Chapter 2: Dative Ligands
2.1 Introduction

Chapters 2-4: presents illustrative summary of the types of complexes
Chapter 2: steric and electronic properties of neutral ligands(Chapters 3&4: formally anionic ligands)

2.2 Carbon Monoxide and Related Ligands
2.2.1. Properties of Free Carbon Monoxide
small dipole moment with negative end located on carbon
strong vibration in IR at 2143 cm-1
neutral ligand, commonly binds to metal C's lone pair elecrtrons
normally binds to one metal, but bridging coordination is possible(M-C-M angle is much less than 120 )CO2.2.2. Types of Metal Carbonyl Complexes
Preparation of metal-carbonyl complexes
from bulk metalfrom reduction of complexes in higher oxidation state

1atm CONi(CO)4Ni

Re2O7 + 17CO

Re2(CO)10 + 7CO2

WCl6 + CO + Et3Al

W(CO)6

polynuclear carbonyl complexes

Fe(CO)5Os(CO)5Os3(CO)12

homoleptic carbonyl complexes
in 2nd and 3rd row

stablemuch less stable

more stable

polynuclear structure
is more favorable

Classification of metal carbonyl complexes: Figure 2.2

2.2.3. Models for CO Binding: Introduction of Backbonding
CO binds strongly to electron-rich, low valent metals (backbonding, soft metal and soft ligand)2.2.4. Evidnce for Backbonding in Terminal Carbonyl
2.2.5. Infrared and X-ray Diffraction Data for Complexes with Bridiging Carbonyls

IR, C-O length2.2.6. Thermodynamics of the M-CO Bonds

dissociation of CO ligands: key step in many reactions

Table 2.3
Cr(CO)6 < Mo(CO)6 < W(CO)6Ni(CO)4 < Cr(CO)6Ir(Pi Pr3)2Cl(CO) (particulary strong)higher enegy of orbitals, backdonation
third row, electron-rich alkylphosphine

2.2.7. Isoelectronic Analogs of CO: Isocyanides and Thiocarbonyls
CNRstronger -donor, weaker -acceptor than CO
weaker C-X -bond than CO

isocyanide

2.3 Dative Phosphorous Ligands and Heavier Congeners
2.3.1 Tertiary Phosphines and Related Ligands

2.3.2 Chelating Phosphinesbidentate, asymmetric bisphosphine, P-M-P angle
Htrans coordination
PPh2

PPh2Ph2P

MeFeFeH
MeTransphos

PPh2

Ph-Trap

2.3.3 Properties of Free Phosphines

PR3vsbinding: to transition metals more strongly (soft-soft)steric effect: less pronounced (larger atom, longer M-P)high inversion barrier => P-chiral phosphine

NR3

Ar3P, (RO)3P less sensitive to air (less electron rich)
2.3.4 Properties of Phosphine Complexes
2.3.4.1 Bonding and Electronic Properties

Electron-donating ability:

R3P > Ar3P > (RO)3P

greater s-character of sp2-hybridized orbital of aryl=>weaker electron donor than alkyl
electron-donation: alkyl > alkoxy-acceptor orbital: hybrid of P-X *-orbital and phosphorus d-orbital (Figure 2.9)

-acceptor ability:
24

NMe3 NH3

PyPH315 17P(OMe)3

NF3PF3PCl3223848

16 18
PPh3 PMe3100

COtheoreticalscaleelectron donating ability ( CO in [Ni(CO)3L])
LPtBu3
PCy3
PMe3
P(C6H4-4-OMe)32.3.4.2 Steric Properties-1
CO (cm )L-1
CO (cm )

2056
2056
2064
2066

PPh3
P(OMe)3P(OPh)3
PF3

2069
2079
2085
2110

cone angle, solid angle (figure 2.11)

2.3.4.3 Effect of Phosphine Steric and Electronic Properties on Structure and Reactivity
ligand dissociation in NiL4(and related Pd complexes)

PMe3 < PMe2Ph < PMePh2 < PEt3 < PPh3 < PiPr3 < PCy3 < PPhtBu2

bulky phosphines bind trans to one anotherdeviation from the ideal coordination geomety because of bulky phosphinese.g.) Wilkinson complex RhCl(PPh3)3=> nonplanar (normally, square planar for d8 Rh(I))

2.3.5 Pathways for the Decomposition of Phosphorus Ligands
oxidation: more electron rich phosphine
P-C bond cleavage
Ar3P
Pd
X

