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CH307 Organometallic Compounds of d-block elements

Organometallic compounds contain at leastone metal-carbon bond.

LigandsOrganometallic complexes have a wide range of ligandtypes from simple M-R to the -cyclopentadienyl ringsof ferrocene.Hapticity of a ligandThe hapticity of a ligand indicates the atoms that aredirectly bonded to the metal.

The Greek letter (eta) is used,(5-C5H5) M indicates that all five carbons are bonded to M.

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5-C5H5 ligands are usuallydrawn as in (23,1b)

- bonded or 1-alkyls of the M-R type are well known.

Examples are WMe6, TiMe4 and MeMn(CO)5. WhichContain simple (2c-2e) M-R bonds.

The compound (5C5H5)(1C6H5)Fe(CO)2also contains one (2c-2e) bond.

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The Carbonyl Ligand, CO

CO is an unlikely ligand as it does not alter the pH of H2O(i.e. will not complex a H+)

It also has a very small dipole moment and the strongestknown chemical bond (1100 kJmol-1).Mopac calculation of CO HOMO and LUMO orbitals

HOMO LUMO

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The accepted bonding scheme has two steps:

2. M to CO back –donationfrom a filled metal d-orbitalto an empty CO (*) orbital.

1. -donation from a filledC based orbital to an empty metal orbital.

Step 2 enhances step 1 and thus the scheme is often saidto be synergic.

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In this scheme CO is acting as a -acceptor or -acid ligandand step 2 enchances step 1 (a synergic effect).

CO ligands may be terminal, doubly bridging (2) or triplybridging (3).

MC-O IR spectra are very useful as the bands are veryintense and narrow.Free CO is at 2143cm-1 and neutral M(CO)xare at ~ 2000cm-1 ~ 100cm-1 lower than free CO.

This indicates a reduction in CO bond order on complexationand shows the importance of step 2 in the bonding scheme.

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C-O bond lengths from electron diffraction and X-raydiffraction studies show thatfree CO is 1.13Å MC-O is 1.17 and 2M2C-O is 1.20Å.

IR spectra show this even more clearlyCO 2143 2000 1800cm-1

Within an isoelectronic series (same no. of valence electrons)The following is generally truea +ve charge increases MC-O ~ 100cm-1 and a –ve charge decreases MC-O ~ 100cm-1.

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Hydride LigandsMany molecular hydrides are known which involvetransition metal organometallic systems.

M-H systems vary from hydridic (basic) types whichreact with H+ to M-H systems which are strong acids.

1H NMR is the best way to characterise M-H systems.The chemical shift range is from -8 to –30ppm.

This is for the most part outside the normal chemical shiftrange of 0 to 10 ppm and it is easy to detect M-H by NMR.

The M-H stretching vibration is observed close to 2000cm-1

However it is often weak and hard to see if CO is present.

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Phosphine ligandsSubstituted PH3, PR3, derivatives are good ligandsPF3 ~ CO due to extensive back bonding enhanced by F.

PR3 in general are weaker -acceptors than CO.

In terms of the bonding scheme used for CO PR3uses a P d-orbital as an acceptor.

The R groups can be used to alter the importance of -donation and -acceptor behaviour.This is an electronic effect.

Along the series Me, Ph, OMe and OPh the R group isincreasing in electron withdrawing power.

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Steric effectsThe ability of different R groups in PR3 to producedifferent steric effects has become very important.

C.A. Tolman was the first to study this extensively.Thecone-angle construction can be used to quantify this property.

This is very useful for fine tuning catalysts

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Bi and tri-dentate phosphines are also very importanttwo well known examples are bis(diphenylphosphino)methane,dppm, and bis(diphenylphosphino)ethane, dppe.The latter is also called diphos.

The phosphine cone angles cover a very wide range with sometaking up more than 180 of the metal’s coordination sphere.

P(OMe)3 107ºPMe3 118PMe2Ph 122PPh3 145

P(4-MeC6H4)3 145ºP(3-MeC6H4)3 165PtBu3 182P(2-MeC6H4)3 194

(Ph)2P P(Ph)2 (Ph)2P P(Ph)2

dppm dppe (diphos)

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-bonded organic ligands2 alkenes bond to the metal “side on”and are two electron donors.

A scheme similar to that used for CO can also be invoked here.The alkene to M donation is from the filled C=C orbital and theback bond from M is into the * C=C orbital

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If the substituents on the olefin are strongly electronwithdrawing (e.g. CN) back-bonding is enhanced to such anextent that C2(CN)4 complexes are often considered to bemetallacyclopropane complexes.

