Terminologies Pertaining to Coordination Compounds Video Lecture | Chemistry Class 12 - NEET

the fact that transition metals often
exist as positive cations while
electrons which are often found as lone
pairs and organic compounds are negative
means that the two can get together to
form what are called coordination
compounds a coordination compound
includes a central metal ion or atoms
such as ni 2 plus surrounded by a set of
molecules or ions that are bonded to the
central metal atom through what are
called coordinate covalent bonds what's
unique about a coordinate covalent bond
is that both electrons of the covalent
bond are donated by the same atom so
electron sharing is still going on it's
just that we think of both electrons in
the bond is coming from the nonmetal
partner a great example of a transition
metal coordination complex is the hexa
aqua nickel 2 ion which contains a
central nickel 2 plus cation surrounded
by six water molecules and this has the
octahedral geometry so these two waters
are actually pointing out towards you
this water that I'm drawing as well as
this one here are both pointing away
from you and we often use dashed lines
like this to represent that while these
two waters above and below are just
pointing straight up and down in the
plane of the screen the entire what's
called complex the entire metal complex
has a charge of +2 and we think of each
of the bonds within the coordination
complex between the nonmetal in this
case oxygen and metal the nickel as
involving a pair of electrons from the
nonmetal atom so you'll often see these
drawn with arrows pointing to the metal
atom indicating that both electrons are
coming from the nonmetal atom this
particular complex is pretty famous for
appearing green so when nickel 2 finds
itself in aqueous solution for example
we end up with a green solution
coordination compounds are a fairly
unique class of compound because this
sphere of groups that surrounds the
central metal atom has a bent big impact
on the properties of the metal and the
properties of the compound
all so we can tune this outer sphere of
molecules and we can tune the central
metal atom to achieve a variety of
properties color is one of those
properties but coordination compounds
also exhibit unique reactivity often as
Lewis acids to begin exploring the
properties of coordination compounds in
more detail though we need to define
some terminology the molecules are ions
that surround the central metal atom are
known as ligands and ligands are usually
small or moderately sized organic
molecules not all of the atoms within
the ligands are actually bonded to the
metal we define the specific atom that
donates a lone pair in a coordination
compound as the ligand donor atom
so for example in the nickel to hexa
aqua complex that we looked at
previously the oxygen atom is what's
actually donating a lone pair to the
metal so if we were to draw out the lone
pairs explicitly one of the oxygen lone
pairs is actually not involved in
coordinate covalent bonding while the
other is and you should notice that
we've kind of turned some of the water
molecules around so that the ligand
donor atom is drawn is directly bonded
to the metal
this helps us clarify which atom within
the ligand is actually bound to the
metal since a coordinate covalent bond
involves the donation of two electrons
from the same atom it makes sense in
this case that oxygen the only atom
within water that bares lone pairs
serves as the ligand donor atom it will
sometimes be the case that a ligand
contains multiple atoms bearing lone
pairs and only one of those atoms
actually can serve as a ligand donor
atom another situation that's worth
appreciating and something we'll come
back to later is that some ligands
include multiple atoms that can serve as
donors and that actually do serve as
donors so a great example and one that
will work with a little bit later is
ethylene diamine which contains two
nitrogen's with Lewis basic lone pairs
that can both serve as donors to the
same metal center this is what's known
as a chelating or poly dentate ligand
the concept of a coordinate covalent
bond and the idea of a lewis acid-base
reaction or interaction go hand-in-hand
so it's worth appreciating that the
formation of coordination compound is a
Lewis acid based process in which a
lewis acidic metal cation combines with
a Lewis basic ligand which may be
neutral or negative but which must bear
a lone pair or a pair of electrons that
can be donated more generally to form a
coordinate covalent bond in which the
new bond is composed of two electrons
from the Lewis base in this case L and
those are donated to the Lewis acid in
this case M and the overall charge
remains at +1 in this case since the
ligand was neutral so we can see that
the Lewis base
has donated a pair of electrons to the
Lewis acid to form the resulting complex
the Lewis acid will always be