Ar3P

PAr3
Ar3P

Pd

X

PAr2Ar'Pd

Ar'PAr3

ArX

Ar'

P-X clevage (X =OR, NR2) by water or alcohol

2.3.6 NMR Spectroscopic Properties of Phosphines
ClMe3P
31

P: 1/2 spin, 100% abundant
relative receptivity: 0.0665 (H = 1, C = 0.000175)

Me3P

RuCl

PMe3
PMe3

trans: - 6.63 (s)2.3.7 Heavier Congeners of Phosphorus Ligands
M-L bond strength
Steric effect of substituents
P-C/As-C cleavage

P > As > Sb > Bi
P > As > Sb (M-L bond length)
P > As

Me3P
Me3P

PMe3
ClRu
Cl
PMe3
cis: 9.0 (t) -12.7 (t)J = 35 Hz

2011/5/12 Kimura

Organometallics Study Meeting #6
2.4 Carbenes
2.4.1. Classes of Free and Coordinated Carbenes
2.4.1.1 Properties of Free Carbenes
more stable insinglet state
triplet statecarbenes with electron negative substituentswith only hydrogen or alkyl groups

RNN

R
nitrogen acts as strong -donor to unoccupied p-orbital
NHC

2.4.1.2 Properties of Carbene Complexes
Schrock Carbene

Fischer Carbenee.g.)

e.g.)
OMe

t-Bu

t-Bu
t-Bu(CO)5 CrTa

Met-Buelectrophilic at carbene carbon
carbene with electronegative substituentsgroups 6-8 (late) metal in a low oxdation statebearing -accepting ligands such as CO
M-C bond: not strongly polarized
considered as neutral ligands( -donation and -backdonation like CO)

nucleophilic at carbene carbon
carbene with hydrogen or alkyl groups
groups 4-6 (early) metal, high oxidation state
more polar M-C bondconsidered as dianionic ligands

Hybrid of Fischer and Schrock Carbene

Vinylidene complexe.g.)

e.g.)

Ru

CH2

R3P
R3P

H

Ru

Ph2P

MeMe

PPh2
HCCHPhlate metal, low oxidation statecarbane ligand lacking heteroatom substituent
with strongly donating ligand inatead of CO

one methylene substituent at carbene carbon
free vinyldene: electrophilic (like dihalocarbene): prone to rearrange to alkyne

unsaturated hydrocarbyl groupsuch as phenyl or vinyl
considered as neutral ligand

tautomers of alkyne and vinylidene (eq. 2.10a,b)(relative stability depends on
the number and identity of ligand)electrophilic at carbene carbon (C )nucleophilic at metal and C

2.4.2. Bonding of Carbenes

siglet carbene

triplet carbene
RRRR

closely related with CO
donate two electrons to metal through a dative bondaccept d-electrons in -backbonding
often have weak p-bonds (large E bet. d M and p )low rotation barrier (8~10 kcal/mol)

NHCRN

considered as dianionic
donation of two electron pairs to metal ()2.4.3. Spectroscopic Characteristics of Carbene Complexes
NR
13C

strong -donor, weak -acceptor: carbon is soft and less electronegative thanmost heteroatom Lewis base
: p-orbital of carbene carbon participates instrong -bonding with amino subsituentslike phosphine, but stronger -donor
two fold symmetry ("fences rather than cones")FischerSchrock1

H

X=O
X=N

290 ~ 365 ppm
185 ~ 280 ppm

240 ~ 330 ppm

10 ~ 20 ppm

2.5 Transition Metal Carbyne Complexes

MCGeneral Preparation Method(1) via metathesis of M-M or M-L triple bond with alkyne(2) transformation of existing ligand (carbene) into carbyne(3) modification of existing carbyne to exchange substituents at carbyne CRcarbyne
Reactivity[2+2] cycloaddition (Chapters 13 and 20)
attack by nucleophiles (Chpter 11)
by electrophiles (Chapter 12)

2.5.1.Bonding and Structure of Carbyne Comlexes
mostly, group 5~7 metal complexes
carbene is typically considered trianionic(cf. monocationic like linear nitrosyl)
bonding interaction: carbene + additional -bondMCcarbyne

RCRCR

in the case of heteroatom-substituted carbyne
HOMO: metal
LUMO: one of the *-orbitals of M-C bond=> nuclephiles attack carbyne carbon

