lecture 4 transition metal organometallics ligands

LECTURE 4TRANSITION METAL ORGANOMETALLICS LIGANDS

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TRANSITION METALSEarly

Middle

Late

3

EARLY TRANSITION METALS Groups 3, 4

Strongly electrophilic and oxophilic Few redox reactions (exception: Ti) Nearly always < 18e Polar and very reactive M-C bonds (to alkyl and

aryl)

Few d-electrons: preference for "hard" s-donors (N/O/F) weak complexation of p-acceptors (olefins,

phosphines) Typical catalysis: Polymerization

EARLY TRANSITION METALS Groups 3, 4

MeM M

Me

MeM etc

5

"MIDDLE" TRANSITION METALS Groups 5-7

Many accessible oxidation states Mostly 18e Ligands strongly bound Strong, not very reactive M-C bonds

Preference for s-donor/p-acceptor combinations (CO!)

Typical catalysis: Alkene and alkyne metathesis

"MIDDLE" TRANSITION METALS Groups 5-7

M CH2

CH2 CH2

MCH2

CH2

CH2

CH2 CH2

M CH2

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LATE TRANSITION METALSGroups 8-10 (and 11)

Many accessible oxidation states Mostly 18e or 16e

16e common for square-planar complexes

Easy ligand association/dissociation Weak, not very reactive M-C bonds Even weaker, reactive M-O/M-N bonds

Preference for s-donor/weak p-acceptor ligands (phosphines)

Typical catalysis: Hydroformylation

HM M

HM

COCO

M

M

O

MH2

O

M

H

O

H

H

O

H2

LATE TRANSITION METALSGroups 8-10 (and 11)

Transition-metal Organometallics

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ROWS

1 rowst

2 rownd

3 rowrd

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GOING DOWN

1st row: often unpaired electrons different spin states (HS/LS) accessible "highest possible" oxidation states not very stable

MnO4- is a strong oxidant

2nd/3rd row: nearly always "closed shell" virtually same atomic radii (except Y/La) highest oxidation states fairly stable

ReO4- is hardly oxidizing

2nd row often more reactive than 3rd

THE CARBONYL LIGAND11

• In 1884 Ludwig Mond found his nickel valves were being eaten away by CO. An experiment was designed where he deliberately heated Ni powder in a CO stream thus forming the volatile compound, Ni(CO)4, the first metal carbonyl. It was also found that upon further heating Ni(CO)4 decomposes to give pure nickel. This Ni refining process still used today is known as the Mond process.

• Having no net dipole moment, intermolecular forces are relatively weak, allowing Ni(CO)4 to be liquid at room temperature.

THE CARBONYL LIGAND CO groups have a high tendency to stabilize M−M

bonds; not only are CO ligands relatively small but they also leave the metal atom with a net charge similar to that in its elemental form (electroneutrality principle).

“Stable complexes are those with structures such that each atom has only a small electric charge. Stable M-L bond formation generally reduces the positive charge on the metal as well as the negative charge and/or e- density on the ligand. The result is that the actual charge on the metal is not accurately reflected in its formal oxidation state” - Pauling; The Nature of the Chemical Bond, 3rd Ed.;1960, pg. 172.

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CARBONYL CO also has the ability to stabilize polyanionic species

by acting as a strong acceptor and delocalizing the negative charge over the CO oxygens.

Na4[Cr(CO)4] has the extraordinarily low ν(CO) of 1462 cm−1, the extremely high anionic charge on the complex, and ion pairing of Na+ to the carbonyl oxygen contribute to the reduced CO bond order by favoring the MC−ONa resonance

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THE CARBONYL LIGAND As the CO ligand is small and strongly bound,

many will usually bind as are required to achieve coordinative saturation, e.g. V(CO)7

Metal carbonyls, in common with metal hydrides, show a strong preference for the 18e configuration.

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METAL CARBONYLS – STRUCTURE and BONDING

CO is an unsaturated ligand, by virtue of the CO multiple bond.

CO is classed as a soft ligand because it is capable of accepting metal d electrons by back bonding, i.e. it is a -donor -acceptor ligand.

Overview of Organometallic Chemistry

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This contrasts to hard ligands, which are σ donors, and often donors, too.• CO can act as a spectator or an actor ligand.

METAL CARBONYLS – STRUCTURE and BONDING

M C O

The CARBONYL LIGAND In the CO molecule both the C and the O

atoms are sp hybridized. The singly occupied sp and pz orbitals on

each atom form a σ and a bond, respectively.

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Frontier orbitals of free CO showing thepolarization of the z orbital.

THE CARBONYL LIGAND This leaves the C py orbital empty, and

the O py orbital doubly occupied, and so the second bond is formed only after we have formed a dative bond by transfer of the lone pair of O py electrons into the empty C py orbital.

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• This transfer leads to a C−−O+ polarization of the molecule, which is almost exactly canceled out by a partial C+−O− polarization of all three bonding orbitals because of the higher electronegativity of oxygen.

• The free CO molecule therefore has a net dipole moment very close to zero.

