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18- Electron Rule.
Recall that for MAIN GROUP elements the
octet rule
is used to predict the
that the total number of electrons around the central atom isThis rule assumes that the central atom in a compound will make bonds suchformulae of covalent compounds.
THIS IS THE
MAXIMUM CAPACITY OF THE s and p orbitals.
This rule is only valid for
Period 2 nonmetallic elements.
The
(^) 18-electron Rule
is based on a similar concept.
The central TM can accommodate electrons in the s, p, and d orbitals.
s (2) , p (6) , and d (10) = maximum of 18
This means that a TM can add electrons from Lewis Bases (or ligands)
in
addition to its valence electrons to a total of 18.
Note that it only applies to metals with low oxidation states.^ This is also known Effective Atomic Number (EAN) Rule
18 Electron Rule cont’d
[Co(NH Example 1.
3 ) 6 ] +
Total electrons?Electrons from Co?Electrons from Ligands?Electron configuration of Co?Oxidation state of Co?
[Fe(CO) Example 2.
5 ]
Total electrons?Electrons from Fe?Electrons from Ligands?Electron configuration of Fe? Oxidation state of Fe?
What can the EAN rule tell us about
[Fe(CO)
5 ] ?
It can’t occur…… 20-electron complex.
EAN Summary
Works well only for d-block metals.
It does not apply to f-block
metals.
Works best for compounds with TMs of low ox. state.
Ligands which are good
σ -donors and
π -acceptors utilize all the
valence orbitals and thus such compounds obey this rule.
Complexes which contain a combination of
σ -donors and
(^) π -acceptors
conform to this rule. (e.g. Cr(NH
3 ) 3 (CO)
3 , Cr(
η 6 -C 6 H 6 )(CO)
3 ).
reactions.Compounds which obey this rule are kinetically inert to substitution
whereExceptions to the rule occur at the two ends of the transition series
nd, (n+1)s, and (n+1)p
valence orbitals are less well matched
in energy.
Let’s talk about electron counting briefly.
Sandwich Compounds Obeying EAN
Let’s draw some structures and see some new ligands.
Each of these ligands is
π -bonded above and below the metal center.
Half-Sandwich Compounds Obeying EAN
Let’s draw some more structures.
CO, NO, H, and PR
3 (^) can be brought together in combination to give 18 electrons.
Ferrocene is an interesting example.
Chemistry and “Magic Numbers”
The Octet Rule:
Period 2 nonmetallic elements tend to form compounds
this is because elements desire a pseudo-noble gas configuration.resulting in eight electrons around the central atom. You have been told
This is a VAST simplification.
Stable Fullerenes:
The allotrope of Carbon known as fullerenes (C
60
or
compounds.been observed that particular numbers of C atoms yield more stable“Bucky-ball” is the most famous) take on a cage structure and it has
C
60 , C
70 , C
76 , C
84 , C
90 , C
94
Nanoparticles:
Metal Nanoparticle are really COOL! It has been observed
Bonding stable structures.that “magic numbers” of atoms preferentially come together to form
in
TM
Complexes:
Many
TM
complexes
will
form
with
electrons
around
the
central
metal
atom.
It
was
first
observed
by
Sedgwick in 1927.
18- Electron Rule.
Recall that for MAIN GROUP elements the
octet rule
is used to predict the
formulae of covalent compounds.
Think about Na
+ and Cl^
-
that the total number of electrons around the central atom is This rule assumes that the central atom in a compound will make bonds such
THIS IS THE MAXIMUM CAPACITY OF THE s and p orbitals.
This rule is only valid for
Period 2 nonmetallic elements.
The
18-electron Rule
is based on a similar concept.
The central TM can accommodate electrons in the s, p, and d orbitals.
s (2) , p (6) , and d (10) = maximum of 18
This means that a TM can add electrons from Lewis Bases (or ligands)
in
addition to its valence electrons to a total of 18.
This is also known Effective Atomic Number (EAN) Rule
Simple Examples of the 18 Electron Rule
[Co(NH Example 1.
3 ) 6 ] +
Total electrons?Electrons from Co?Electrons from Ligands?Electron configuration of Co?Oxidation state of Co?
[Fe(CO) Example 2.
5 ]
Total electrons?Electrons from Fe?Electrons from Ligands?Electron configuration of Fe? Oxidation state of Fe?