CR

R = NR2 ( -donor group): decrease -acidity
R = R3Si (electropositive group): increase -acidity

MCR

R = alkyl ( -donor group): increase the enegies of all the orbitals=> more polarized M-C bond, C becmes more nucleophlic2.5.2.Spectroscopic Characteristics of Carbyne Comlexes13

C: 200 ~ 350 ppm, triple bond vibration: 1250 ~ 1400 cm-1

2.6 Organic Ligands Bound Through More than One Atom
2.6.1 Olefin Comlexes*C-C

Chatt-Dewar-DuncansonmodelC-C

2.6.1.1 Stability of Metal-Olefin Comlexes
electron-rich metal => olefin bering EWG (e.g. Ni(0))
M in higer oxdation state (with charges greater than +1 and d 0 ):olefin complexes are less common (cf. olefin polymerization)
also sensitive to steric effects (binding: ethylene > -olefins)2.6.1.2 Structures of Metal-Olefin Comlexes
2.6.1.2.1 Structural Changes Upon Binding

Cr(CO5)* backbonding: sp2 => sp3++Cr(CO)5trans-cycloctene

cis-cycloctene

2.6.1.2.2 Structural Changes Upon Binding

electronic preference for orientaion about M-olefin axis-donor + -acceptorLLLM

L

orientation of olefin:
controlled by metal HOMO

Ld 10
trigonal planarLLMLMLL

d8
trigonal bipyramidal

d6
trigonal bipyramidal

2.6.1.3. Spectral Properties of Metal-Olefin Comlexes

electron-poor metal complex: close to free olefin
electron-rich metal complex: upfield shiftNMR

2.6.2 Alkyne ComlexesM-Bonding-Donation-Backbonding

2.6.2.1 Structural Characteristics of Alkyne ComplexesR

if -backdonationis strong enough

MRM

M

MRR
distorted to thegeometry of cis-olefin"metallacyclopropene"strained alkynes stabilized upon
coordinationto the metal

2.6.2.2 Physical and Chemical Properties of Alkyne Complexes

2.6.3 Complexes of Organic Carbonyl Compounds
electron-rich metal fragments(low-valent late metal)

e.g.)
O

Ph3PO

Cp2Mo
CH2

bind to aldehyde/ketone
in 2-fassion

2.6.4

6-NiPh3P

C

CF3
CF3

Arene and Related Complexes
arene is stronger electron donor than three CO ligands
electron-releasing substituents on aromatic ring invariably stabilize 6-arene ligand
steric interactions counteract this electronic effectcoordination with TM deplets arene of much of its -electron density

RMLn

NH2

CO2H

Cr
OC

COCO

less basic than aniline

CrOC

COCO

more acidic than benzoic acid4Ru-arene complexes
distortion from planarity, large loss of aromaticity
prepared by reduction of ( 6-arene) dication

2.7 Complexes of Ligands Bound Through N, O and S
2.7.1 Neutral Nitrogen Donor Ligands
2.7.1.1 Amine Complexes
ammonia, amines: classic ligand in coordination chemistry
these Lewis bases are less commonly used
NH proton of coordinated amine tend to be reactive
tertiary amines bind weakly (hard-soft mismatch)tertiary amins are morer sterically congested than tertially phosphine(shorter C-N bond, larger C-N-C angle)

RHNRuNHH
R2

HO1R* examples: figure 2.39

2.7.1.2 Pyridine and Imine Complexes

they lack reactive N-H bondmonoimines tend to be reactive species

nitrogens in pyridine, imine, oxazoline are softer
heteroarenes can act as -acceptors

2.7.1.3. Dinitrogen Complexes
N2 is isoelctronic with CO, binds most often as anless basic than CO, less -acidic than CON2N-ligand

accepting ability of N2 is stronger than -donating propety
N2 is generally more electron-accepting than it is electron-donating
most dinitrogen complexes contain electron-rich metal center
mostly, mononuclear complex, weak interaction

M N2

Cp* Zr1

Zr Cp*NN2
Cp* ZrN2

NN

Zr Cp*

dinuclear complex
N-N bond is much longermuch more activated N2 in side on complex

