophilic attack at carbon

LiBHEt3[Cp(NO)(PPh3)ReCO]+Cp(NO)(PPh3)Re(CHO)

Metal Carbonyls Reactions

• Electrophilic attack at oxygen

Cl(PR3)4ReCO Cl(PR3)4ReCOAlMe3

AlMe3

Metal Carbonyls Reactions

• Migratory insertion

PMe3MeMn(CO)5 (PMe3)(CO)4Mn C-Me

O

Bridging CO Groups

Unequivalent Bridging CO

triply Bridging CO

Isonitriles

• M=C=N-R• Stabilize higher oxidation state

[Pt(CNPh)4]2+ no [Pt(CO)4]2+ is known

• The lone pair in CO is almost nonbonding while in CNR it is more of antibonding, so when σ donation take place the CN bond become stronger, π back donation weaken the bond, so the shift in the IR will depend on the strength of σ or π donation. (unlike CO)

Isonitriles

• M=C=N-R

• If back bonding is not strong, M-CΞNR should be linear

• M=C=N-R bent molecule is also known which means strong back bonding

• NbCl(CO)(CNR)(dmpe). The ligand is bent at N (129o-144o)

Thiocarbonyls

• CS ligand• CS is not stable by itself above -160oC• It is known in some compounds as a ligand

bonding through C• Also bridging CS is also known

• Usually prepared from CS2

RhCl(PPh3)3 Trans-RhCl(CS)(PPh3)2 + SPPh3

CS2

Thiocarbonyls

• Frequency range

• Free CS is 1273

• μ3 CS 1040-1080

• μ2 CS 1100-1160

• M-CS 1160-1410

Nitrosyls

• NO is a stable free radical

• Also as NO+ in NOBF4

• NO+ is isoelectronic with CO

• It can bind as NO+ and it will be three electron donor

• When NO is bent then it will be one electron donor

• NO is a fifteen electron molecule • with one unpaired electron residing in the π* molecular orbital: (σ1)2(σ1*)2(σ2)2(σ2*)2(σ3)2(πx, πy)4(πx*, πy*)1(σ*3)

• This electronic configuration explains the high reactivity of the NO molecule, particularly the formation of nitrosonium cation (NO+) on oxidation and the reduction

to nitroxide anion (NO–), making it a "non-innocent" ligand

• Most of the known stable "nitrosyl" complexes are assumed to contain thediamagnetic π acceptor ligand nitrosonium, NO+,but there are cases whenNO• or NO– (nitroxide) can be reasonably postulated as ligands in transition metal complexes.

• Establishing the actual form of coordinated NO often requires a variety of physical methods such as IR, EPR, NMR, UV/VIS, X-rays, resonance Raman, magnetic circular dichroism (MCD), etc., and theoretical calculations.

NO Bonding

• NO binds in two ways• Either as NO+ then it will

give linear molecule and will be three electron donor

• Or as NO- then it will give bent molecule and will be one electron donor

M N O

M N

O

..

Reaction

NuMo(CO)2Cp No Reaction

NOBF4

Mo(CO)(NO)Cp Nu

Nu

Mo(CO)(NO)Cp

When NO+ is added it makes reaction with Nu- more probable

Electron Count

• When NO change from linear to bent both the number of electron on the metal and the oxidation state of the metal will change

• CoCl2L2(lin-NO)

• CoCl2L2(bent-NO)

Electron Count

Preparation- NO+ is a powerful oxidation agent

- Migratory insertion is also possible for NO

Phosphine Ligands

• Phosphine ligands have the general formula PR3 • where R = alkyl, aryl, H, halide etc. • Closely related are phosphite ligands which have

the general formula P(OR)3. • Both phosphines and phosphites are neutral two

electron donors that bind to transition metals through their lone pairs.

• There are many examples of polydentate phosphine ligands, some common examples of which are shown below.