H H

H H

M MCN

NC

CNNC

H H

H HCl

ClCl Pt

_

Zeise’s salt was reported in 1827 (structure in1960s)K2PtCl4 + ethene or ethanol = K+[(2-ethene)PtCl3]-

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Dienes and trienes may also bond.(4C4H6)Fe(CO)3 (6-cycloheptatriene)Cr(CO)3C4H6 = buta-1,3-diene

CrCC C

OO

O

FeCC C

OO

O

(3-allyl)(4-C4H6)(5-Cp)Mo

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Molecular orbitals of ethene

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Molecular Orbitals of Buta-1,3-diene

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BenzeneMolecularOrbitals

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Odd numbered -systems have a non-bondingorbital in the middle of the energy rangewith a node in the middle.

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Metal complexes in low oxidation states with high fieldligands tend to have 18-valence electrons on the metal.[The 18 electron rule does not work for high oxidation states(>2) and weak field ligands, e.g. [Cr(H2O)6]3+].

18-electron rule

To simplify electron counting the metal is always in oxidationstate 0 sharing 1e in covalent bonds and accepting 2ein donor bonds.

..M PR3..M Cl

In M-Cl the metal gains one electron from the ClAnd in MPR3 the metal gains 2e from PR3

Some 16e compounds are stable at the RHS of the transition series Rh, Pd, Pt, Ir.

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Listing ligands by number of electrons they provide

1e M-H, M-Cl, M-Br, M-I, M-R, bent M-NO and M-M2e CO, PR3, R2C=CR2, R2C (carbene)

3e 3-allyl, NO (linear), RC (carbyne), -X (Cl, Br, I)

4e 4-diene

5e 5 -dienyl

6e 6 -C6H6

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Example (6-C6H6)Cr(CO)3

Cr(0) group 6 6e6-C6H6 6e

3CO 3x2 6e

18e

This complex would be expected to be stable.

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((2 ethene)2RhCl)2 has a -Cl structure

RhCl

RhCl

Rh(0) group 9 9eCl –Rh 1eClRh 2e2 x 2-ethene 4e

16e

16e compounds are often stable for Rhas it is at the RHS of the d-block.

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Formulae of the binary metal carbonyls M(CO)x

Using the 18 electron rule the formulae of the simplestbinary carbonyls can be predicted.

Remember the 18 electron rule is a principle of maximumorbital use.

It is not easy to increase a metals valence electron countbeyond 18 as all s, p and d-orbitals are filled by 18e.

The simplest possible formulae are;

M(CO)x when M has an even number of valence electrons

M2(CO)x when M has an odd number of valence electronsand

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Metal valence electron count

The number of valence electrons = Group number for d-block

No. 3 4 5 6 7 8 9 10 11 12Sc Ti V Cr Mn Fe Co Ni Cu Zn

Example with even number of electrons

Cr Cr(CO)x Cr 6e + (xCO) 2x = 18

x = (18 –6) / 2 = 6

Predicted formula Cr(CO)6

X = no. of CO per M

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Example with odd number of electrons

Co Co2(CO)2x

These metals get 18e by forming an M-M bond which giveseach metal 1 extra electron.

The formula is then for each metalM(e) + 1(from M-M) + 2x = 18

x = (18 –1 – M(e)) / 2

Which for Co is x = (18 – 1 – 9) / 2 = 4

Cobalt is in group 9

Thus the formula is Co2(CO)8

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Simple Stable Binary Metal Carbonyls

M(e) 6 7 8 9 10Cr(CO)6 Mn2(CO)10 Fe(CO)5 Co2(CO)8 Ni(CO)4

Mo Tc Ru RhW Re Os Ir

There are no simple carbonyls known for Pd or Pt

While other formulae are known the 2nd and 3rd rowsfollow the first row for the most part.

What about Ti and V ?

Ti Group 4 x = (18 – 4) / 2 = 7

Ti(CO)7 does not exist, there are not enough orbitals to bond7 CO groups. However salts of [Ti(CO)6]2– have been made

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V Group 5

M(e) + 1(from M-M) + 2x = 18

x = (18 – 1 – 5) / 2 = 6

This suggests a formula V2(CO)6

This would be 7-coordinate which would exceed 6and be too sterically hindered.

Salts of 18e [V(CO)6]– are known and the very unstable V(CO)6 radical has been isolated.