What can the EAN rule tell us about
(^) [Fe(CO)
5 ] ?
It can’t occur…… 20-electron complex.
Approach 1 to counting
Oxidation State Electron Count.
Ligands are viewed as “close-shelled” entities.
(No radicals).
This is what we did in the earlier examples.
We dissect the structure
When neutral Lewis base ligands (like NH
3 ) are considered they are viewed as
Ligands like methyl (CHneutral molecules with 2 electrons for donation to the metal.
3 and Cl) are viewed as anions….
NOT AS NEUTRAL
RADICALS. (By definition H is viewed as H
After removal of the ligands the metal is assigned a formal charge. [Ni(CO)
4 ]
Ni 0 10 e^
**- , CO 2 e
each (8) = 18**^
[PtCl
2 (PMe
3 ) 2 ]
Pt 2+ 8 e^
**- , Cl
2 e
each (4), PMe**^
3 (^) 2 e
- each (4) = 16
[Ta(Me)
5 ]
Ta
5+ 0 e^
**- , Me
2 e
each (10) = 10**
Fe(
η 5- C 5 H 5 ) 2
Fe
2 6 e^
η 5- C 5 H 5 6e
- each (12) = 18 Ferrocene
Look at CO complexes of Mn
MnCO
OC
OC
CO
CO
structure for a CO complex of Mn.You may expect to have the following
Total 17 electrons3 CO Terminal 10 Mn 7
Prediction of Structure.
(metal carbonyls)
Is this the only possible structure for bis[tetracarbonylcobalt]? but it CAN provide possibilities for investigation.The EAN Rule cannot differentiate structures of compounds
1 Co-Co 12 CO Bridging 23 CO Terminal 6 Co 9
CoCO
OC
OC
CO
structure for a CO complex of Mn.You may expect to have the following
Co
OC
OC
OC
CO
Co
CO
OC
CO
CO
What about?
Compounds and the EAN Rule
1. We can divide compounds into three groups.
EAN rule.Electronic configurations are completely unrelated to the
The central metal may have >, <, = 18
electrons.
have >18 electrons, but may have less.Electron configurations follow the EAN rule and never
A group that follows EAN rule rigorously.
(This is what I have shown you so far)
How can we understand this?
Group I
d s p
σ
filled
M
ML
6
6L
∆ o
Weak sigma interaction and NO pi interaction by 6L
Little or no pi interaction between metals and ligands. Energy of the t
2g orbitals is the same
Thereas the free metal.
are
low
energy
bonding
MO’s,
medium
energy
MO’s
and
and
strongly
12 electrons from the ligands fill the lowest energy orbitals (blue).antibonding MO’s (too high energy to be occupied).
Up to 6 metal electrons
reside in the t
2g set (nonbonding) without any destabilization of bonding.
o (^) is so small that up to 4 electrons can be put into the e
g (^) set with only a small penalty.
Valence electrons from 12 to 22.
(d-electrons, valence)
TiCl
4 (THF)
2
(O,12)
Ti(H
2 O) 6 3-
(1 ,13)
V(urea)
6 3-
(2 ,14)
CrCl
6 3-
(3 ,15)
CrI 2 (DMSO)
4
Mn(H
(^2) O)
6 2+
CoF
6 3-
CuCl
5 3-
Ni(H
2
(^6) 2+
Cu(H
2
6 2+
ZnCl
2 (biuret)
2
You figure out.
Ligands are weak field,
o (^) is small.
EAN Summary
Works well only for d-block metals.
It does not apply to f-block
metals.
Works best for compounds with TMs of low ox. state.
Ligands which are good
σ -donors and
π -acceptors utilize all the
valence orbitals and thus such compounds obey this rule.
Complexes which contain a combination of
σ -donors and
(^) π -acceptors
conform to this rule. (e.g. Cr(NH
3 ) 3 (CO)
3 , Cr(
η 6 -C 6 H 6 )(CO)
3 ).
reactions.Compounds which obey this rule are kinetically inert to substitution
whereExceptions to the rule occur at the two ends of the transition series
nd, (n+1)s, and (n+1)p
valence orbitals are less well matched
in energy.
This Rule allows for prediction of structures, reactivity, and reaction mechanisms.
