Coordination Chemistry 1 Flashcards
central atom in
i) chlorophyll
ii) haemoglobin
iii) vitamin b12
i) Mg
ii) Fe
iii) cyanocobalamine (cobalt)
Difference between a double salt and a complex
Both double salts as well as complexes are formed by the combination
of two or more stable compounds in stoichiometric ratio. However, they
differ in the fact that double salts such as carnallite, KCl.MgCl2.6H2O,
Mohr’s salt, FeSO4.(NH4)2SO4.6H2O, potash alum, KAl(SO4)2.12H2O, etc.
dissociate into simple ions completely when dissolved in water.
However,
complex ions such as [Fe(CN)6]
4– of K4 [Fe(CN)6] do not dissociate into
Fe2+ and CN–
ions.
Complex ions retain their identity in solution.
Postulates of Werner’s Theory
- In coordination compounds metals show two types of linkages (valences)-primary and secondary.
- The primary valences are normally ionisable and are satisfied by negative ions. It is equal to its oxidation state.
- The secondary valences are non ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to
the coordination number and is fixed for a metal. - The ions/groups bound by the secondary linkages to the metal have
characteristic spatial arrangements corresponding to different
coordination numbers. - Every metal must satisfy both valencies.
difference between primary valency and secondary valency
primaryvalency is non directional
secondary valency is directional
He further postulated that octahedral, tetrahedral and square planar
geometrical shapes are more common in coordination compounds of
transition metals.
Coordination entity
A coordination entity constitutes a central metal atom or ion bonded
to a fixed number of ions or molecules.
For example, [CoCl3(NH3)3] is a coordination entity in which the cobalt ion is surrounded by
three ammonia molecules and three chloride ions.
Other examplesare [Ni(CO)4], [PtCl2(NH3)2], [Fe(CN)6]4–, [Co(NH3)6]3+.
Central atom/ion
In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement
around it, is called the central atom or ion.
For example, the central atom/ion in the coordination entities: [NiCl2(H2O)4], [CoCl(NH3)5]2+ and [Fe(CN)6] 3– are Ni2+, Co3+ and Fe3+, respectively.
These central atoms/ions are also referred to as Lewis acids.
Ligands
The ions or molecules bound to the central atom/ion in the
coordination entity are called ligands.
These may be simple ions
such as Cl–
, small molecules such as H2O or NH3
, larger molecules
such as H2NCH2CH2NH2
or N(CH2CH2NH2
)3
or even macromolecules,
such as proteins.
These are lewis bases as they donate lp.
what is Chelation
A ligand may contain 2 or more donor atoms positioned in such a way that 5 or 6 membered ring is formed with the metal ion.
it is either tridentate, bidentate ligand and is said to be chelate ligand.
The resulting complex is called as a metal chelate and this property is called as chelation.
Such complexes, called chelate complexes
tend to be more stable than similar complexes containing unidentate
ligands.
Denticity
The number of coordinating/ligating donor atoms present in a ligand is called as the denticity of the ligand.
types of ligands based on denticity
When a ligand is bound to a metal ion through a single donor atom, as with Cl–, H2O or NH3 , the ligand is said to be unidentate.
When a ligand can bind through two donor atoms as in H2NCH2CH2NH2
(ethane-1,2-diamine) or C2O4 2– (oxalate), the
ligand is said to be didentate
when several donor atoms are
present in a single ligand as in N(CH2CH2NH2)3 -[3 sites]
, the ligand is said
to be polydentate. Ethylenediaminetetraacetate ion (EDTA4–) is
an important hexadentate ligand. It can bind through two
nitrogen and four oxygen atoms to a central metal ion.
what is an ambidentate ligand? give examples
Ligand which has two different donor atoms and either of the two ligetes in the complex is called ambidentate ligand.
Examples of such ligands are the NO2– andSCN– ions. NO2– ion can coordinate either through
nitrogen or through oxygen to a central metal
atom/ion.
c/a nitrito-N or nitrito-O
Similarly, SCN– ion can coordinate through the sulphur or nitrogen atom.
S- thiocyanato (or) thiocyanato-S
N-isothiocyanato (or) thiocyanato-N
Coordination number
The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded.
- It is important to note here that coordination number of the central
atom/ion is determined only by the number of sigma bonds formed by
the ligand with the central atom/ion. Pi bonds, if formed between the
ligand and the central atom/ion, are not counted for this purpose. *
in the complex ions, [PtCl6]
2– and [Ni(NH3)4] 2+, the coordination number of Pt and Ni are 6 and 4
respectively.
Similarly, in the complex ions, [Fe(C2O4)3]3– and [Co(en)3]3+, the coordination number of both, Fe and Co, is 6 because C2O4 2– and en (ethane-1,2-diamine) are didentate ligands.
Coordination sphere
The central atom/ion and the ligands attached to it are enclosed in
square bracket and is collectively termed as the coordination
sphere. The ionisable groups are written outside the bracket and
are called counter ions.
Coordination polyhedron
The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines a coordination
polyhedron about the central atom. The most common coordination polyhedra are octahedral, square planar and
tetrahedral.
For example, [Co(NH3)6]3+ is octahedral, [Ni(CO)4] is tetrahedral and [PtCl4]2– is square planar.
Oxidation number of central atom
The oxidation number of the central atom in a complex is defined
as the charge it would carry if all the ligands are removed along
with the electron pairs that are shared with the central atom. The
oxidation number is represented by a Roman numeral in parenthesis
following the name of the coordination entity.