CARBONYL LIGAND A metal orbital forms a bond with HOMO orbital of CO. The HOMO is a s orbital based on C (due to the higher

electronegativity of O its orbitals have lower energy). The metal orbitals form a bond with the CO * LUMO (again

polarized toward C) The metal HOMO, the filled M d orbital, back donates to the CO

LUMO increasing electron density at both C and O because CO * has both C and O character.

The result is that C becomes more positive on coordination, and O becomes more negative. This translates into a polarization of the CO on binding.

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THE CARBONYL LIGAND This metal-induced polarization chemically activates

the CO ligand. It makes the carbon more sensitive to nucleophilic and

the oxygen more sensitive to electrophilic attack. The polarization will be modulated by the effect of the

other ligands on the metal and by the net charge on the complex.

In LnM(CO), the CO carbon becomes particularly + in character if the L groups are good acids or if the complex is cationic, e.g. Mo(CO)6 or [Mn(CO)6]+, because the CO-to-metal -donor electron transfer will be enhanced at the expense of the metal to CO back donation.

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CARBONYL If the L groups are good donors or the complex is anionic, e.g.

Cp W(CO) or 2[W(CO)5]2−, back donation will be encouraged, the CO carbon will lose its pronounced + charge, but the CO oxygen will become significantly −.

The range can be represented in valence bond terms the extreme in which CO acts as a pure s donor, through to the extreme in which both the ∗

x and ∗y are both fully engaged

in back bonding.

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CARBONYLs and IR BANDS The high intensity of the CO stretching bands

(a result of polarization on binding) means that IR spectroscopy is extremely useful.

From the band position, we can tell how good the metal is as a base.

From the number and pattern of the bands, we can tell the number and stereochemistry of the CO’s present.

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CARBONYL LIGAND We can tell the bond order of the CO ligand by

recording the M-CO IR spectrum. The normal range of the C-O stretching frequency, (CO) is 1820–2150 cm−1. Free C-O stretch at 2143 cm-1. Lower energy for stretching mode means C-O bond is weaker.

As the metal to CO * back bonding becomes more important, we populate an orbital that is antibonding with respect to the C=O bond, and so we lengthen and weaken the CO bond, i.e. the M−C bond is made at the expense of the C=O bond.

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CARBONYL LIGAND Strong donor co-ligands or a negative charge on the metal result

in CO stretches at lower frequency. Why?v(CO) cm-1

[V(CO)6]- 1859

Cr(CO)6 2000

[Mn(CO)6 ]+ 2100

[Fe(CO)6 ]2+ 2204 The greater the ability of a metal to donate electrons to the *

orbitals of CO, the lower the energy of the C-O stretching vibration.

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SAMPLE EXERCISE On the basis of the carbonyl complexes

in the table shown, predict the approximate position (in cm-1) of the C-O streching band in [Ti(CO)6]2-

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CARBONYL LIGANDS Carbonyls bound to very poor -donor metals have very high

frequency ν(CO) bands as a result of weak back donation.

When these appear to high energy of the 2143 cm−1 band of free CO, the complexes are sometimes called non-classical carbonyls.

Even d0 species can bind CO, for example, the nonclassical, formally d0 Zr(IV) carbonyl complexes, [Cp2Zr(S2)(CO)] has a ν(CO) stretching frequency of 2057 cm−1.

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CARBONYL LIGANDSThe highest oxidation state carbonyl known is trans-[OsO2(CO)4]2+ with ν(CO) = 2253 cm−1.

Carbonyls with exceptionally low ν(CO) frequencies are found for negative oxidation states (e.g., [Ti(CO) ]2−; ν(CO) = 1747 cm−1) or where a single CO is accompanied by non -acceptor ligands (e.g., [ReCl(CO)(PMe3)4]; ν(CO) = 1820 cm−1); these show short M−C and long C−O bonds.

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CARBONYL LIGAND One of the most extreme weak -donor examples

is [Ir(CO)6]3+ with ν(CO) bands at 2254, 2276, and 2295 cm−1.

The X-ray structure of the related complex [IrCl(CO)5]2+ shows the long M−C [2.02(2)A° ] and short C−O [1.08(2)A° ] distances expected.


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SYNTHESIS1. Direct reaction of a transition metal with CO. Ni + 4CO Ni(CO)4

This method requires that the metal already be in a reduced state because only -basic metals can bind CO.

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SYNTHESIS2. Reductive carbonylation (reducing agent plus CO gas):

CrCl3 + 6CO + Al Cr(CO)6 + AlCl3 (catalyzed by AlCl3)

Re2O7 + 17CO Re2(CO)10 + 7 CO2 (CO as RA)

NiSO4 + CO + S2O42- Ni(CO)4

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SYNTHESIS3. Thermal or photochemical reaction of other binary carbonyls.

Fe(CO)5 hv Fe2(CO)9

Lesser known:From organic carbonyls.

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BRIDGING MODESCO has a high tendency to bridge two metals (μ2-CO)Electron count here is unchanged either side of equilibriumIn most cases the M−M bond accompanies the CO bridging group.The CO stretching frequency in the IR spectrum falls to 1720–1850 cm−1 on bridging.Type of CO v(CO) cm -1

Free CO 2143terminal M-CO 1850-2120bridging CO 1700-1850


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BRIDGING Consistent with the idea of a nucleophilic attack by a

second metal, a bridging CO is more basic at O than the terminal ligand.