N2

2.7.1.4. Complexes of Neutral Oxygen Donors

H2O, MeOH, THF, DME, acetone DMSO, Ar3PO etc.
NtBu
Cp2 Zreven dessociate readily from more-oxophilic high-valent transition metals
R3PO tend to bind more strongly than R2O
despite of weak bindings, these can influence reaction chemistry(without forming stable complexes)O
THF, DME, phosphine oxides are common (Figure 2.44)
dissociate readily => temporaly masking of reactive intermediatebindentate ligand with mixture of P and OR (figure 2.45)

2.7.1.5. Complexes of Neutral Sulfur Donors

SMePd X
SMeO R3PSO

neutral sulfur donors: softer and more polarizable than neutral oxygen donors
thioethers and sulfoxides are most common
trans influence of neutral sulfur donor is greater than neutral oxygen donor
compatible with amine
M-SR2 is stronger than M-NR3 bond, but weaker than M-PR3 bondspectrochemical series: PR3 > SR2 > Cl-DMSO
to low-valent late metals: through electron pair on sulfur
to harder high-valent metals: through electron pairs on oxygenRuHPR3DMSO can induce isomerization of square-planar complexes tononrigid five-coordinate intermediate2.8 Sigma Complexes
2.8.1. Overview of Sigma Complexes

dihydrogen, alkanes, silane, boranes (X-H bond)*LnM

electron donation from X-H -bonding orbital
backdonation from metal to X-H *-orbital
like olefin's Chatt-Dewar-Duncanson model( donation from -bonding and backbonding to *-orbital)
bonding intercation is much weaker than olefin-bonding enegy is in lower (less basic)-bonding is higher in enegy (less -acidic)

LnME between andis too large?
-> complexation => longer X-H bond => less EHLnM

H

2.8.2 Dihydrogen Complexes
2.8.2.1 Properties that Lead to Stable H2 Complexes

Ph3P
H

PPh3

HRuH

PPh3

H2-complexes: arrested oxidative addition of dihidrogen
metal backbonds to H2 *, but cleaves H-H only partiallymost metal: in low oxidation state (backbonding)many of these low-valent complexes are cationic(neutral, low-valent metal is too much electron-rich)neutral H2 complexes have -accepting carbonyl groupH[Mo(H2)(CO)(dppe)2] : dihydrogen complex

second-row metals: dihydrogen complex[WH2(CO)(dppe)2] : dihydride complex

third-row metals: dihydride complex

2.8.2.2 Spectroscopic Signatures of H2 Complexes
2.8.2.3 Reactivity of H2 Complexes
dissociation of dihydrogen
addition of dihydrogen to form dihydride
deprotonation of hydrogen by external base* reactivity

Dissociation of H2
PCy3
HOC
W
OC+ P(OMe)3

H
CO

PCy3

PCy3
P(OMe)3

OC
W- H2OCCO

PCy3

H2 is replaced by CO, PR3, nitriles
not replaced by H2O nor ethers
binding entropic term is surprizingly large
rate of substitution reaction => 0 order in incoming ligand
rate of displacement = rate of dissociation of H2

Addition of H2 to form dihydride

PR3
COHOC WHOCOC
OC

PR3

PR3
COHW
H

oxidative addition to cleave H-H bondin many cases. equibrium between H2 and dyhydride
barrier: structural change to accomodate two hydrides
in some cases: dihydrogen as kinetic product at low temp.

PR3

2.8.3 Alkane and Silane Comlexes

HH

property and reactivity: closely related to those of dihydrogen complexes
silane complexes: similar stability to dihydrogen-comlex (or slightly unstable)alkane complexes: not so much study (too unstable in solution)

MMR

Si

2.8.3.1 Stability Relative to H2 Complexes
Hsilane complexes: similar stabiliy to dihydrogen complexes (despite greater steric effect)
Si-H bond is more basic than H-H or R-H bond
Si-H bond is much longer weaker, has lower energy *-orbital

M
SiH

alkane complexses: less stable than H2 or Si-H complexes
steric effect is not compensated by smaller E between

Mand *-orbitals

R
2.8.3.2 Evidence for Alkane Complexes
2.8.3.3 Intramolecular Coordination of Aliphatic C-H Bonds (Agostic Interactions)

M

HH
M

HHH

H

H
H

H

M

LM
H

H-agostic-agostic-agostic

agostic interaction
with a dative ligand

intermediate to-hydrogen elimination(Chapte 10)

electrophilicmetalcarbene in highoxidation state

unsaturated metal(dpme)Cl3

Ti
H

86

severe distortion
from agosticinteraction

agostic interaction: dynamic
C-H in methyl: single peak in NMR