V2(CO)2x

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No. 3 4 5 6 7 8 9 10 11 12Sc Ti V Cr Mn Fe Co Ni Cu Zn

Another approach

Remember 1 simple formula e.g. Fe(CO)5 or Ni(CO)4 andwork from there.Fe(CO)5 is 18e

Mn(CO)5 is 17e + 1 M-M = 18e Formula is (Mn(CO)5)2

Co(CO)5 is 19e Co(CO)4 is 17e + 1 M-M gives (Co(CO)4)2

In the Fe group Fe forms Fe(CO)5 Fe2(CO)9 Fe3(CO)12

Many other formulae are known e.g. -

Ru and Os also form M(CO)5 but are much more stableas M3(CO)12

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Cr(CO)6 Oh Mn2(CO)10 staggered Fe(CO)5 D3h

Co2(CO)8 bridged + non-bridged (soln.) Ni(CO)4 Td ED

Structures of some binary carbonyls

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Synthesis of M(CO)x

Reaction of metal powder (often prepared in situ) with COunder pressures of up to 200 bar is the general method.

CrCl3 + LiAlH4 + CO Cr(CO)6

RuCl3.xH2O +CO +Zn Ru3(CO)12

390K 70bar

400K 90bar

Physical properties

Cr(CO)6, white solid

Fe(CO)5, orange liquid b.p.103º (toxic),

Ni(CO)4, colourless liquid b.p. 45º (very very toxic)

Mn2(CO)10, yellow solid

Co2(CO)8, orange air sensitive solid

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Reactions of Organometallic Compounds

Ligand Substitution

Carbonyl substitution

M(CO)x + L M(CO)x-1L + COh or

h or

18e 16eM(CO)x M(CO)x-1 M(CO)x-1L

18e

L

slow fast

The reaction rate does not depend on the concentrationof L.

The substitution mechanism for 18e complexesis dissociative.

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Oxidative Addition

A reaction in which the metal is oxidized by two units andthe metal coordination number is increased usually by 2.

Addition of a molecule X-Y to a metal centre.

M(L)x

NM(L)x

N+2

Y

XX-Y+

Oxidation state N Oxidation state N+2

Coordination No. x Coordination No. x+2

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The best know examples are provide by the complexes ofRh(I) Ir(I) Pd(0) Pd(II) Pt(0) and Pt(II).

The metal must have 2 stable oxidation statesseparated by 2 units.

e.g. Rh(I) and Rh(III) or Pd(0) and Pd(II)

Perhaps the best known is Vaska’s complex [Ir(PPh3)2(CO)Cl]

This complex reacts with a very large number of X-Y molecules.

[Ir(PPh3)2(CO)Cl] + MeI = [Ir(Me)(PPh3)2(CO)(I)Cl]

16e Ir(I)4 coord.

18e Ir(III)6 coord.

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Oxidation state of the metal

In the [MLaXb]c+ formalism

a is the number of L type ligands

b is the number of X type ligands

c is the charge on the complex

Oxidation state of the metal = b + c

[ML4X2]+ The metal is in oxidation state III

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Conversion of (n-ligand)M complexes to [MLaXb]c+ formalism

Monodentate neutral ligands e.g. PR3 are L typeIn general n-ligands with even n haveLa with a = (no. of electrons / 2) e.g. 6-C6H6 = L3

Ligands which donate an odd number of electrons have anX type interaction and possible L interactions

Ligands which donate an even number of electrons are L type

n-ligands with odd n have an X interaction and (n-1) / 2 L interactions

Only X type interactions affect oxidation state

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(3-allyl)(4-butadiene)(5-cyclopentadienyl)Mo

(X L) ( L2 ) ( X L2 )Mo

MoL5X2 Mo oxidation state = II

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[Mn(5-C5H5)(CO)3]

[Co(5-C5H5) (CO)(PPh3)2],

[Mo(CO)3(PPh3) 2I]

[Co(2-Buta-1,3-diene)(CO)4Br]

[(3-allyl)Mo(CO)4I]

Mn( X L2 ) ( L3 ) MnL5X Mn(I)

CoL5X Co(I)

MoL5X Mo(I)

CoL5X Co(I)

MoL5X2 Mo(II)

[Fe(CN)5NO]2-

nitroprussideNO is more easily oxidized than Fe(II) Fe(II)NO+ [FeX5(L+)]2-

[NO]+ is isoelectronic with CO You can buy [NO]+[PF6]-

Fe(IV)[Fe ( X5 ) ( X L )]2- [FeLX6]2-

The iron is diamagnetic low spin d6

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In this example reductive elimination gives a new C-H bond

Oxidative addition is reversible

the reverse reaction is called reductive elimination

Vaska’s complex is an oxygen carrier

IrCl

COPPh3

Ph3P +OO Ir

PPh3

PPh3

OClOOC

Oxidative addition followed by reductive eliminationis the basic mechanism of many catalytic processes.

