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Origins of Organometallics - Structure & Bonding
the 18 electron rule and common exceptions to the rule




18 electron rule was formulated in an era when organometallic compounds were mostly found

1
...
It is followed most

ully by 2nd and 16/14 electron rules
...
The 18 electron rule provides the

W(CO) (PMe ) , (5-C H ) TiCl , Mo(CO) {CPh(OEt)}, (5-C H ) Co, Ti(CH Ph)4 [RhCl2(CO)2] , Hg(CO)22+, Cr(NO)4, Cr(6C8H8)(CO)3, Ce( 8-C8H8)2, Fe( 4-C8H8)(CO)3 Re2(CO)10, Ir4(CO)12, [(5-C5H5)Fe(CO)]4

4
3 2
5 5 2
5
5 5 2
2
etic or mechanistic organometallic chemist with2 considerable predictive power with regard to

ble ligand combinations around a metal centre, and this is its main use
...

It was devised for mid-low oxidation state metals with π-acceptor ligands, such as CO, for 2 & 3 row compounds
...


s of the 18- and 16-electron rules
...
If all of the metal-ligand

ng and low-energy ~non-bonding (d) orbitals are filled, and the antibonding orbitals are

cupied, then aWhat is the 18-electron rule?
kinetically stable complex will result
...


then a kinetically stable complex will result
...


This gives (9-x) non-bonding/weakly antibonding MOs, of mostly
d-character on the metal
...


How do we determine the valence electron count of a metal
complex?
1
...

2
...

3
...

gure
...
Add electrons for any metal-metal bonds, providing there are sufficient d electrons
...

Transition metals have 9 valence MOs (5 x d, 1 x s, 3 x p)

In any -onlyComplications
complex MLx there will be x bonding and x antibonding MOs
...

This leaves 9–x non-bonding (or weakly antibonding) MOs of mostly d character on the

metal
...

Square planar (D4h ML4) complexes
requires 18 electrons
• Early transition metals
ver, this simplified •
model does not explain why not all transition metal compounds follow the
Sterically encumbered compounds
only 2nd and 3rd row and/or those with low OS and -acceptor ligands),
ectron rule (in general



13








Organometallics - Structure & Bonding





or why there is an equivalent 16 electron rule
...





For an ML6 complex to obey the 18-electron
6
rule, the t2g level (d ) is filled and the 2eg (σ*)
level is left empty
...




In the diagram on the left, the ligand is shown
as a π acceptor ligand (eg CO)
...




The first two of these classes deviate from the
standard 18-electron rule
...
Figure
...

6
Class I
t2g level (d ) and leaving the 2eg ( *) level empty
...


The Figure shows L as a

acceptor ligand
...

whichtthe eg and t2g have
2g non-bonding – un/occupied

0
ding
...

2
...
d0
ding
...

4
...

6
...







and including

14

Class II

nd 5d metals
and including


ligands (e
...

and 5d metals
r complex(e
...

ligands the
onding (thus
l complex the

sentially (thus
onding nonsentially CN)
nding for non-
ed
...
Hence d0

ample  of this
ore possible,

WMe6
...


eg weakly antibonding - un/occupied
3d metals
weak field ligands
12-22 valence electrons
0
10
d up to d are possible

1
...

3
...

5
...


t2g non-bonding – un/occupied
eg antibonding - unoccupied
4d/5d metals
Strong σ-donor ligands
18 valence electrons
d0 to d6 allowed, past d6 not allowed

re possible,

ample  of this

Organometallics - Structure & Bonding

Me6
...
One


notable exception to the 18 electron rule are electron-rich complexes of the later transition metals


lectron with d8 configurations and either strong
rule
...
g
...
The reason in the increased stabilisation of the d shell across the series so
2
that,
bonding, and stabilisation count
upied, while for example, the dz orbital no longer participates so well in 1
...
eg strongly antibonding (σ*
These and steric and/or
2
...
g
...
18 valence electrons
over d ML5 complexes (e
...
Fe(CO)5 and Ru(CO)3(PPh3)2) for these later metals
...
Unless steric/electrostatic reasons,
eric and/or

A schematic MO diagram for D4h is shown below
...

4pz AO is non-bonding by
e attainment
symmetry (the M–L bonds lie in the nodal plane) so that there are only eight M–L bonding and
majority of
metal-based non-bonding MOs to fill (16 ): The 16-Electron Rule Exceptions
Square Planar Complexes (ML4 VE required)
...
strong σ-donor ligands

2
...


Increased stabilisation of the d-shell across the
series

2
dz orbital no longer participates well in
bonding

π back-donation stabilization (eg via CO)
becomes less efficient
...