Thus a bridging CO ligand will bind a Lewis acid more strongly than a terminal CO ligand.

Equilibrium can therefore be shifted in the previous reaction scheme.

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BRIDGING• Triply and even quadruply bridging CO groups are also known in metal cluster compounds.

For example, (Cp∗Co)3(μ3-CO)2

• These have CO stretching frequencies in the range of 1600–1730 cm−1.

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REACTIONS OF METAL CARBONYLS

All reactions of the CO ligand depend on the polarization of the CO upon binding, and so change in importance as the co-ligands and net charge change.

1. Nucleophilic attack at the Carbon:

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Reactions

Hydride attack at the C atom of CO here produces the unusual formyl ligand, which is important in CO reduction to MeOH.

It is stable in this case because the final 18e complex provides no empty site for rearrangement to a hydridocarbonyl complex (a-elimination).

Reactions2. Electrophilic attack at Oxygen

3. Migratory Insertion:

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BRIDGING There also exists the semi-bridging carbonyl in which the

CO is neither fully terminal nor fully bridging but intermediate between the two.

This is one of the many cases in organometallic chemistry where a stable species is intermediate in character between two bonding types.

Below each semi-bridging CO is bending in response to the second metal atom

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LIGANDS SIMILAR TO COCS, Cse, CTe do not exist as a stable free molecule and therefore do not provide a ready ligand source.

CN- and N2 complexes

CN- - stronger donor than CO - very similar to CO when it interacts with metal orbitals - weaker acceptor (consequence of the negative charge)Dinitrogen is a weaker donor and acceptor vs. CO. However, still of interest in reactions that might stimulate nitrogen fixation.

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PROBLEM SETSFrom Spessard and Meissler.

4-1 to 4-5, 4-8, 4-10, 4-11, 4-13

5-1, 5-3, 5-6, 5-7, 5-9, 5-13 to 5-14

6-1, 6-2, 6-3, 6-4, 6-6, 6-8

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IR SPECTRA No. of Bands:Monocarbonyl complexes have single possible C-O stretching mode – single IR band

Dicarbonyl complexes:Linear and bent

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Complexes with 3 or more carbonylsPrediction of exact no. of carbonyl bands complex but can be determined using group theory.

For convenience refer to a table.

Although can predict using a table:- some bands may overlap- may have very low intensity

- if isomers present, difficult to sort out

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POSITIONS OF IR BANDS: Increase in negative charge of the

complex, causes a reduction in the energy of the CO band.

The bonding mode of the CO terminal CO > double bridged (2) > triply bridged (3)

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POSITION OF IR BANDS Other ligands present also affects position of IR bands. For

example for the complex Ni(CO)3L:

L v(CO), cm-1

PF3 2111

PCl3 2097

PPh3 2069

The greater the electron density on the metal, the greater the back bonding to CO, the lower the energy of the carbonyl stretching vibration.

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MAIN GROUP vs. BINARY CARBONYL COMPLEXES

Electron short of filled shell

Examples of electronically equivalent species

Main Group Metal Carbonyl

1 Cl, Br, I Mn(CO)5, Co(CO)4

2 S Fe(CO)4, Os(CO)4

3 P Co(CO)3, Ir(CO)3

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Cl vs. Co(CO)4

Characteristics Cl Co(CO)4

Ion with closed shell configuration

Cl- [Co(CO)4]-

Nuetral dimer Cl2 Co2(CO)8

Interhalogen compound BrCl ICo(CO)4

Binary Acid HCl HCo(CO)4

Insoluble Heavy metal Salts AgCl AgCo(CO)4

Disproportionation by Lewis Bases

Cl2 + Me3N Me3NCl+ + Cl-

Co2(CO)8 + C5H10NH [C5H10NHCo(CO)4]+ + [Co(CO)4]-

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BONDED LigandsAlkene ComplexesAlkyne ComplexesAllyl ComplexesDiene ComplexesCyclopentadienyl ComplexesArene ComplexesMetallacycles


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TRANSITION METAL – ALKENE COMPLEXES


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The report in 1825 by William Zeise of crystals with composition, KCl.PtCl2.Ethylene, prepared from KPtCl4 and EtOH was a topic of controversy for many years due to the nature of Zeise’s structure - only possible by the dehydration of EtOH.

Proof of Zeise’s formulation came 13 years later when Birnbaum isolated the complex from a solution of platinic acid, H2PtCl6.6H20, treated with ethylene.

Zeise’s salt was the first organometallic compound to be isolated in pure form.

This discovery spawned a tremendous growth in organometallic chemistry and still serves as the simplest example of transition metal-olefin complexation.

TRANSITION METAL – ALKENE COMPLEXES

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• The -acid ligand donates electron density into a metal d-orbital from a -symmetry bonding orbital between the carbon atoms.

• The metal donates electrons back from a filled d-orbital into the empty *antibonding orbital of the ligand (similar to dihydrogen s-complexes)

• Both of these effects tend to reduce the C-C bond order, leading to an elongated C-C distance and a lowering its vibrational frequency.