CoCO

OC H

HOCO Me

CoCOOCCO

H

+ MeO

H

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Mechanism of oxidative addition – There are 3 establishedreaction mechanisms.

1. Concerted addition

* filled empty

M

M + H2 MH2

H

H

H

H

MH

H

H

H

This is the most likely mechanism when X=Y in X-Y and the reaction is conducted in a non-polar solvent.

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2. Ionic stepwise

3. Free radical

In the presence of O2 peroxides or other free radicalsources radical mechanisms have been detected. Forexample in the case of Vaska’s complexInit + Ir(I) Init-Ir(II)Init-Ir(II) + RX Init-Ir-X + R

R + Ir(I) R-Ir(II)R-Ir(II) + RX R-Ir(I)-X + R

M + RX MR+ + X- M(R)(X)slow fast

A likely mechanism when X≠Y and the reaction takes place ina polar solvent.

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Alkyl and hydrogen migrations

MnMe

CO COOC

COOC + CO MnCO COOC

COOCMeO

This is an example of alkyl migration

This has also been called CO insertion into a metal alkyl

The reaction mechanism suggests the former

The term migratory insertion tries to satisfy everyone

Using 13CO the mechanism can be deduced.

If 13CO is used none of it ends up in the MeC=O group

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Mn

Me

CO COOCOC C

O

MnCO COOC

OC O

Me13CO

MnCO COOC

OC O

MeCO

13

6-coordinate 5-coordinate 6-coordinate

MnCO

COOC

COOCMe

MnCO

CO COOC

MeOCMnCO

CO COMe

COOC MnCO

CO COOC

COMe

MnCO COOC

OC COCMe O

13

- CO

13 1313+ + +

25% 25% 25% 25%

CO is lost trans to another CO (trans effect of CO > COMe) and Me then moves into the vacant position.Thus the result supports the alkylmigration mechanism.If only CO insertion was involved the only products would be those in the box.

When this 13CO labelled complex is heated it looses CO and the 13C label position in the products can be used to test the mechanism.

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-Hydrogen elimination

-H elimination, giving an unstable [M(alkene)H] complex, is the principle decomposition pathway for metal alkyls which havea -hydrogen.

This is why the most stable alkyl complexes are formedby Me, Ph, CH2CMe3, CH2SiMe3 and CH2Ph.

MH

H

RH H

Ln

MH

H

RH

H

Ln

HH

RH

LnMH

After -elimination the olefin may dissociate (L type ligand) leaving a very reactive metal hydride which itself then reacts/decomposes.

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“Insertion Reactions”

Metal alkyl + CO metal acyl

+H

H

H

HM HLn M

H HHH

H

Ln

-elimination

olefininsertion

Metal hydride + olefin metal alkylOlefin into metal hydride

COM RCO

Ln MCO

RO

Ln

CO into M-R

Olefin into M-R

This is really alkylmigration

R'

H

H

H

M R MH

R'R

Ln Ln

This is reallyH migration

This is another alkylmigration.It is a mechanism forolefin polymerization

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Reactions involving M-M bonds

Oxidative clevageM-M + X2 2M-X(OC)5Mn-Mn(CO)5 + Br2 2 (CO)5Mn-Br

Reductive clevageM-M + reducing agent 2 M¯

(OC)5Mn-Mn(CO)5 + Na / Hg 2 (OC)5Mn¯

These anions are good nucleophiles.

(OC)5Mn¯ + MeI (CO)5Mn-Me + I¯

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(OC)5Mn-Mn(CO)5 + Br2 2 (OC)5Mn-Br

Na / Hg

(OC)5Mn¯

MeI

(OC)5Mn-Me

CO

(OC)5Mn-(CO)-Me

MeMgBr

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Metal carbene complexes

M=CR2 There are two types

1. Hetero atom stabilized or Fischer type carbenes

CrR

OMe(OC)5

2. Unstabilized or Schrock type carbenes

MR

R'

The O atom is the heteroatom in this case.

There is no hetero atom attachedto the carbene carbon.