The 4pz AO is non-bonding, due to symmetry:
the M-L bonds lie in the nodal plane

This leaves 8 M-L bonding/metal-based nonbonding MOs to fill

This only requires 16 valence electrons


Figure
...
Note that the pz AO is non-bonding leaving
only 8 M–L bonding and metal-based non-bonding MOs to fill (16 VE required)

16

M

Cl

Ti

Me
Ar

N

py

Ar

Cl

py

Organometallics - Structure & Bonding
16 VE
M = Ti, Zr, Hf

py

Zr
N



16 VE



18 VE

Early Transition Metal Compound Exceptions
Low numbers of metal valence electrons

OC
CO CO
Exceptions to the 18 electron rule: early transition metal and/or sterically encumbered
CO

CO
OC
compounds
...
g
...
g
...

OC OC
CO
Examples are the group 4 metallocenes :
17 VE
(a) Early transition metal compounds have low number VEmetal valence electrons and so it is
of dimer (7-coord V)
18

(isolable)
inherently difficult to achieve a VE count of 18 without unacceptable steric conflicts
...
V(CO)6 has
CO
electron
...
The hypothetical
CO
CO OC Mn
bond
...
But in this case a 6-coordinate
OC
CO

Mn
CO
OC
dimer Mn2(CO)10 is This Cp*Ti(NR)Cl(py) (Cp* = 5C5Me Mn = pyridine, recently, but I mention it
py
OC
CO
bond strength)
...
point of view has been5;challengedNC5H5) is isoelectronic
Mn(CO)6 has 17 valence electrons, so 1 unpaired
CO
CO
with Cp2TiMe2 and exists as a 16 VE compound but OC zirconium congener formselectron
...
You can see it is
see below)
...

18 VE dimer (6-coord Mn)
quite a balance! (not isolable)


M

Me
Ar

N

17

16 VE
M = Ti, Zr, Hf





Cl

Ti

Me

py

OC

py

Cl

18 VE

OC
OC

CO

For Cp2MMe2, where the M= Ti, Zr, Hf, the compound has

16 valence electrons
...


Cp*Ti(NR)Cl(py) is isoelectronic with Cp2TiMe2 and exists
as a 16 valence electron compound, but the zirconium
congener forms the 18 valence electron bis(pyridine)
species Cp*Zr(NR)Cl(py)2, as the metal can accommodate
another py as Zr is larger than Ti
...
V Describe concisely the (σ and π) bonding in metal-carbonyl and metal-alkene compounds
...


OC OC

CO

increases the CO bond strength)
...
CO itself is a π-acid
...
The bonding in a metal carbonyl complex
CO coordinates exclusively as a C-donor ligand
...
The(not isolable)donates electrons18to the (6-coord Mn)
ligand
HOMO),
The HOMO of CO is considered to be slightly antibonding in
nature – LHS is MO diagram for CO, showing the frontier
tal's charge and its propensity to back donate electron density (into the *
molecular orbitals relative to a typical d atomic orbital
...
The one interaction reinforces the other
...
The frontier as a orbitals of carbon monoxide relative to a typical nd AO energy
molecular
r orbitals it is known
acid ligand
...

(a) General features of the metal-carbonyl bonding
...
The ligand donates electrons to the metal (from the 5 HOMO),
The ligand donates electrons to the metal from the 5σ HOMO, and:
increasing the metal's charge and its propensity to back donate electron density (into the *

LUMOs) of the ligand
...
As CO is accepting electron density
1
...
Pictorially the situation be can be
2
...


These two interactions reinforce each other
...



Figure
...
e
...

The dominance of the M-CO bonding interactions also explains why CO complexes of main
Organometallics - Structure & Bonding




group metals are virtually non-existent
...

their bonding but their interaction with the CO 2 (LUMO)
Metal-CO π back bonding stabilizes the metal
dπ orbitals – eg a t2g orbital
...

This is why π acid (aka π acceptor) ligands like
CO are able to stabilize metals in their low
oxidation states
...

Larger lobes are based on the carbon “end”, so
there is better π overlap with the metal dπ
orbitals
...

compared with the M-CO π component
...
 
Since the 5σ HOMO of CO is slightly C-O σ*
The presence of terminal or -coordinated CO can be inferred from the (CO) values (the more
antibonding, this donation strengthens the C-O
bond
...