TRANSITION METAL –ALKENE COMPLEXES

In the nickel compound Ni(CH2CH2)(PPh3)2 the C-C bond distance is 143 pm (vs. 134 pm for free ethylene).

The interaction can cause carbon atoms to "rehybridize“, for e.g in metal alkene complexes from sp2 towards sp3, which is indicated by the bending of the hydrogen atoms on the ethylene back away from the metal.

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Molecular geometry of Zeise’s salt (neutron diffraction)


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The PtCI3 moiety forms a nearly planar group with the Pt atom.

The Pt-CI bond trans to the ethylene group (2.340 A) is significantly longer than the cis Pt-CI bonds (2.302 and 2.303 A) – trans effect !!

The C atoms are approximately equidistant from the Pt atom (2.128 and 2.135 A).

The distance from the midpoint of the C-C bond to the Pt atom is 2.022 A.

The C-C distance, 1.375 A, is slightly longer than the value found in free ethylene (1.337 A), indicating some d-p* back-bonding from the platinum atom to C2H4.

Back-bonding is also indicated by a bending of the four hydrogen atoms away from the Pt atom.

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ALKYNE COMPLEXES Alkynes behave in ways broadly similar to

alkenes, but being more electronegative, they tend to encourage back donation and bind more strong

The substituents tend to fold back away from the metal by 30–40 in the complex, and the M−C distances are slightly shorter than in the corresponding alkene complexes.


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ALKYNES• Metals can stabilize alkynes that cannot be observed as free compounds.

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ALKYNES• Alkynes can also form complexes that appear to be coordinatively unsaturated.

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ALKYNES Coordinatively unsaturated alkynes?


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ALKYNE• In such cases the alkyne also donates its second C=C π-bonding orbital, which lies at right angles to the first.• The alkyne is now a 4e donor.

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ALKYNES Four electron alkyne complexes are rare

for d6 metals because of a 4e repulsion between the filled metal d and the second alkyne C=C π-bonding pair.

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BRIDGING METAL ALKYNE COMPLEXES


61

ALKYNES Alkynes readily bridge an M−M bond, in

which case they can act as conventional 2e donors to each metal.


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ALKYNES The alternative tetrahedrane form is the

equivalent of the metalacyclopropane picture for such a system.


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TAUTOMERIZATION


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TRANSITION METAL ALLYL COMPLEXES

The allyl group, commonly a spectator ligand, binds in one of two ways.

In the 1 form it is a simple X‐type ligand like Me

In the 3 form it acts as a LX -enyl ligand.

It is often useful to think in terms of the resonance forms (2e + 1e donor)


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ALLYL


66

ALLYL67

• As the number of nodes on the allyl ligand increase the MOs of the free ligand increase in energy, i.e. become less stable.

ALLYL68

ALLYLFrontier molecular orbitals of the metal allyl fragment:

1 is occupied by 2 electrons and has appropriate symmetry, energy and orientation to overlap with a suitable metal d orbital.

2 is also occupied by 2 electrons (ionic model) and has appropriate symmetry, energy and

orientation to overlap with a suitable metal d orbital. 3 is unoccupied and has appropriate symmetry, energy and

orientation to overlap with a suitable metal d orbital for back donation.

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ALLYL70

• The plane of the allyl is slanted at an angle with respect to the coordinationpolyhedron around the metal ( is usually 5◦–10◦ ).• The extent of orbital overlap between 2 and the dxy orbital on the metal isimproved if the allyl group moves in this way.

ALLYL


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• The terminal CH2 groups of the allyl are twisted about the C−C vector so as to rotatethe anti hydrogens (Ha) away from the metal, and the syn hydrogens (Hs) towardthe metal.• This allows the bonding p orbital on these carbons to point more directly towardthe metal, thus further improving the M‐L overlap.• The 3‐allyl group often shows exchange of the syn and anti substituents. Onemechanism goes through an 1‐allyl intermediate.• This kind of exchange can affect the appearance of the 1H NMR spectrum and alsomeans that an allyl complex of a given stereochemistry may rearrange with time.

ALLYL72

SYNTHESIS1. From an alkene via oxidative addition:

2. Nucleophilic attack by an allyl compound (transmetallation)

3. Electrophilic attack by an allyl compound:


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SYNTHESIS4. From diene complexes:

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TRANSITION METAL DIENE COMPLEXES

The diene ligand usually acts as a 4e donor in its cisoid conformation.

Analogous to metal alkene systems the LX2 (enediyl or σ2π) form to the metalacyclopropane extreme.


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• The L2 form is rarely seen with the LX2 form becoming more important as the back donation increases.

Butadiene


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• The frontier orbitals of the butadiene, 2 (HOMO) and 3 (LUMO), are the most important in bonding to the metal.

• Depletion of electron density in 2 by donation to the metal and population of 3 by back donation from the metal lengthens the C1‐C2 bond and shortens the C2‐C3 bond because 2 is C1‐C2 antibonding and 3 is C2‐C3 bonding.

BUTADIENE Binding to a metal usually depletes the ligand

HOMO and fills the ligand LUMO. This is the main reason why binding has such a profound effect on the chemical character of a ligand.

