CHEM 522 Chapter 04 Carbonyl, 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, S 2 O 4 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 Me 3 NO give a free bonding site for metal

Metal Carbonyls Reactions Nucleophilic attack at carbon [Cp(NO)(PPh 3 )ReCO] + Cp(NO)(PPh 3 )Re(CHO)

Metal Carbonyls Reactions Electrophilic attack at oxygen Cl(PR 3 ) 4 ReCO Cl(PR 3 ) 4 ReCOAlMe 3

Metal Carbonyls Reactions Migratory insertion MeMn(CO) 5 (PMe 3 )(CO) 4 Mn

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 (129 o -144 o )

Thiocarbonyls CS ligand CS is not stable by itself above -160 o C It is known in some compounds as a ligand bonding through C Also bridging CS is also known Usually prepared from CS 2 RhCl(PPh 3 ) 3 Trans-RhCl(CS)(PPh 3 ) 2 + SPPh 3

Thiocarbonyls Frequency range Free CS is 1273 μ 3 CS μ 2 CS M-CS

Nitrosyls NO is a stable free radical Also as NO + in NOBF 4 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 the diamagnetic π acceptor ligand nitrosonium, NO+,but there are cases when NO 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

Reaction 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 CoCl 2 L 2 (lin-NO) CoCl 2 L 2 (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 PR 3 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

Ti2+ 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

The 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 LigandCone Angle PH 3 87 o PF o P(OMe) o PMe o PMe 2 Ph122 o PEt o PPh o PCy o P(t-Bu) o P(mesityl) o

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(PCy 3 ) 2 ]

Chelates Cis and trans phosphines

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 eu (entropy unit)

stereochemistry O h can go to SP or distorted TBP (DTBP)

stereochemistry O h can go to SP or distorted TBP ML 6 d 6 seems to prefer SP or DTBP ML 6 d 8 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) + PPh 3  W(CO) 5 (PPh 3 )

Associative Mechanism L n M  L n M-L’  L n-1 M-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 (I d ) 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 (I a )

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 PPh 3 to give W(CO) 3 (PPh 3 )(Phen)

Hydride Cp 2 WPhH + H 2 Cp 2 WH 2 Reductive elimination enforced by hv followed by oxidative addition

Hydride ReH 5 (PR 3 ) 2 + PR 3 ReH 5 (PR 3 ) 3 Some times loss of phosphine can occur instead

M-M Bond Disproportionation The metal when bonded to the NH 3 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