In the vast majority of cases coordinated CO has (CO) less than free CO due to back-donation of
The dominance of the π bonding explains why
electron occupancy into the CO * orbitals
...
Note that (CO) for
(c) Infra-red spectroscopy as a probe of M–CO bonding and metal centre electron-richness
...
The decrease in (CO) down each isoelectronic group reflects better d →CO  
d-electron numbers (as these have electrons to
As   mentioned,   IR   spectroscopy   is   a   useful   “  spectroscopic  handle”  for  metal  carbonyl  complexes
...
Thusbe inferred frombecome both less contracted
populate the dπ-CO bonding MOs, like t2g in an
The presence as Zeff for the -coordinated CO can the 3d orbitals the (CO) values (the more
(sometals CO is coordinated CO 2 more d →CO   higher in M →CO   back-donation energy match
overlap better with the to the LUMOs) and * back-donation)
...


weakens C–O due to
In the vast majority oflittle effect on C–OCO has (CO) less than free CO bond back-donation of
cases coordinated bond
The dominance of the M-CO π bonding
strengthens M–C bond
strengthens M–C bond
electron occupancy into the CO -1* orbitals
...
Note that (CO) for
n+/d6 compound
/cm
interactions also explains why CO complexes of
CO
CO(g) = 2143 cm-1
...
Only the np atomic orbitals of these metals can be π-like in their bonding, but
+ orbital interaction diagram for the - and
Figure
...
Thus the 3d orbitals become both less contracted
M
their interaction with the CO 2π (LUMO) acceptor orbitals is very poor
...
Factors influencing
d 6 [M(CO)6]n +/The presence of a terminal or µ-coordinated Co can be inferred
6
-1
n+/d compound
/cm
CO
from (CO) values
...

OC M
[Cr(CO)6]
2000
CO
[Co(CO)4]– –
1890
Coordinated CO has a lower (CO) value than free CO, due to the
CO
[V(CO)6]
1860
CO
OC6
back-donation of electron occupancy into the CO π* orbitals
...

10
n +/d [M(CO)4 ]
n+/(CO) decreases across the period, as atomic orbital energies
d10 compound
/cm-1
The (CO) values also tell us about the donor/acceptor properties of other ligands present
...

CO
[Ni(CO)4]
2060
series of fac-Mo(CO)3(PR3)3 compounds below the donor properties increase from PF3 (which is
(CO) decreases down each isoelectronic group
...

1890
almost like CO itself)
3
stronger dπ→CO π* back-donation as the Zeff for the metal
CO 3
2–
OC
1790
Thus the Mo[Fe(CO)4]
in the Mo(PR3)3 fragment becomes more electron-rich and a better -donor to the 3 x
increases down the group
...


1
...
In the

2
...

almost like CO itself) to PPh3 while the 2090
in the (CO) values also tell us about the donor/acceptor properties
OC
of other ligands present
...
3 = P(OMe)3
OC
PR
1888, 1977
PR3
- σ donor properties increase from PF3 (which is almost like CO
CO
PR3 = PPh3
1835,/cm-1
1934
fac– Mo(CO)3(PR3)3
itself) to PPh3
PR3
- π acceptor properties of PR3 decrease in the same order from PF3
PR3 = PF3
2055, 2090
to PPh3
...
The 5σ HOMO of CO is traditionally thought to be antibonding, as judged by (CO) for CO
OC
PR3 = P(OMe)3
1888, 1977
PR3
and CO*
...

PR3 = PPh3

1835, 1934
42

CO

abstraction
and for OC
[Hg(CO)2]2+ Me asymof an alkyl ligand can be effective, in
...
The product: to form a neutral6F-propene product:
isopropyl ligand a cationic compound to form a neutral a cationic compound
isopropyl ligand of -propene electron-deficient C 2 5 ligands
2+
Isopropyl
Isopropene
H3B←COin terms of their electronic properties
...
for many years attributed to partial(cf
...
The alkeneThe explanation of an also formed was -H elimination from alkyls removal of electron
(CO) via
Exceptions where coordinated CO has - HCPh
(CO) greater than that of free CO
- HCPh3
3
CPh3
H
-1
Fe -1 CH H
density from theCompound
weakly antibonding are 2level
...

route structureproducts and so Fe
other calculations OC may
OC
/cm
CompoundOC Fe
/cm
CH
CH
hydride
8 hydride
The d square planar Pd(C
OC
OC
abstraction
have suggested that  the  degree  of  M←CO   bonding playsabstraction Me OC raising (CO)
...

CO (g)
2143 Me
Pd(C6F5)2(CO)2
2186
Me
Me
that there is negligible back-donation, as the electron-deficient
Isopropyl
Isopropyl
the increased (CO) can be explained in terms of a positive charge close to CO stabilising the C–O
Isopropene
Isopropene
+
2+
CO (g)
2184
[Pd(CO)4]
2248
C6F5 ligands are halide-like, in terms of their electrical properties
...
elimination from alkyls (cf
...
The alkeneAlkene compounds are
via -H elimination from alkyls (cf
...