The structure of the bound form of a ligand is often similar to that of the first excited state of the free ligand because to reach this state we promote an electron from the HOMO to the LUMO.


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CYCLOPENTADIENE - Ferrocene The two cyclopentadienyl (Cp) rings of ferrocene may be

orientated in the two extremes of either an eclipsed (D5h) or staggered (D5d) conformation.

The energy of rotation about the Fe Cp axis Fe‐is very small (~ 4 kJmol‐1) and ground state structures of ferrocene may show either of these conformations.

There is also very little difference in electronic states between the D5h and D5d symmetries however the D5d point group irreducible representations are used here in the description of the electronic structure of ferrocene as they simplify the symmetry matching of ligand molecular orbitals and metal atomic orbitals.

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FERROCENE The primary orbital interactions that form the metal‐

ligand bonds in ferrocene occur between the Fe orbitals and the ‐orbitals of the Cp ligand.

If D5d symmetry is assumed, so that there is a centre of symmetry in the ferrocene molecule through the Fe atom there will be centro‐symmetric (g) and antisymmetric (u) combinations.

The five p‐orbitals on the planar Cp ring (D5h symmetry) can be combined to produce five molecular orbitals.


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CYCLOPENTADIENYL


80

CYCLOPENTADIENYL For a bis‐cyclopentadienyl metal complex

(5‐Cp)2M , such as ferrocene, the ‐orbitals of the two Cp ligands are combined pairwise to form the symmetry‐adapted linear combination of molecular orbitals (SALC’s).


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82


83


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CYCLOBUTADIENE Most of the neutral ligands we have studied (apart from

carbenes) have been stable in the free state. Cyclobutadienes on the other hand are highly reactive

when not complexed to a late transition metal. The free molecule, with four electrons, is antiaromatic

and rectangular, but the ligand is square and appears to be aromatic.

By populating the LUMO of the free diene the ligand is stabilized by metal back donation.


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CYCLOBUTADIENE Thus by gaining partial control of two more

electrons the diene attains an electronic structure resembling that of the aromatic six ‐electron dianion.

Ligand‐to‐metal σ donation prevents the ligand from accumulating excessive negative charge.

This again is a clear example of the free and bound forms of the ligand being substantially different from one another.


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CYCLOBUTADIENE


87

METAL HYDRIDE COMPLEXES Main group metal hydrides play an important role as

reducing agents (e.g. LiH, NaH,LiAlH4, LiBH4).

The transition metal M-H bond can undergo insertion with a wide variety of unsaturated compounds to give stable species or reaction intermediates containing M-C bonds

They are not only synthetically useful but are extremely important intermediates in a number of catalytic cycles.

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METAL HYDRIDE PREPARATION1. Protonation (requires an electron rich

basic metal center)

2. From Hydride donors (main group metal hydrides)


90

HYDRIDE3. From H2 (via oxidative addition – requires a coordinatively unsaturated metal center)

4. From a ligand (-elimination)


91

HYDRIDE A metal hydride my have acidic or basic character depending on

the electronic nature of the metal involved (and of course its ligand set).

Early transition metal hydrides tend to carry significant negative charge on the H atom whereas later more electronegative transition metals favour a more positive charge on the H atom (the term Hydride should therefore not be taken literally).

Reactivity can also depend upon the substrate, e.g. CpW(CO)3H is a H+ donor to simple bases, a H• donor to toward styrene and a H− donor to carbonium ions.


92

HYDRIDES HCo(CO)4 is a strong acid due to the

electron withdrawing effect of the -donating, -accepting CO ligands on the Co(I) center.

With -donating, -donating ligands the hydride can become quite basic and reactive towards H− transfer.


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HYDRIDES Applied in catalysis

to promote ketone reduction e.g. acetone to isopropanol. The formate ion (HCO2

− ) is used as a source of H− with liberation of CO2.


95

HYDRIDES The same

catalyst can be used for reduction of CO2 under the appropriate conditions.


96

DIHYDROGEN COMPLEXES An electrophile E+ can react with an X-H

bond to give a s complex 1, in which the X-H bond acts as a 2e donor. (not to be confused with -bonding Hydrides)


97

DIHYDROGEN COMPLEXES 1 a and 1 b show two common ways of

representing 1. Coordination to E+ alters the chemical properties of the X-H bond and can activate it either for nucleophilic attack at X or deprotonation.


98

DIHYDROGEN COMPLEXES The X-H bond is always coordinated side-

on to E+, as in 1. complexes bind by donation of the X-H

-bonding electrons in a 2e 3 center bond to the metal.


99

• X = H, Si, Sn, B, or P…..at least one H must always be present.• The H atom has a small atomic radius and carries no lone pairs or other substituent's, allowing the hydrogen end of the X-H bond to approach close to the metal and so allow the filled M d orbital to back-bond relatively strongly onto the lobe of the X-H * orbital that is located on the H atom.


100

DIHYDROGEN COMPLEXES


101

The bonding picture for a s complex.a) Only the 1 orbital which

bonds over all three centers, is occupied. Occupation of 2 would lead to the opening of one edge of the triangle (nodal plane marked as a dotted line).

b) In an M-(H-X) complex the electrons of the X-H bond are donated to an empty metal d-orbital. This is analogous to the binding of the lone pair on NH3 to a metal atom.

c) Electron density from the M(d) orbital is donated to the X-H * orbital (back-donation). This resembles M(d) + CO(*) back-donation and is unique to transition metal complexes.