accompanied by some counter ion usually
it's something that's not very basic
something like chloride or sulfate
something fairly innocuous
in that case X minus comes along for the
ride and is considered part of the
coordination compound but we put the
complex in brackets to distinguish
between the ligands that are actually
covalently bonded coordinate ly
covalently bonded to the metal and the
counter ions which just balance the
charge on the complex another way to
state this same point is that many
coordination compounds are salts this
means that the complex part the part
containing the metal ion and the ligands
that are coordinate ly covalently bonded
to it has a charge that charges counter
balanced by the counter ions but very
often those counter ions at least
theoretically have lone pairs right if
we think about sulfate for example
sulfate has tons of lone pairs within
its Lewis structure that could in theory
be used to form coordinate covalent
bonds with a metal center right any of
these two singly bonded oxygens with
negative charge are just right to donate
electrons to a metal center in theory
because of that possibility we want to
take care to distinguish what's actually
bonded to the metal and what's not and
that's why we use brackets to do this so
square brackets enclose everything
including the metal center and the
ligands that are actually coordinate ly
covalently bonded to it another
important point here is that the sulfate
ion in this case is not a ligand it's a
counter ion it just balances the charge
of the complex and engages in an ionic
bond with the entire nickel h2o 6 2 plus
cation as we've talked about in many
different contexts before here as always
it's important to be able to look at the
formula and be able to separate this
into its component ions so4 2 - and in
this case the newcomer nickel h2o 6 -
plus because ligands are often neutral
and metals are often positive we
do often see that the coordination
complex itself is positive but this is
by no means required for example here's
an anionic complex containing three
potassium ions this is just k+ and one
copy of the FEC and six ion which
includes iron six cyanide ligands and an
overall charge of minus three to balance
the three K pluses within the formula
ultimately we can unpack this hexxus Ino
ion even further to notice that since c
and minus is itself negatively charged
six of them amount to negative six
charge overall and the iron cation here
has the oxidation state of plus 3 or Fe
3 plus you'll want to be able to
decompose a complex like this into its
component ions so that you can tell the
oxidation state of the metal center
which is often key and which species are
actually ligands versus mere counter
ions that's especially important in
cationic complexes where the counter ion
is going to look a lot like a ligand
that in some cases can serve as a ligand
when other metal centers or other ligand
sets are involved one important
characteristic of coordination compounds
that plays a big role in their
properties is the coordination number
and we want to define the coordination
number very specifically it's not
necessarily just the number of ligands
around the metal center because a single
ligand may have multiple binding points
we define the coordination number very
specifically as the number of ligand
donor atoms in the metal complex so for
example the complex silver NH 3 2 with a
+1 charge has two ammonia ligands bonded
to a central silver cation for a
coordination number of two nickel CN for
two - has a coordination number of four
since four cyano ligands surround the
central nickel cation the hexa aqua
cations such as the chromium version
with six waters surrounding the central
metal atom in this case having a charge
of 2 + overall has a coordination number
of six as evidenced by the six
waters within the formula for the
complex looking at these three examples
you may believe that the number of
ligands within the complex formula tells
you the coordination number directly
however that's not the case
the reason is ligands can have multiple
binding points so for example the
similar-looking complex CR ethylene
diamine 3/2 plus appears to have a
coordination number of three but in fact
ethylene diamine is a bidentate ligand
that is it has two binding points on it
and so rather than three of the
coordination number here is six and in
fact structurally this red complex has a
lot in common with the hexa aqua complex
right above a table in your text has
other examples of coordination complexes
and their associated coordination
numbers do keep in mind that ligands
with multiple binding points are going
to complicate assignment of the
coordination number we define
coordination number as the number of
ligand donor atoms and that sometimes
requires us to think about the number of
binding points in the ligand or a finer
examination of the structure of the
complex rather than just its formula