2164
2278
hydride
are via intermediates en route way are often intermediates
hydride donation in this -type fashion from route to other products
alkene acts as H3B←CO compounds metal centre often compounds formeda to other products and so mayenthe filled C=C and so may
a 2e ligand to a formed this way [Hg(CO)2]

not be directly observed
...

The increased (CO) can be explained in terms of a positive
onding MO to a vacant increased (CO) was for many yearsAt the same time, to a of electronor lesser
acceptor AO on the metal
...
However,a recent electronicfrom via filledcalculations
orbital occurs ligand to metal form
...
This particular
bonding MO to a vacant acceptorbonding MO to ano the same time, toraising metal
...
vacant acceptor AO greater (CO)
...
This particular
model
...
This occurs
the increased (CO) can be explained in terms of a positive charge close to CO stabilising the C–O

bonding scheme is known representedscheme is known as It Dewar-Chatt-Duncanson
bonding thus:
ms of all you have seen so far and is as the Dewar-Chatt-Duncanson model
...
It looks very familiar to you in
triply bonded resonance form A below: far and is represented thus: seen so far and is represented thus:
Metal-carbonyl bonding explanations have
terms of all you have seen so
terms of all you
Additional reading: Photoelectron (PE) spectroscopy as a probe of M–CO bonding
...
Atomic tungsten has a much lower ionisation energy
σ-type donation from the filled C=C π-bonding MO
than the HOMO of CO (7
...
01 eV) so we expect the t2g
to a vacant σ acceptor atomic orbital on the metal
...
The band at about 8
...
This is called the Dewar-Chattis predominantly W 5d
...
g
...
g
...

potential for two extremes of character of the the MOs
...

potential for on extremes of The depending overlapping
atom Additional reading: Photoelectron (PE) spectroscopy as 2gextent of bondingseries of on the extent of
and reflects the significant 5d orbital bonding depending two1t a probe of M–CO bonding
...


These are a formally neutral alkene (below are a formally neutral alkene (below left) and a dianionic representation (right) where the
These left) and a dianionic representation (right) where the

bands from about 13 eV to beyond 18 eV are assigned to all of the orbitals that are ionisation energy
The PE spectrum of W(CO)6 is shown above
...

(L)M←alkene  “adduct”  is  really  best  viewed  as  a  “metallacyclopropane”  structure
...
g
...

s thethan the with many ligands as compared to the HOMO of CO at 14
...
98
RR R
R R
R
R R
orbitals of extremes be much less stable than theon g,the1gextent1uof (L)M→alkene back-donation
...
a dianionic representation,
1
...
The band at about 8
...
The slight splitting is due to spin-orbit coupling caused by the heavy W
R
R
R R
RR R
R
M←alkene  “adduct”  is  really  best  viewed  as  a  “metallacyclopropane”  structure
...
The series of
2
2
Metallacyclopropane
Metallacyclopropane
AO are especially important with -alkene to setting the formal-alkene of the alkene
...
describe the bonding As usual we can describe the bonding situation using aThis is orbital interaction diagram
...
fragment
In these complexes, the C2H4 C=C π-bonding (σ donor) MO and the (L)M σ acceptor atomic orbital are not especially
the discussiondepicted below
...
C=Cbelow
...
acceptor
sp2 C
sp3 C
60
60
(L)M
The energies of the C=C π* (acceptor) MO and
(L)M
formally
formally
the d π (donor) AO are critical
...
This is
The extent of back bonding depends on:
cted below
...
And the other ligands and their electron
donating/withdrawing ability)
– The electron-acceptor strength of the alkene,
so the donating/releasing strengths of the
substituents R in C2R4
...

– Steric effects
2

-alkene
2e donor, neutral ligand

43

Metallacyclopropane
2e donor, 2- ligand

Figure
...

In general terms, back-bonding into the * MO of an alkyne has these effects:
A lengthening of the C=C bond
...
Also useful are involved in the bonding)
...

13
13

1 Organometallics - Structure & Bonding
2
1 C
For example, JCC in free CC 2Hsensitive to Hz degree of 2s character of39 Hz in Os(CO)4( bonding)
...
6 the but this reduces to orbitals involved in the -C2H4
alkene carbons ( J is 4

above) is the
C– C coupling constant which in gives information ) the hybridisation
r example, JCC in free C2H4and 67
...
C=C stretching
C–C bond (see
Factors influencing lengthened H 67
...
C=C
4
quencies are It is difficult to assign this region of the spectrum
...

a significantly reduced alkene and lengthened C–C bond (see structure below)
...