Bonding

π Acidity

π AcidityTi2+ is a d2 ion in octahedral field so it should be paramagnetic, however it is diamagnetic. The reason is because of the strong back bonding

Tolman Cone Angle

Tolman Cone AngleThe stronger donor phosphine increase the electron density on metal which increase it on CO by back donation

Cone angles for some common phosphine ligands are:

Phosphine Ligand Cone Angle

PH3 87o

PF3 104o

P(OMe)3 107o

PMe3 118o

PMe2Ph 122o

PEt3 132o

PPh3 145o

PCy3 170o

P(t-Bu)3 182o

P(mesityl)3 212o

Factors Effecting Bonding

• There are two important factors effecting the bonding of the phosphines– Electronic – Steric

• The advantage of using bulky ligands compounds of low coordination number can be formed [Pt(PCy3)2]

Chelates

• Cis and trans phosphines

Dissociative Substitution

Dissociative Substitution

Usually the larger the cone angle the faster the dissociation

This mechanism is usually preferred for 18-electron molecule

Transition state has a positive ΔS‡ and in the range 10-15 eu (entropy unit)

stereochemistry

• Oh can go to SP or distorted TBP (DTBP)

stereochemistry• Oh can go to SP or distorted TBP

• ML6 d6 seems to prefer SP or DTBP

• ML6 d8 seems to prefer TBP

stereochemistry• Phosphines usually do not replace all CO

in the complex

• The fac structure is usually prefer over the mer for electronic reason

Dissociative Substitution

• Bulky ligands usually enhance dissociation

• Protonation can be used to remove an alkyl or hydride group

• Weakly bonded solvent is a good leaving group

W(CO)5(thf) + PPh3 W(CO)5(PPh3)

Associative Mechanism

LnM LnM-L’ Ln-1M-L’

This mechanism is usually adapted for 16 e complexes

The Trans Effect

• This is observed in square planar complexes where the incoming ligand will occupy certain position depending on the ligand trans to it

The Trans Effect

• The solvent may have some effect

Ligand Rearrangement

• This take place for 18-e complexes

Ligand Rearrangement

• This also observed for indenyl complexes better than their Cp analogs because of the benzene ring

Ligand Rearrangement

• This also observed for other complexes

Redox Effects

Sometime mechanism can be catalyzed by oxidation

The 17, and 19 e species are very difficult to study they are unstable and usually only a transition state

Redox Effects

This could lead to chain reaction

Redox Effects

A trace of a free radical can abstract a 1e ligand

The Interchange Mechanism

• It is intermediate state in which the ligand will be in the area around the complex but will not substitute before the leaving of one of the ligands from the complex (Id) this is usually observed when an 18 electron complex exist and it is thought that an associative mechanism take place

• There is also interchange associative mechanism (Ia)

Rearrangement

• This take place with coordinatively unsaturated species

Rearrangement

• This take place with coordinatively unsaturated species

Rearrangement

Coordinatively unsaturated species is using a ligand from other specie

Cyclometallation

• This is one of the reductive elimination process• W (IV) W (III)

Cyclometallation

• This is one of the oxidative addition process

Agostic Ligand Substitution

• This is one of the ligand substitution process

Photochemical Substitution

• Usually used for carbonyl complexes

Photochemical Substitution

• Charge transfer process

• W(CO)4(Phen) at 546 nm there will be

charge transfer transition to give W.+

(CO)4(Phen.-)

• Then irradiation will lead to substitution by PPh3 to give W(CO)3(PPh3)(Phen)

Hydride

Cp2WPhH + H2Cp2WH2

hv, benzene

Reductive elimination enforced by hv followed by oxidative addition

Hydride

ReH5(PR3)2 + PR3ReH5(PR3)3

Some times loss of phosphine can occur instead

hv

M-M Bond

• Disproportionation

Mn2(CO)10 + NH3 [Mn(CO)3(NH3)3]+[Mn(CO)5]-

The metal when bonded to the NH3 it can not take the electron density no more. electron density will be provided by NH3 to an extent it may be oxidized

Solvents

DMSO

DMF

THF

Diethylether

Acetone

Ethanol

Halocarbon

Solvents

Solvents