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Summary metal organic chemistry (NWI-MOL096)
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Summary the course metal organic chemistry.
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Summary metal organicchemistry
,Chapter 1 Introduction to
organometallic complexes
Introduction
In order for a complex to fall into the category organometallics, it must by definition contain at least
one metal carbon bond though complexes with hydrogen-metal bonds are also often regarded as
organometallic complexes
Metals are very useful in the lab and in industry as catalysts. They can serve both in homogenous as
well as heterogenous catalysis produce less waste than many other catalysts and is highly efficient
Besides being catalysts, metals can also be a stable part of the reaction product(s) such as
in silicone rubbers
Trends in the periodic table
,In order to analyse the behaviour and complexes formed by certain metals, one must know some of
the characteristics of the particular metal
A lot of a metal’s characteristics follow certain trends along the periodic table therefore
knowing the metal’s position in this table provides clues about important properties to
determine for example:
stability reactivity
ligands, number of ligands and geometry frontier orbitals
electronegativity reactivity and reaction mechanisms
From the left to the right:
the first column are the alkali metals belongs to the s-block: prefers a two electron count
in its valence shell
the second column are the earth alkali metals belongs to the s-block as well: prefers a two
electron count in its valence shell
the middle block – columns 3 to 12 – are the transition metals, often referred to as the d-
block elements as their frontier orbitals are d-orbitals These generally prefer an 18
electron count in their valence shell these can be subdivides into:
early – columns 3 and 4: exhibit complexes often with less than 18 electrons low
d-electron count so need a lot of ligands in order to acquire 18 electrons which is
sterically almost impossible
middle – columns 5 6 and 7: obey the 18 electron rule
late – columns 8 to 12: these metals are often more electronegative (so less
electropositive) and have already a very high d-electron count therefore these
often already form stable complexes with 16 electrons
the columns 13 to 17 form the p-block obey the 8 electron rule
column 18 are the noble gases that are unreactive
Electronegativity increases
from the left to the right because of increasing nuclear charge by addition of 1 proton
each step to the right while there is no change in shell and thus no change in electron
distance from the nucleus
from the bottom to the top because the further to the bottom an element is, the further
its valence orbitals are separated from the nucleus and thus the less nuclear charge the
electrons in these orbitals experience – or the other way round: the earlier a row in the
table, the closer an electron is to the nuclear attractive charge
, Chapter 2 Reactivity of
organometallics
The reactivity of organometallics is significantly different from that of organic molecules bonds are
not static, oxidation states vary and partial charges are divided differently
Bond polarity
The metals of the organometallic complexes are electropositive and therefore very electron donating
as the metals are much more electropositive than carbons that coordinate with them, the carbon
receives electron density
Therefore, the carbon atoms form the partial negative end of the bonds and the metals the
partial positive ones carbon reacts as nucleophile, metal as electrophile
The more electropositive the metal is, the more reactive the coordinating carbon will be
though reactivity also depends on
M-C, M-X bond strengths
number of d-electrons factors that determine
coordination number complex stability
steric hindrance
It is this difference in electronegativity that
causes some organometallic complexes to be
reactive either with water or oxygen
Substitution reactions
Metal complexes do undergo substitution reactions of their ligands, these can be classified as:
associative: the incoming ligand attaches before dissociation
of the leaving ligand often encountered with complexes
that are:
less than 18 electrons
dissociative: the leaving ligand dissociates before attachment of the incoming ligand often
encountered with complexes that are:
sterically hindered
have already 18 electrons (as <18 electrons is more
stable than >18 electrons)
Electron counting on organometallic complexes
Electron counting is the most essential skill in organometallics because the electron count explains
the major part of the complexes’ observed reactivity
The electron count of the complex is the total number of electrons present in the valence
orbitals of the metal combined with the electrons from the ligands that bond with these
orbitals
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