1
...

frequencies are also used but it can be hard to assign that region of the spectrum
...

Cl CH2
CH2

1
...
337 A
H
Cl CH2
1
...
354 AP
1
...
43 A
1
...
337 A
Cl
1
...
46 A Fe
1
...
46 A
1
...
54 A H
CH2
CH
OC 2
OC
H
OC 1
...
46 A
1
...
54 A
CH2
OC

D
OC
C
CH2
H
D
C
H
– Better acceptor ligands are used on the alkene, the distance will increase
...
44 A F
D
C C F
F C

F 1
...
44 A FC H
Pt
C
H C
C C F1
...
36 A
H H
H HH
PtE
2
...
The J Figure
...
C–C distances for
1
C
C
E
the J value will be reduced for binded alkenes as the C-C bond is lengthened
...
36 A
C2H4 and C2H6 are given for comparison
...
Comparison of C=C bond lengths for alkenesH
bondingH a metal centre
...
In
5 3
...
η C2H4 and C2H6 are given for comparison
...
moiety is not
5 5 2

5d

very and certain alkene complexes
the C=C
bonding interactions in your diagram
...
the contrast,above
...
Comparison of C=C bond lengths Marealkenes bonding toFigure centre
...

About Cp
ligands (i
...
PPh3)
...
In contrast, the C=C distance in
Cp complexes – or metallocenes – are known for the first row metals, V through to Ni, spanning counts of 15-20 valence
Co The C=C distances4 for C H and certainbond is longer for M = Osare than for M = Fe (C) as expected above
...
The C=C rd
(D)
nd
2
2 4
electrons
...
Of the 2 and 3 alkene complexesPt(0) summarised in the Figure
PPh3)2( -C2H4) (B) is significantly lengthened as C2H row metals, only 18 valence electron complexes are stable at room
with good donor
4 is now bound to
(5d vs 3d)
...

Zeise’s  compound  A the C=C distance is little changed from free C2H4 as the PtCl3 moiety is not
2
ands (i
...
PPh3)
...

The Cp2M compounds are all monomeric in the solid state, except Cp more more extensive
19 VE
--Cp---Mn units
...
(Pt(2+), late metal, electronegative co-ligands)
...
M
...
of changing C2H4 for now bound to Pt(0) with good donor
aps with metal s, p or d atomic orbitals
...
e
...
(C, D) suggest more extensive
the C=C distance 3)
...

complex
...
The C=C bond is longer for M = Os (D) than 5for M = Fe (C) as expected
The molecular orbitals on C5H can be drawn (see left)
...
The tris(alkene) compound E shows 62 effect of changing C2H4 for the better acceptor
The lowest energy orbital, a1, does not have any favorable overlap with
2 nodal surfaces
ligand C2F4, the C=C distance in the latter showing that there is more back-bonding
...


It has little interaction with the dz2 because the ligand p-orbitals lie on
the dz2 conical nodal plane
...

The e1u interaction between the metal px and py also gives some
stabilization
...

62

0 nodal surfaces

Figure: The

molecular orbitals of planar C5H5 (D5h)

Organometallics - Structure & Bonding






Bonding in a Cp2M complex

The 10 Cp electrons fill the 6 lowest orbitals,
The next five unoccupied MOs have little/no
bonding character, which explains why
metallocenes are known for a variety of delectron counts
...


Exception: The C-C distance in ferrocene is
longer than in many other metallocene
complexes
...

The C5 ring on ferrocene is planar, but the
hydrogens are bent down towards the metal
o
by about 5 , which lowers the energy of the
carbon p orbitals, which permits better overlap
Bent metallocenes, Cp2M(L)n
...

(a) General overview
...
Equally importantly, the Cp2M moiety is used to bondbond dissociation is large, the ligands tend not
with
to get involved chemically, the ligand blocks
other ligands so that Cp2 becomes a supporting ligand set to allow chemistry to take place at other
several coordination sites, and it has good
M–ligand bonds present
...

Sandwich and half-sandwich cyclopentadienyl compounds
for instance)
half-sandwich cyclopentadienyl compounds

structure of Cp Fe  (“Ferrocene”)  by  G
...
B
...
Wilkinson and R
...
Woodward in the
Survey structure & bonding in cyclopentadienylmetal compounds in the s, p, d & f-blocks
...
Ferrocene chemistry
...

entadienyl compounds are given below
...
Examples of some bent metallocene compounds illustrating the ability of Cp2M to bind up
guidelines, as shown below
...

more Cp groups, so the structure will either be bent or a half-sandwich (a two-legged
piano stool) complex
...