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Synthesis of metal carbene complexes

The Fischer type are obtained by reaction of Li alkylswith coordinated CO. Works best for Cr, Mo and W.

+ Me LiCr C O(OC)5 Cr C OMe

(OC)5 Li+-

The anion is then alkylated using Me3O+ BF4¯

Cr CR

OMe(OC)5

The hetero atom stabilizes the molecule and polarizes theCr=C bond

Cr CR

OMe(OC)5

+- The carbene carbon is

attacked by nucleophiles

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The Schrock type are synthesised by -H abstraction

Ta(CH2tBu)3Cl2

LiCH2tBu

-LiCl -CMe4

TaH

tBu(tBuCH2)3

Schrock type carbenes involve early t-metals in highoxidation states.

Cr CR

OMe(OC)5

+- Fischer type are polarized M¯C+ and are

not olefin metathesis catalysts, C attacked by nucleophiles

MR

R'+ -Schrock type are polarized M+C¯ andare good olefin metathesis catalysts,C attacked by electrophiles.

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Metal carbyne complexes

MOMe

Ph(OC)5 + BX3 +M C PhX(OC)4 BX2OMe + CO

Fischer carbenes react with BX3 (loss of OMe¯)

M C Ph(OC)5+ + BX3OMe- M C PhX(OC)4 + CO

The cation formed is susceptible to X¯ attack and CO loss

-H abstraction from a Schrock carbene can also yieldcarbyne complexes.

PMe3 Ph3P=CH2

- Ph3MePClTa

Cl Cl

CH

tBuTa

Cl PMe3

C tBu

M = Cr, Mo, W and X = Cl, Br, I

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Bond length and bond order

An M-C single bond should be close to the sum of thecovalent radii.

M-C double and triple bonds should be shorter than M-C

This is the case

M C M C M C> >M = Cr ~2.10 2.04 1.69 Å

Metal carbonyls

M-C bond lengths are shorter than M=C in carbenes

M C O M C O A resonance form withsome MC multiplecharacter is involved

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Metallocenes

di-cyclopentadienyl sandwich complexes

Ferrocene is by far the best known and the most stable

[Fe(5-cyclopentadienyl)2] Fe( X L2 )2 FeL4X2

FeL4X2 Fe 8L4 8X2 2

18

Only Ru and Os can also do this

All non-iron group metallocenesare less stable than ferrocene

Cobaltcene has 19e and is unstable in air

The cobaltcinium cation is 18e and air stable

[Co(5-cyclopentadienyl)2]+ [PF6]¯

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Bonding in 5-cyclopentadienyl complexes

cyclopentadienyl molecular orbitals Chapt 18 box 18.2

+

+ - +-

++-

- + +--

D5d

a1

e1

e2

Metal orbitals

dz2

dxz, dyz

Each C is sp2

hybridized andafter the -frame 5pz orbitals remain

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Chemistry of ferrocene

Fe FeAc2O + H3PO4

Me

O

FeLi

nBuLiRCOClAlCl3

FeMe

OMe

O

mono acetylationmild conditions

di-acetylation morevigorous conditions

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Zirconocene derivatives

Zirconium is stable in oxidation state IV and forms theZirconocene derivative (5-C5H5)2ZrCl2

ZrCl

Cl

This compoundIs 16e Zr Group 4( X L2 )2 Zr X2ZrL4X4 = 16e

WCl

Cl

The analogous Wcompound is 18eas W is in Group 6

This type of compoundis sometimes called abent metallocene

ZrClCl

A related Zr systemhas led to importantnew chiralMetallocene Zieglercatalysts

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Questions

Give metal valence electron counts for the followingsystems and indicate those which are likely to be stableand those which are not;[Mn(5-C5H5)(CO)3], [Co(5-C5H5)(CO)(PPh3)2],[Mo(CO)3(PPh3) 2I], [Co(2-Buta-1,3-diene) (CO)4Br]and [Mo(3-allyl)(CO)4I].

What is meant by the term -acid ligand ?

Explain the term "ligand cone angle". Which of the followinghave the largest and smallest ligand cone angles, P(Ph)3, P(o-tolyl)3 and PH3.

Give metal valence electron counts for the following systems andindicate those which are likely to be stable and those which are not; [Cr(5-C5H5)(CO)2], [Mn(5-C5H5)(CO)4],[Mo(CO)3(PPh3)I2], [Co(2-Butene)(CO)3Br] and [Mo(3-allyl)(5-C5H5)(CO)2].