Download Coordination compounds :

Coordination compounds :
Coordination compounds have been recognized by chemists as a special type
of compound since late in the eighteenth century . Throughout the Nineteenth
century , a wide variety of "complex inorganic compounds" were prepared by
reacting metal salts with neutral organic or inorganic molecules, such as
ethylene or ammonia, or with halide Ions and other simple anions, such as
cyanide and nitrite . In 1892 Alfred Werner, proposed the Coordination Theory.
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Werner's Coordination Theory :
1. In coordination compounds, central metal atoms exhibit primary valence
and secondary valence.
primary valence is ionic . Secondary valence is not ionic. The primary valence
corresponds to oxidation state. The secondary valence corresponds to
coordination number.
2. Every metal atom has a fixed number of secondary valences (coordination
numbers).
3. The metal atom tends to satisfy both its primary valence as well as its
secondary valence. Primary valence is satisfied by negative ions (metal ion
has a positive charge) whereas secondary valence (coordination number) is
satisfied either by negative ions or by neutral molecules).
4. The coordination number or secondary valences are always directed
towards the fixed positions in space and this leads to definite geometry of the
coordination compound
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A coordination compound is one in which a central atom or ion is joined to one
or more ligands through what is called a coordinate covalent bond in which
both of the bonding electrons are supplied by the ligand. In such a complex the
central atom acts as an electron-pair acceptor (Lewis acid) and the ligand as an
electron-pair donor (Lewis base ). The central atom and the ligands
coordinated
to
it
constitute
the
coordination
sphere.
Thus
the
salt
[Co(NH3)5Cl]Cl2 is composed of the complex ion [Co(NH3)5Cl]2+ and two Cl–
ions; components within the square brackets are inside the coordination
sphere, whereas the two chloride ions are situated outside the coordination
sphere. These latter two ions could be replaced by other ions such as NO3– .
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The central atoms of coordination complexes are most often cations (positive
ions), but may in some cases be neutral atoms, as in nickel carbonyl Ni(CO)4.
Ligands composed of ions such as F– or small molecules such as H2O or CN–
possess more than one set of lone pair electrons, but only one of these pairs
can coordinate with a central ion. Such ligands are said to be monodentate
(―one tooth‖.) Larger ligands may contain more than one atom capable of
coordinating with a single central ion, and are described as polydentate. Thus
ethylenediamine (shown below) is a bidentate ligand. Polydentate ligands
whose geometery enables them to occupy more than one coordinating
position of a central ion act as chelating agents and tend to form extremely
stable complexes known as chelates.
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. For example: in the complex K4[Fe(CN)6]،[is the coordination sphere
[Fe(CN)6]4- .
Counter ions: The ions present outside the coordination sphere are called
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counter ions. For example: in the complex K4[Fe(CN)6], K+ is the counter ion.
A complex is a structure composed of a central metal atom or ion, generally a
cation, surrounded by a number of negatively charged ions or neutral
molecules possessing lone pairs. A complex may also be called a coordination
compound or metal complex.
The ions or molecules surrounding the metal are called ligands. A ligand that is
bound to a metal ion is said to be coordinated with the ion. The process of
binding to the metal ion with more than one coordination site per ligand is
called chelation. Compounds that bind avidly to form complexes are thus called
chelating agents (for example, EDTA).
Simple ligands like water or chlorine form only one link with the central atom
and are said to be monodentate. Some ligands are capable of forming multiple
links to the same metal atom, and are described as bidentate, tridentate etc.
EDTA is hexadentate, which accounts for the great stability of many of its
complexes.
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Donor atom: An atom of the ligand attached directly to the metal is called the
donor atom. For example: in the complex K4[Fe(CN)6 ] carbon is a donor atom.
Coordination number: The coordination number (CN) of a metal ion in a
complex can be defined as the number of ligand donor atoms to Get the Power
of Visual Impact on your side which the metal is directly bonded. For example:
in the complex K4[Fe(CN)6], the coordination number of Fe is 6.
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Naming Coordination Compounds:
The coordination compounds are named in the following way.
A. To name a coordination compound, no matter whether the complex ion is
the cation or the anion, always name the cation before the anion.
B. In naming the complex ion:
1. Name the ligands first, in alphabetical order, then the metal atom or ion.
Note: The metal atom or ion is written before the ligands in the chemical
formula.