H
H
In these compounds the Cp ligands have bent back, decreasing the Cp–M–Cp angle from 180 to
o Fe
Re
Ir Re
Ni
Mo
Ir
Ni

OC
H
OC
o
Formula
Description
COH This   allows   mixing   (“rehybridisation”)   of   the   metal   d-based orbitals
usually between 130-140
...
The
O
[(η-C5H5)2MLx]
Bent or tilted Cp rings with additional ligands, L

general picture can be schematically shown as follows with three new M–L bonding frontier orbitals
3 legged
2 legged
[(η-C5H5)MLx]
Only one Cp ligand with additional ligands, L

tallocene
3 legged
2 legged
tallocene in the bent metallocene between the two Cp rings
...
The coordinate system
"half-sandwich" or "piano stool"
conventionally Bent Metallocene StructureC2v–symmetric Cp2M
...
x (see alsoligands
...

2
ligands, so that Cp2 becomes a supporting ligand set
...
Cp* and the
bonds present
...
Cp* and the
0
y
decreasing the Cp-M-Cp angle from 180 to between
more sterically demanding and useful for larger metals
...
Cp* is also
...
Indenyl is related to Cp but has a
onor than Cp due to D5d +I effects of the methyl groups
...
page 4)
...
page 4)
...
Me
Me
SiMe3
Me
Me
Me
Me

SiMe3

Organometallics - Structure & Bonding





Cp2M
...

(b) Bonding in bent metallocenes
...


(eventually) the new ligand(s)
...

How the frontier orbital energies of a parallel-ring metallocene change when the rings are bent back to fit the ligands is
shown in a Walsh Diagram
...


* antibonding

pective views of the frontier MOs are helpful to visualise their shapes and orientation,

ore usually depicted as follows projected onto the equatorial (yz) plane
...
The 2a1, The important frontier orbitals of the C Cp2Mvisualise their and 2a orbitals, which are derived from the former 1e and
b2 views of the frontier MOs of helpful M are the 1a , b shapes and
e the perspectiveand 1a1 frontier orbitals are C2v2v Cpto viewed in the yz plane
...
Walsh diagram showing how the equatorial (yz) plane
...

usually depicted as follows projected onto the frontier orbital energies of Cp2M change on going from a
ow shows the bonding situation in a Cp2M–L complex constructed from a Cp2M

coplanar to a bent metallocene
...
Note that the Cp2M b2 MO is non-bonding by symmetry in this case
...
The labels on the right correspond to C2v
...
The 2a1 b2 the 1
2
The important the lower energy 1a1 bent C2v Cp2M areaccommodate the remaining metal d-electrons
...
This is because
important thing is that all bonding interactions take

The Figure below These three orbitals can be usedin a up 2M–L complex
shows the bonding situation for Cp
the former 1e2g and
gives a much worse overlap
...
place in the equatorial (yz) plane, between the Cpto
2 rings
...
Note that the Cp2M b MO
The left-hand diagram is how these three frontier
three additional ligands (or bonding interactions)
orbitals – 2a , b and 1a – look, projected onto the

1
2
1
Importantly, the best interaction between take place in the
(d) electrons
...


poorer energy match
...
Perspective views of these orbitals are shown below
...



Figure
...

The 2a1, b2 and 1a1 frontier
For the Cp2M-L complex, the Cp2M
b 2
molecular orbital is nonbonding by
Figure below shows the bonding situation in a Cp2M–L complex constructed from a Cp2M
M
symmetry
...
Note that the Cp2M b2 MO is non-bonding by symmetry in this case
...
This is because the lower energy 1a1 orbital isorbital, even though is has a
poorly oriented for M-L
energy match
...

b2
1a1
2a1
This is because the lower energy 1a1 orbital is poorly orientated
for M-L bonding, and so gives much worse overlap
...
Perspective views of the 1a1, b2 and 2a1 frontier orbitals of C2v Cp2M

Organometallics - Structure & Bonding





Cp2M-L2 complex bonding

For a Cp2M-L2 complex, the SALCs for the (L)2 fragment transform
to a1 + b2 due to the C2v symmetry of the complex
...


σ-only donor Cp2ML2
0
1
compounds take d , d
2
and d configurations
...
-only donor Cp2ML2 compounds can therefore accommodate a d , d or d configuration
...
The carbonyl group
Real compounds to which these MOs apply are Cp2MCl2 (M = Zr (d0), Nb (d1), Mo (d2))
...
For heavier
metals, like Mo or Os, the
carbonyls do not bridge
...