Bridging or Terminal CO
Terminal
CO
bonding
at
cm
and
1975.7 cm
also,
because
of
very
small
carbon monoxides.symmetry differences between
Terminal CO
(^) bond1887 cm
Bridging CO
bond at 1770 cm
Fe
Fe
OC
CO
OC CO
Fe
OC
CO I
Bonding in TM Carbonyls
CO bonding-the orbital picture
10 valence electrons
C (4), O(6)
Filled FilledFilled
C
O
HOMO
d s p
σ
filled
M
ML
6
6L
∆ o
Strong sigma interaction and
strong
pi acceptor interaction
(^) by 6L.
t 2g (^) vacant
π
A cartoon of M-CO bonding.
is the high energy The HOMO in carbon monoxide
(^) σ NB
which is
This2p orbital.primarily derived from a carbon
means
a
lone
pair
of
The LUMO on CO is theatom.electrons is residing on the C
π
2p
which
are
antibonding orbitals
TheLewis Acid.CO acts as a Lewis Base and awith significant 2p character.
back
bond
appearing
in
this
systems
is
known
as
a
synergistic effect
M-CO photochemistry Examples
(CO)
4
Ru
(OC)
4 Ru
Ru(CO)
4 hv,
>370nm L
3 LRu(CO)
4
(L= olefin)
Orange, colour arises from
(Ru-Ru)
~390nm
Another example involving Fe and an 18 electron transition state
Fe
OCOC
CO CO
18 electrons
alkyne 2e
donor
Fe
OCOC
CO
18 electrons
alkyne 4e
donor^
It substitutes 10a 4e- donor alkyne.16e- and is stabilized byThis intermediate is not
13 x faster
than Fe(CO)
5 .
L
Reduction of TM Carbonyls
What will happen if electrons are added to 18e
TM carbonyls?
High energy 19 or 20 electron systems will result and CO will be ejected. (This can be viewed as the two electrons taking the place of the CO or
breaking M-M bonds)
Fe(CO)
4 2-
2Mn(CO)
5
2Na/Hg 2Na/Hg
These
anions
are
of
andThey are nucleophilessignificant importance.
react
further
to
form
M-C
and
M-H
bonds.
Mn(CO)
5
RX
A
MnCO
OCCO
CO
CO
R
MnCO
OCCO
CO
CO
C
R
O
Formation of M-H and M-C bonds
B
RCOX
CO between M and R.and B is the presence of The difference between A
CO and heat
MnCO
OCCO
CO
CO
CO
MnCO
OCCO
CO
CO
R
MnCO
OCCO
CO
R CO
R
CO
bonding Empty site.
This is referred to as “CO insertion” although the mechanism involves migration of R.
Mn(CO)
5
H-Mn(CO)
5
Collman’s Reagent
Application of “carbonylmetallates” in organic synthesis.
Na
2 Fe(CO)
4
Fe
OC OC
CO R
CO
Fe
OCOC
CO CO
CO
R
Disodium tetracarbonylferrate is useful in the functionalization of organic halides.
R
R'
O
R
OH
O
R
X
O
RD
R'X
D
+
RX
RCOCl
CO
R'X
O
2
O
2
X
2
X
2
R'OH
H
2 O
HNR
2
The Mond Process
Nickel carbonyl, a gas formed from carbon monoxide and metallic nickel.
Scientific Serendipity
nickel valves used in apparatus for the In 1890 Ludwig Mond, was investigating the rapid corrosion of
Solvay process*
, and
discovered Ni(CO)
(^4).
solids, Ni(CO)In contrast to many nickel compounds which are usually green
4 is a colourless, volatile, toxic liquid with a very
"Mond process". He used it as the basis of a method to purify nickel, called the"organic character".
Ni reacts with CO (leaving the impurities behind), to form Ni(CO)
(^4).
The
Ni(CO)
4 is passed through a tower filled with nickel pellets at a high velocity and 400 K.
Pure Ni plates out on the pellets.
- A commercial process for the manufacture of Na
2 CO 3
. NH
3 and CO
2 are passed into a sat’d NaCl
(aq)
solution to
form soluble (NH
(^4) )(HCO
(^3) ), which reacts with the NaCl to form soluble NH
(^4) Cl and solid NaHCO
3 if the reactor
temperature is maintained below 15°C. The NaHCO
3 is filtered off and heated to produce Na
2 CO 3 .
Hemoglobin and Heme
