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