Cyclopentadienyl Metal Nitrosyls can form, as NO is a very versatile ligand – it can function as a coordinated
+
NO ion (isoelectronic with CO) or as a coordinated NO ion (isoelectronic with O2)
...
Interaction diagram for a Cp2ML2 molecule
...
-only donor Cp2ML2 compounds can therefore accommodate a d0, d1
substitution become possible
...
Isoelectronic complexes are
obtained by substituting three CO groups for two NO ligands, or by compensating the Zr (d0), Nb (
Real compounds to which these MOs apply are Cp2MCl2 (M =
substitution of one CO for one NO, by the introduction of a positive charge
...
using the non-bonding a1 MO and the 2p AO of CH2:
being formed


Cyclopentadienyl Metal Hydrides
are formed mainly for the 4d and 5d
metals, as the 3d complexes tend to
dimerize and lose H2
...



Cyclopentadienyl Metal Halides are far more varied than the Cp-metal hydrides, even though halide and hydride ions share
the property of both being anionic 2-electron ligands
...
Interaction diagram for a Cp2ML2 molecule
...
(include: definition,

C

Agostic interactions:
M'
M
M'
M
M'
M
M'
examples, experimental evidence for existence; comment upon significance
discovered by Malcom Green!
of agostics)
C
A
B
D
E

The bondingDefinition: An agostic reaction is… described as a 3 centre – 2 electron interaction (as in a
in the first four cases (A –D) is
M

The interaction of a coordinatively-unsaturated (electron-deficient) transition metal with a C-H bond, when the two

B–H–B bond)
...



M´
...

donated  to  the  second  one  (“2e  donor”)
...
  This   term   was   introduced by
This is otherwise known as stabilization by intramolecular chelation
...
Electron-deficient metals can gain
stabilized complexes, due to the chelate effect
...

stabilisation by intramolecular chelation
...
Depending on the situation -, - or other
C–H bonds of alkyl ligands (and also C–Si and (but much less commonly) C–C)interaction with the hydrogen
can coordinate to
H
agostic interactions are possible
...
  This   term   interaction
...

C or C–Si) for the purposes of electron counting
...
Depending on the situation -, - or other
determined by single crystal diffraction methods (X-ray or, preferably, neutron for accurate H atom
A β-agostic interaction is the
agostic interactions are possible
...
The   “half  
1
interacton with the hydrogen
JCH coupling constants and IR (C-H) frequencies (for the agostic H)
...

C or C–Si) for the purposes of electron counting
...

Me 2
Me2
C preferably,
CH2
determined by single crystal diffraction methods Cp
or, H
CH H

P donating density (X-ray metal (see CO neutron for accurate 2 atom
P
Ph angle where the Cipso of CH2 Ph ring is
the
to the
below)
...

P
P
H
Agostic interactions are observed throughout organometallic chemistry, in alkyl, alkylidene & polyenyl ligands
...
The equilibrium position between a Cl -agostic ethyl

compound and the isomeric ethylene-hydride can be a delicate one as illustrated by this
2
-agostic
-agostic
-benzyl
-agostic
isoelectronic series:
The equilibrium position between a β-agostic ethyl compound and the isomeric ethylene-hydride can be a delicate one
...
The equilibrium position between a

-agostic ethyl

CH2

compound and the isomeric ethylene-hydride can CH2 a delicate one Rh illustrated by this
be
as CH2
Co
Fe
CH2
CH2
CH2
isoelectronic series:
R3P
R3 P
R3P
H
H
H
Ethylene-hydride

-agostic ethyl

Ethylene-hydride

Experimental Evidence for Existence
CH2
CH2
CH2
Co
Fe
Rh
The presence of C–H···M agostic interactions are determined by:
CH2
CH
Single crystal diffraction methods (X-ray or, preferably, neutron for accurate H atom location)
...
JCH is 68Hz for agostic H, and 151Hz for a terminal H
...

-agostic ethyl
Ethylene-hydride
Ethylene-hydride




Cy2P
OC

H

W
Organometallics - Structure & Bonding
OC

CO

H2 or HD

PCy3
CO H
W
H

OC



OC

or

OC
OC

PCy3
CO D
W
H

1

JHD = 33
...
2 Hz in HD)



d(H-H) 0
...
74 A in H2)

PCy3
PCy3
Significance of Agostics PCy3
1
...

2
...
2
3
...
5 Hz consistent with a direct H–D bond in
4
...

solution
...
  For  
5
...

example:

CO, NO, N2 and H2 ligands: metal dihydrogen (H2) compounds
Write some brief notes on each of the following Compounds of Interest
...

