2011/5/6 Kimura

Organometallics Study Meeting #5
Chapter 1: Structure and Bonding
1.1 General Properties of the Ligands
1.1.1. Classification of Ligands

Covalent ligands

Dative ligands

Two systems for classification of ligands

1e- from ligand
1e- from metal

2e- from ligand(e.g. BF3-NH3)(1)

ligand(2)

neutral (dative) orcharged (covalent)

ligand = neutral

2e- donator: L-type ligands
1e- donator: X-type ligands
LX, L2, L2X... etc.

1.1.2. Classification by Number of Electrons Donated to the Metal
1.1.3. -Bonded Ligands
1.1.4. Combinations of - and -Donors
1.1.5. Cationic Ligands (nitrosyl)

1.2 Properties of the Ligands
oxidation state: far from the true charge on the metale.g. [Fe(CO)4]2- : Fe(- II), iron atom bears little if any negative charge

1.2.1. Oxidation State

1.2.2. The Relationship Between Oxidation State and the Number of d-electons
1.2.3 Trends in the Properties of Transition Metals
1.2.3.1 Trends in Ionization Potentials
Ionization Potential (figure 1.9)e.g.)Zr(II)

incrases from left to right(electronegativity, effective nuclear charge)

more easily oxidized
more basic
more electron-richthan Pd (II)For middle to late transition metals:<secondrow

basicity ofmetal complexes

firstrow<secondrow>firstrow>accessibility ofoxidation statehigher than +3

thirdrowe.g. Pt(IV) vs Ni(IV),Os(VIII) vs Fe (VIII)

thirdrow

Os(CO)4(H)2
is less acidic thanRu(CO)4(H)2
Chapter 3

1.2.3.2 Trends in Sizesize(figure 1.10)

firstrow<secondrowthirdrow~lanthanoid contraction

sizefor low-spin metal complexes 4~7 groups
with same charge and oxidation state

decreases from left to right1.2.3.3 Trends in Bond Strength

firstrow

Strength of M-L bond<secondrow<thirdrow

greater orbital overlap
smaller E of M and L orbitals

1.3 Metal-Ligand Complexes
1.3.1. Electron Counting
1.3.2. The 18-Electron Rule1.3.3. Metal-Metal Bonding and Electon Counting in Polynuclear Complexes
Number ofM-M bonds

1=2(18M N)

M: number of metals
N: total number of valence electronse.g.) figure 1.12
1.3.4. Geometries of Transition Metal Complexes

coordination number

electroniceffects= the number of atoms directly linked to the metal (IUPAC)= the number of electron pairs donated to the metal (conventionally)

often
>

stericeffects

figures of several geometries => figure 1.13

four-coordinate d 8 complexesof second and third row

square planar

howeverin the first row complexes.

tetrahedral
e.g. Ni(PPh3)2Cl2

extremely high d x2-y2
in square planar

cf. square planar
Ni(PPh3)2Me2
strong donation=> strong electronic
preference

five-coordinate d 8trigonal bipyramidal

five-coordinate d 6square-based pyramidal

1.3.5. Isoelectronic and Isolobal Analogies

isoelectronic:

e.g.

group of metal fragments that havesame structure and number of electrons[V(CO)6]Cr(CO)6[Mn(CO)6]+d 6 octaheral

isolobal: (by Hoffmann)group of molecules with similar frontier orbital(number, symmetry, approx. energy, and shape)e.g. (1)

Ni(CO)2

CH2

HCOCCO

NiH

CO

a1

a1

a1

b2

Fe(CO)4b2

FeCOCOb2OC(for CH2, E(b2) > E(a1))

e.g. (2)Cp- and three mutally cis carbonyl(Fig. 1.15)

isolobal analogy:Mn(CO)5, [CpMn(CO)2]- and CpFe(CO)2
are isolobale.g.) Me radical and Mn(CO)5
bonding between Me group and Mn(CO)5

Me radical dimerizes to form ethane
orbital interaction like C-C bond in ethane

1.3.6. Molecular Orbitals for Transition Metal Complexes
SALCs = symmetry-adapted linear combinations => Figure 1.16

Valence bond theory (Ref. JACS 1998, 120, 12650)

hybrid orbitals are constructed from s- and d-orbitals (p-orbitals: not used)

MLnN : the number of total electronsn: the number of ligands

N12; when there are n metal-ligand bonds => hybrid orbitals are generated from s- and (n-1) d-orbitalsN

12: some M-L bonds are considered as delocalized 3c-4e bonding

lone electron pairs of matal occupy purely d-orbitals

N

12
hybridization of each M-L bonds: sdn-1
remaining electrons occupy otherpure d orbitals to make metal lone pairs

N : the number of total electrons
n: the number of ligandse.g.) WMe6 (N = 12, n = 6)
sd5 (optimal 63 and 117 L-M-L angle)(Figure 1.17, Table 1-3)

N12
considered as hypervalent
give rise to delocalized 3c-4e- bonding(3 center-4-electrons)
every pair of electrons greater than 12
requires one 3c-4e- bonding interactionm = 1/2 *(N-12)
m x (3c-4e- bond) => sdm-1 hybridizatione.g.) [PtH6]2 (N = 18, n = 6)three 3c-4e- bonds, sd2 hybridization
remaining 6e- are in metal's d-orbital

1.3.7. -Bonding in Organotransition Metal Complexes
1.3.7.1. -Bonding in CO and Its Analogs

MO, -backdonation

1.3.7.2. -Bonding in Carbene and Carbyne Complexes

MM

XR'R

M
XR'RCRMCRM
CRXR'MMRRcarbene (Fischer type)one -orbital and one -orbital
L: -donor and -acceptor

carbyneone -orbital and two -orbital
two dM interact with two pL
carbyne ligand is often assigned as trianionic
L: -donor and two -donort

CO COOCCO COMeOCCrOC

CONMe2

Cr(0)

OCBu

PhCrt BuTaPhCOt BuCr(0)

tBu
Ta(V)

"Fischer" carbene
carbene ligands bearing at least onehetero atom orother substituents capable of -interaction"Schrock" carbene (alkylidenes)
carbene ligands containing alkyl groups or hydrogens
considered to be dianinoic (chapter 3)

ORR
Formal charge: 0
Electrons donated: 2

MFormal charge: -2
Electrons donated: 4

MRR

1.3.7.3. -Bonding in Olefin Complexes

electron rich: more -back donation

1.3.7.4. -Bonding with Other Unsaturated Ligands

alkyne,ketone, imine(softer metal capable of backbonding)

1.3.8. -Donor Ligands

MO of arene, metallocene-donation in some alkyne complexalkyneMalkyneM-BondingalkyneM-Backbonding-Donation

donation in alkoxide and amide (Chapter 4)alkoxideamideRMORM

M-O-C angle ~ 180
sp hybridization at OORMNR

planar at nitrogensp2 hybridization at N
other -donors:
halides oxide, nitride ligands

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 Monoxidesmall 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 )CO

2.2.2. 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 complexesFe(CO)5Os(CO)5

Os3(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 2.3Cr(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
CNR

stronger -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 angleHtrans coordination
PPh2PPh2

Ph2PMeFeFeHMeTransphos

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:
24NMe3 NH3PyPH3
15 17P(OMe)3NF3PF3PCl3

2238

48

16 18PPh3 PMe3

100CO

theoreticalscale