An isolable complex must have some backbonding, but strong back-donation leads to cleavage of the X-H bond by oxidative addition to give an X-M-H complex.


102

In complexes with weak back-bonding, the length of X-H bond is similar to that in free X-H.

The acidity and electrophilicity of X-H can be strongly enhanced, however, because bonding reduces the electron density in the X-H unit.

Stronger back-donation can lead to complexes with elongated X-H bonds and reduced electrophilicity of the X-H group.


103


104

“We propose the term “agostic” which will be used to discuss the various manifestations of covalent interactions between carbon-hydrogen groups and transition metal centres in organometallic compounds. The word agostic will be used to refer specifically to situations in which a hydrogen atom is covalently bonded simultaneously to both a carbon atom and to a transition metal atom.”


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AGOSTIC BONDS The β C−H bond is bound to the metal in a way that

suggests that the alkyl is beginning the approach to the transition state for β elimination.

These agostic alkyls can be detected by X-ray or neutron crystal structural work and by the high-field shift of the agostic H in the proton NMR.

The lowering of the J(C,H) and ν(CH) in the NMR and IR spectra, respectively, on binding is symptomatic of the reduced C−H bond order in the agostic system.


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107

• The reason that β elimination does not occur is that the d0 Ti has no electron density to back donate into the σ∗ orbital of the C−H bond.

• This back donation breaks the C−H bond in the β-elimination reaction, much as happens in oxidative addition.

Agostic binding of C−H bonds also provides a way to stabilize coordinatively unsaturated species.

They are also found in transition states for reactions such as alkene insertion/β elimination either by experiment or in theoretical work.


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109


110

M-H and M-C s-bonds

M H

M C

M C

C

M C C

M

Hydride

Alkyl

Vinyl (alkenyl)

Acetylide (alkynyl)

Aryl


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Synthesis of metal alkyls Metathesis

Electrophilic attack on metal

Insertion

TiCl4 + 4 BzMgCl TiBz4 + 4 MgCl2

(Bz = benzyl, C6H5CH2)

Mn(CO)5MeI

MeMn(CO)5

C2H4N

N

N

Co

Ar

Ar

EtN

N

N

Co

Ar

Ar

H


112

Synthesis of metal alkyls Oxidative addition

often starts with electrophilic attack

L

O

OL

RhL

O

OL

Me

I

L = P(OPh)3

RhMe

O

OI

L

L

L = PPh3

MeIRh

I


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Decomposition of metal alkylsDominant: b-hydrogen elimination

Alternatives: homolysis a/g/d-eliminations reductive elimination (especially with H or another alkyl) ligand metallation

MH

MH

MH

MH


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How to prevent b-hydrogen elimination ? No b-hydrogen

CH3, CH2CMe3, CH2SiMe3, CH2Ph

No empty site cis to alkyl

Product of elimination unstable

OH2

Co

ON

NOH

NN

OH

Et

?


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How to prevent b-hydrogen elimination ? Planar transition state inaccessible

even for 5-membered metallacycles b-elimination is difficult !(basis of selective ethene trimerization)

L2PtH

L2Pt H L2PtH

???


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Reactions of metal alkyls Insertion, of both polar and non-polar

C=X bonds: olefins, acetylenes, allenes, dienes (ketones etc) CO, isocyanides

Reductive elimination

CARBENE COMPLEXES The concept of a double bond between transition metals and

carbon constitutes one of the most important elements in the field of organometallic chemistry

The notion of a metal–carbon double bond was first brought forward by Fischer and Maasbol in 1964 with the synthesis of (CO)5W=C(Ph)(OMe)

Soon after the discovery of Fischer type complexes their chemistry was systematically explored and they have been since well established as valuable species in organic synthesis as well as in catalytic processes

Schrock later prepared a number of tantalum complexes including (Np) Ta=CH(CMe ) and (5-Cp)2MeTa=CH2

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CARBENES

Two different patterns of reactivity emerged during the development of these systems resulting in their classification as Fischer and Schrock type carbenes.

Each represents a different formulation of the bonding of the -CR2 group to the metal and real cases fall somewhere between the two.


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CARBENES


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CARBENES


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• Free carbene CH2 has two distinct spin isomers: singlet and triplet– not resonance forms (sinlget ↔ triplet resonance forbidden)• Singlet and triplet forms have different H-C-H angles• In the singlet state 2e- are paired up in the sp2 orbital leaving the pz orbital unoccupied• In the triplet state both the sp2 and p orbitals are singly occupied

CARBENES


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• (a) Singlet and triplet forms of a carbene• (b) In the Fischer case, direct CM donation predominates and the carbon tends to be positively charged.• (c) In the Schrock case, two covalent bonds are formed, each polarized toward the carbongiving it a negative charge.