R3P
OC
Ru
Fe
containing a bound H2 ligand
...
Starting from a -agostic bis(phosphine)
[W(CO)3(PCy )2η -H2)] - Kubas’ (Dihydrogen) Compound
Ph3P
PR3
CO
Here, H and adding H gave W(PCy ) (CO) ( 2H ):
species2 has displaced N2 and formed a dihydrogen compound, containint a bound H2 ligand
...

Obviously a main point of interest is the nature of the H2→M bonding
...
5 Hz

CO H
CO D
H -type H2 or HD
(43
...
DihydrogenHD)
OC
or
W
OC
W
OC
W

CO
H
H
d(H-H) They
OC
sigma complexes because of the nature ofOC bonding picture
...
82 A are also known as nonthis
OC

(0
...


-agostic cyclohexyl

The first

2

-H2 compound


The H–H distance (determined by neutron diffraction) was only2)(H)2(PR3)3, the heaviest H2; using element Os forms the Os
M( 2-H a little longer than in free Group 8
The H-H distance is
determined by neutron
HD in place of H2 allowed measurement of 1HHD as 33
...

diffraction, and is only a
solution
...
  For   compound   in   fact   exists   as
Another   example   is   shown   below
...
However, the cationic (but isoelectronic
H2
...

compounds
...
Dihydrogen complexes are known as sigma
H
Obviously a main point of interest is the nature of the H2→M bonding
...
Dihydrogen compounds form where there is not enough density at the metal
PR3
PR3
complexes, because of this bonding, or sometimes as nonfollowing donor-acceptor way where donation to metal is from the filled H–H bonding MO with
classical hydrides
...
There are several examples in the literature that show this
W(0), dihydrogen
W(+2), dihydride
Kubas’ compound actually exists in equilibrium between a MO
...
For example, whereas Fe
dihydrogen and a dihydride form, as shown to the right
...
They are also known as non
Finally we note that dihydrogen compounds can act as intermediates
classical H4)]- – Zeise’s Salt
[PtCl3(C2 hydrides
...
Here the rate of protonation of the termina
Zeise’s salt was one of the first examples of a transition metal alkene complex
...

σ-bonding
...


The salt is synthesised by K2[PtCl4] and ethylene (C2H4), with a small amount of SnCl2 catalyst
...

Fe
Fe
P




P

H
P
Ph 2

H

- 80 oC

P
Ph 2

- 40 oC

H

Evidently H2 compounds (A above) may be viewed as a point on the way to oxidative addition
Fe(+2), hydride

Fe(+2), dihydrogen

where full donation of metal electron density occurs to form two 2 electron – 2 centre M–H bonds product
Kinetic
(as in B above)
...

n


med alkene-hydride routes kinetically accessible
...


energy process:
Ph3 P
Ph3 P

Rh

PPh 3

Ph 3P

Cl

Ph3 P

16 VE
Wilkinson's catalyst

+ PPh3

Rh
Cl

14 VE

ce at all; estimated Ti–C bond enthalpy 190 kJ mol-1) has low


It loses a ligand, forming the T-shaped 14valence electron Rh(+1) centre, prior to the
oxidative addition of H2
...
This is ligand dissociation
...

hydrogenation alkene catalysts
...
In solution it exists in three isomeric

coordinate RhCl(PPh3)3(H)2
mposition routes for transition metal alkyls include bimolecular mechanisms involving -

s (such as is proposed for TiMe4)
...


Strategies for stabilising metal alkyl complexes are based on the following


forms: cis, trans and unbridged, with the cis and
trans isomers being the most abundant
...


When the dimer is heated, a carbonyl is lost on each Fe atom and a tetra-cluster of Td symmetry
is formed, with 2 Fe-Fe bonds on each atom
...
Use of additional ligands to block
the left
...
It gives an electron-precise structure with 12 bonding electrons in the
a1 + e + t2 M-M HOMO bonding orbitals, which are also involved in backbonding to the CO π*
d increase in VE count
...

nsition metal – see include bimolecular mechanisms involving s TiMe4(L2) alkyls below – are stable at room temperature
...
Again these are not readily available for main group
[TiCl3(dmpe)Et] based on the following
stabilising metal alkyl complexes are
This is an example of a kinetically stabilized transition metal alkyl by blocking vacant coordination sites
with donor ligands
...
Use of additional ligands to block

H elimination which involves an increase in The dmpe ligand blocks the vacant coordination sites, and inhibits β-H elimination
...
Therefore, while TiMe4 is unstable aboveincrease in the coordination number and an increase in the valence electron count
...

0
Therefore, TiMe4 is unstable about -50 C, but the dmpe adduct is stable at room temperature
...


27



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