2. The names of some common ligands are listed in Table 1.
For anionic ligands end in "-o"; for anions that end in "-ide"(e.g. chloride),
"-ate" (e.g.sulfate, nitrate), and "-ite" (e.g. nirite), change the endings as
follows: -ide -o; -ate -ato; -ite -ito For neutral ligands, the common name of
the molecule is used e.g. H2NCH2CH2NH2 (ethylenediamine).
Important exceptions: water is called ‗aqua‘, ammonia is called ammine‘,
carbon monoxide is called carbonyl‘, and the N2 and O2 are called
dinitrogen‘ and dioxygen‘.
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2. Greek prefixes are used to designate the number of each type of ligand in the
complex ion, e.g. di-, tri- and tetra-. If the ligand already contains a Greek prefix
one binding site) the prefixes bis-, tris-, tetrakis-, pentakis-, are used instead.
The numerical prefixes are listed in Table 2.
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3. After naming the ligands, name the central metal. If the complex ion is a
cation, the metal is named same as the element. For example, Co in a
complex cation is call cobalt and Pt is called platinum.. If the complex ion is
an anion, the name of the metal ends with the suffix –ate.. For example, Co
in a complex anion is called cobaltate and Pt is called platinate. For some
metals, the Latin names are used in the complex anions e.g. Fe is called
ferrate (not ironate).
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4. Following the name of the metal, the oxidation state of the metal in the
complex is given as a Roman numeral in parentheses.
C. To name a neutral complex molecule, follow the rules of naming a complex
cation. Remember: Name the (possibly complex) cation BEFORE the
(possibly complex) anion.
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[Co(NH3)6]3+
Hexammine cobalt(III)
K3[Fe(CN)6]
potassium hexacyano ferrate(III)
[Co(NH3)4Cl2]Cl
Tetrammiine dichlorocobalt(III) chloride
[FeNO(NH3)5]Cl3
Pentammine nitrosyl iron(II) chloride
[Cr(CO)6]
hexacarbonylchromium(0)
K[V(CO)6]
potassium hexacarbonyl vanadate(I)
Q/name the following complexes :
[Mn(NO)3CO] , NH4[Cr(NH3)2(NCS)4] , [Cu(en)2(H2O)2]SO4
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1. Do ligands act like Lewis acids or
Lewis bases? Why?
2. Do ligands form ionic bonds with the
central metal atom?
3. What are chelating agents?
4. What is a monodentate ligand?
5. Describe polydentate ligands and
provide an example.
6. What are hexadentate ligands?
7. Name this complex [Cu(NH3)4]SO4.
8. Name this complex [Co(en)3](NO3)2.
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Q /For the complex K3[Cr(C2O4)3]·3H2O
1) What is the central metal ion?
2) What is its oxidation state?
3) What is its electronic configuration?
4) What is its coordination number?
5) What is the IUPAC name of the complex?
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Q/Name the following compounds
Answers:
[Cu(OH)4] 2tetrahydoxocopper(II)
Na3AuCl4
Sodium tetrachloroaurate(I)
[Mo(CN)6 ]4hexacyanomolybdenate(II)
Fe(CO)6ScCl6
hexacarbonyliron(III) hexachloroscanadate(III)
[Cr(NH3)3(H2O)3 ]Cl3
triamminotriaquachromium(III) chloride
[Pt(NH3)5Cl]Br3
pentaamminochloroplatinum(IV) bromide
[Pt(en)2Cl2]Cl2
dichlorobis(ethylenediamine)platinum(II) chloride
[Co(en)3]2(SO4)3
tris(ethylenediamine)cobalt(III) sulfate
K4[Fe(CN)6]
Potassium hexacyanoferrate(II)
Na2[NiCl4]
Sodium tetrachloronickelate(II)
Pt(NH3)2Cl4
diamminoplatinum(IV) chloride
(NH4)2[Ni(C2O4)2(H2O)2] Ammonium diaquabis(oxalate)nickelate(II)
[CoBr(NH3)5]SO4
pentamminobromocobalt(III) sulfate
[Fe(NH3)6][Cr(CN)6]
hexamminoiron(III) hexacyanochromate(III)
[Co(SO4)(NH3)5]+
pentamminosulfatocobalt(III)
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