CARBENES Taylor and Hall used ab-initio calculations to differentiate between the

electronic structures of Fischer and Schrock type carbene complexes. Calculations on a variety of free carbenes indicated that: heteroatom

and phenyl substituents preferentially stabilize a singlet ground state. alkyl and hydride substituents stabilized a triplet ground state at the

carbene carbon. Carbenes are both thermodynamically and kinetically unstable

therefore forming very strong metal-carbene bonds disfavoring dissociation e.g. just as pramagnetic triplet :CH2 can dimerize to form diamagnetic H2C=CH2, it also binds to a triplet LnM fragment to give a diamagnetic LnM=CH2 complex.

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FISCHER CARBENES


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SHROCK CARBENES125

• A Schrock carbene forms two covalent bonds via unpaired electrons.• Each M-C bond is polarized towards the carbene carbon because C is more electronegative than M, leading to a nucleophillic carbene carbon.


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SCHROCK CARBENE SYNTHESIS High valent metal alkyls of the early transition metals can

undergo proton abstraction at the a carbon to give nucleophillic Schrock carbenes

• This reaction is believed to involve an -proton abstraction (possibly agostic) by a neighbouring Np ligand liberating tBuMe.

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• One requirement of this a-abstraction reaction is that the molecule must be sterically crowded.• For example, simple substitution of Cl in Np2TaCl3 with the bulky Cp or PMe3 ligands induces -abstraction and tBuMe elimination producing the corresponding Schrock carbene complex.


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• On replacing the Np ligand with the benzyl ligand a more sterically demanding ligand set is required to induce a-proton abstraction liberating toluene to produce corresponding Schrockcarbene complex.

• Typically 2 Cp rings can be used or even pentamethylcyclopentadiene (Cp*)

REACTIONS

• Their nucleophillic character allows them to form adducts with Lewis acids.

• They react with ketones in a similar fashion as Wittig (Ph3P=CH2) reagents


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REACTIONS

Similar to Fischer carbenes Schrock type complexes also react with alkenes and alkynes to form metalacycles


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Schrock type complexes react with alkynes to form metalacyclobutenes which can rearrange to form the p-extended carbene-ene systems

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METAL CARBYNES

Have similar bonding formulations as per Fischer and Schrock carbenes

The free carbyne can be of doublet (Fischer) or quartet (Schrock) multiplicity


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METAL CARBYNES The carbyne ligand is linear Carbyne carbon is sp hybridized The MC bond is very short (1.65 – 1.90 Å) Characteristic low-field 13CNMR

resonance in the range +250 to +400 ppm


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FISCHER CARBYNES


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• A doublet (Fischer) carbyne is sp hybridized• Contains one filled sp orbital capable of donating 2e- to a metal centre• Contains one singly occupied p orbital capable forming an additional bond• The remaining empty p orbital is capable of MC p back donation• 3e- donor covalent model / 4e- donor ionic model

SCHROCK CARBYNE


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• A quartet (Schrock) carbyne is also sp hybridized• Contains three singly occupied orbitals (one sp and two p) capable of forming three covalentM-C bonds (one and two bonds)• This class of ligand is X3-type• 3e- ligand in covalent model (or 6e- ionic model)

SYNTHESIS

Fischer first prepared metal carbyne complexes by the electrophilic abstraction of methoxy from a methoxy methyl Fischer carbene.

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SYNTHESIS

In a more general approach, Schrock carbynes can be prepared by deprotonation of an a-CH

Intramolecular oxidative addition of a bound Schrock carbene (a elimination)


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SYNTHESIS

Metathesis of tertiary butoxide (tBuO) complexes (triple bi-nuclear oxidative addition, i.e. +III change in oxidation state)

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REACTIONS Fischer carbynes are electrophillic and

thus prone to nucleophillic attack. Nucleophiles such as PMe3, pyridine,

alkyl lithiums, and isonitriles react with Fischer carbynes to give the corresponding Fischer carbene complex.


140

REACTIONS• Alternatively the nucleophile may attack the metal centre producing a ketenyl complex


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REACTIONS• In contrast, Schrock carbynes are nucleophillic and prone to attack by electrophiles


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PHOSPHINES Tertiary phosphines, PR3, are important because they constitute

one of the few series of ligands in which electronic and steric properties can be altered in a systematic and predictable way over a very wide range by varying R.

They also stabilize an exceptionally wide variety of ligands of interest to the organometallic chemist as their phosphine complexes (R3P)nM−L.

Phosphines are more commonly spectator than actor ligands.


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PHOSPHINES


144

PHOSPHINES Like NR3, phosphines have a lone pair on the central atom

that can be donated to a metal.

Unlike NR3, they are also -acids, to an extent that depends on the nature of the R groups present on the PR3 ligand.

For alkyl phosphines, the acidity is weak; aryl, dialkylamino, and alkoxy groups are successively more effective in promoting acidity.

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PHOSPHINES In the extreme case of PF3, the acidity becomes

as great as that found for CO!

In the case of CO the * orbital accepts electrons from the metal.

The σ* orbitals of the P−R bonds play the role of acceptor in PR3.

146

PHOSPHINES Whenever the R group becomes more electronegative, the

orbital that the R fragment uses to bond to phosphorus becomes more stable (lower in energy).

This implies that the σ* orbital of the P−R bond also becomes more stable.

