Organometallics Study Seminar
Chapter 13: Metal-Ligand Multiple Bonds
Contents
1. Carbene Complexes
2. Silylene Complexes
3. Metal-Heteroatom Multiple Bonds

1. Carbene Complexes
1.1 Classes of Carbene Complexes

1.2 Origin of the Electronic Properties of Fisher and Schrock Carbenes
vinylidene complex
Melectrophilic:CCH2
singlet state is more stableby 42kcal/mol.

M: low oxidation state

Fisher type carbene

LUMO: ligand character
electrophilic at ligand
HOMO: metal character
nucleophilic at metal

LUMO: metal character
electrophilic at metal
HOMO: ligand characternucleophilic at ligand-bond: polarize toward the carbon-bond: polarize toward the metal-bond: polarize toward the carbon-bond: polarize toward the carbon

11.3 Synthesis of Carbene Complexes
Fisher carbene complexes
General method

Metal dianion

Isocyanide complex

Diazoalkanecf) rhozium-carbene complex

Surfur ylide2Vinylidene carbene complexes
Alkyne complexes
alkyne: 4e donor (without CO)
18 e complex
alkyne: 2e donor (with CO)
vinylidenetautomer is more stable on
this relatively electron-rich, low-valent
metal fragment

In all cases, these vinylidene complexes contain transition metals in relatively low oxidation state.
Metal-acetylide complexes
Vinylidene complex is thermodynamicallyunstable.

Vinylidene complex is stable with more
electron-rich ligand.

Alkylidene complexes
General procedure (synthesis of first Schrock carbene)-hydrogen elimination from ahigh-valent, early metal , dialkyl
complexSchrock alkylidene catalysts

active catalyst for olefin metathesis (reactivity: Mo > W)
Tebbe's reagent31.4 Synthesis of N-heterocyclic carbene complexes
Synthesis of N-heterocyclic carbene complexes

imidazolium salt with a complex
containing a basic ligandtransfer from silver carbene

imidazolium-2-carboxylate
Synthesis of free carbene

These carbene are stable at monomeric species when the substituents at nitrogen are large enough toprevent dimerization (t-Bu, Ad, di-ortho-substituted aryl groups).

1.5 Reactivity of carbene complexes
Fisher carbene complexes: related to the reaction chemistry of organic ester
Reaction with nucleophile

Conversion to carbyne complexes

4Reactions related to those of enolates

Fisher carbene complexes react like enolates when treated with base.
Cyclopropanations

mechanism

Both pathways are possible.

cyclopropanation + olefin methathesis
metallacycle intermediate

R1Ln(OC)MR2OR
Annulations: The Dotz reaction

zwitterionic intermediate

Vinylidene complexes: similarity to Fisher carbenecomplexes
Basicity of -carbon[2+2] reaction

stepwise: nucleophilic attack, ring closure5

Alkylidene and alkylidyne complexes
Examples[2+2]

Mechanism

Direct detection of ruthenacyclobutane intermediate

open coordination site

(Schrock-type carbene complexes also undergo
the [2+2] reaction.)

NMR time scale(without dissociation)
Factor of the reaction speed

16e

14e
dissociation of ligand
reassociation of ligand

Formal [2+2] reactions with C-H -bond
opposite of the -eliminations that forms alkylidene complexes
less common

reactive intermediate6

initial C-H activation-hydride elimination
extrusion of neopentane
second C-H activation

2. Silylene Complexes
2.1 Overview of Silylene Complexes
In general, multiple bonds to silicon and other second-row main group elements are less stable thanmultiple bonds to carbon, oxygen and nitrogen.
ex) Si=O, M=Si: unstable

via metal-silylene intermediate

metal-silylene complex is stabilized bycoordination of a Lewis base.

2.2 Bonding of Silylene Complexes
Orbital interaction: same as carbene complex
Back donation from the metal d-orbital to silicon:
weaker than that of carbene complex
silicon remains Lewis acidic, and thesilylene complexes are often stabilized
by Lewis bases.

2.3 Examples of Isolated Silylene Complexes
Silylene complexes stabilized by Lewis bases
elimination of a salt

abstraction of triflate7Base-free silylene complexes
soft -donating thiolate substituenets on siliconstabilization of silicon center

Preparation of silylene complexes
Free silylene

Alkyl elimination

Photo extrusion of silylene

Hydrogen elimination

H2 elimination via Si-H activation

dissociation of CO,
silyl migration

dissociation of CO,
Si-Si bond cleavage

2.4 Reactivity of Silylene Complexes[2+2] addition with isocyanate

alkyne insertion

migration of Ph to silylene unit,
C-C cleavage of the nitrile,insertion of the cyano group into M-Si bondgeneration of an aryliridium silyleneintermediate83. Metal-Heteroatom Multiple Bonds
3.1 Scope of the Section
3.2 Overview
3.3 Bonding of Oxo and Imido Complexes
Formal charge
dianioniccf) carbene: neutral
6e donor
Qualitative molecular orbital diagramC4v symmetry: -bonding of the O or N with the M leads to splitting the degenerated eg and t 2g.
octahedral: dxz, dyz orbitals are -antibonding.
complexes possessing more than two d-electrons must contain these electrons in a d-orbitalthat overlaps with the filled -orbitals on the oxo, nitride, or linear imido complexes.

Bond angle between M, N and C

M NC

Analysis implies that linear imido compounds would have a larger degree of -donation thanbent imido compounds. (most of the complexes are linear)

3.4 Synthesis of Metal-Imido and Metal-Oxo Complexes
Metal-imido complex
From metal-oxo complexes

9-elimination

elimination of ligand

metathesis

alkylation of nitrogen

Electron-rich nitrido:
alkylation with electrophile

Electron-poor nitrido:
alkylation with nucleophiletransfer of imide group
unsaturated transition metalazide complex,
N2 elimination

Metal-oxo complex

parallel to imido complexes
oxidation of a lower valent metal
metathesis-eliminationoxidative addition... etc

103.5 Reaction of Imido and Oxo Compounds[2+2] and [3+2] cycloaddition[2+2](early metal oxo complex: only one example)

alkyne: thermodynamically favorable
alkene: reversible[2+2], reductive elimination

direct reaction[3+2]
computational study:
barrier [2+2] > [3+3]13

C istope effct:
similar magnitudes for both carbonsof the olefine11

Atom transfer of oxo and imide groups to olefins
Sharpless asymmetric dyhydroxylation

Manganese-salen catalyst

via metal oxo complex

Mechanism of epoxydation of olefin
non-radical concerted process

Reactions with C-H bonds[2+2] reactions with C-H -bondSee Chapter 6

direct interaction of imide group withC-H -bond

12

Reaction with electrophiles
protonation

organic electrophiles

polar C=X bond

Metallacycle was isolated orcharacterized by NMR.

Migration of alkyl and hydride groups from M to O or N
proton transfer

13aryl transfer

Catalytic reactions of imido and metal-oxo compounds through organometallic
intermediates
olefin metathesis/ alkene and alkyne hydroamination

3.6 Nitrido ligands
Bonding of nitrido ligands
trianionicone -bond, two -bondstrong -donor ligand

Structural and spectral features
X-ray crystallography, IR, NMRSynthesis of metal-nitrido complexes

14metathesis and atom transfer processes

Reactions of metal-nitrido complexes
The uncoordinated lone pair on nitrogen can act as a nucleophile, as a lewis base, or as abridge toform -nitrido complexes.
[4+1] cycloadditioncleavage of C-C double bond15