At the same time, the phosphorus contribution to σ* orbital increases, and so the size of the σ* lobe that points toward the metal increases

Both of these factors make the empty σ* more accessible for back donation.

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PHOSPHINES The final order of increasing -acid

character is:

PMe3 ≈ P(NR2)3 < PAr3 < P(OMe)3 < P(OAr)3 < PCl3 < CO ≈ PF3

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PHOSPHINES149

• The empty P−R * orbital plays the role of acceptor in metal complexes of PR3.• As the atom attached to the P atom becomes more electronegative, the emptyP−X * orbital becomes more stable (lower in energy) making it a better acceptor of electron density from the metal center.

PHOSPHINES Occupation of the P−R σ* orbital by back donation from the metal

also implies that the P−R bonds should lengthen slightly on binding.

In practice, this is masked by a simultaneous shortening of the P−R bond due to donation of the P lone pair to the metal, and the consequent decrease in P(lone pair)–R(bonding pair) repulsions.

Once again, as in the case of CO, the M−L bond is made at the expense of a bond in the ligand, but this time it is a σ, not a , bond

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PHOSPHINES


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PHOSPHINES – TOLMAN ELECTRONIC PARAMETER

The electronic effect of various PR3 ligands can be adjusted by changing the R group as, quantified by Tolman, who compared the ν(CO) frequencies of a series of complexes of the type LNi(CO)3, containing different PR3 ligands.

The increase in electron density at the nickel from PR3 σ-donation is dispersed through the M-L system via -backbonding. Much of the electron density is passed onto the CO * and is reflected in decreased v(CO) stretching frequencies which corresponds to weaker CO bonds.

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PHOSPHINE153

TOLMAN – CONE ANGLES The second important feature of PR3 as a ligand is the variable

steric size, which can be adjusted by changing R.

CO is so small that as many can bind as are needed to achieve 18e. In contrast, the same is rarely true for phosphines, where only a certain number of phosphines can fit around the metal.

This can be a great advantage in that by using bulky PR3 ligands, we can favor forming low-coordinate metals or we can leave room for small but weakly binding ligands,


154

PHOSPHINEThe usual maximum number of phosphines that can bind to a single metal is

two for PCy3 or P(i-Pr)3

three or four for PPh3

four for PMe2Ph five or six for PMe3


155

PHOSPHINECoordination Number (CN) – the number of bonding groups at metal center.Low CN favored by:1. Low oxidation state (e- rich) metals.2. Large, bulky ligands.


156

Although Pd(P(tBu)2Ph)2 is coordinatively unsaturated electronically, the steric bulkof both P(tBu)2Ph ligands preventsadditional ligands from coordinating tothe metal.

TOLMAN CONE ANGLE

The cone angle is obtained by taking a space-filling model of the M(PR3) group, folding back the R substituents as far as they will go, and measuring the angle of the cone that will just contain all of the ligand, when the apex of the cone is at the metal.

Although the procedure may look rather approximate, the angles obtained have been very successful in rationalizing the behavior of a wide variety of complexes.

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An important part of organometallic chemistry consists in varying the steric and electronic nature of the ligand environment of a complex to promote whatever properties are desired: activity or selectivity in homogeneous catalysis, reversible binding of a ligand, facile decomposition, or high stability.

Using the Tolman plot we can relatively easily change electronic effects without changing steric effects

e.g., by moving from PBu3 to P(OiPr)3]

Also, we can relatively easily change steric effects without changing electronic effects

e.g., by moving from PMe3 to P(o-tolyl)3

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1H and 13C NMR13C:

- for ligands that do not contain hydrogen (CO)- decoupled spectra shows singlets for each atom

Saturated Carbon appear between 0-100 ppm with electronegative substituents increasing the shifts. CH3-X : directly related to the electronegativity of X.

The effects are non-additive: CH2XY cannot be easily predicted

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Shifts for aromatic compounds appear between 110-170 ppm

-bonded metal alkene may be shifted up to 100 ppm: shift depends on the mode of coordination

Metal carbonyls are found between 170-290 ppm. (very long relaxation time make their detection very difficult)

Metal carbene have resonances between 250-370 ppm


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1HNMR


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Review QuestionsFor each of the following pairs of complexes, which will have the lowest average CO infrared stretching frequency? briefly explain your reasoning.

Cp2Y(CH3)(CO) vs. Cp2V(CH3)(CO)

CpFe(CO)2(PF3) vs. CpOs(CO)(PMe3)2 Overview of Organometallic Chemistry

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The most electron-rich metal with d-electrons will -backbond the most with the CO ligands and have the lowest CO stretching frequency


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Cr(CO)4(PEt3)2 vs. W(CO)4(PPh3)2

PtCl4(CO)2 vs. NiBr2(CO)2


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Sketch out a neutral 18-electron structure showing the geometry about the metal center as accurately as you can at this point in the course for the following metals and ligands. Use at least one metal and each type of ligand shown. Try to keep your structure as simple as possible .Show your electron counting.

A) W, μ-PR2, CO, H

B) Pt, CH, Cl, PMe3

C) Nb, O, CH3, Cp, PMe3 Overview of Organometallic Chemistry

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lecture 4 transition metal organometallics ligands

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