See for a set of questions on this topic the post
http://iit-jee-chemistry-ps.blogspot.com/2007/10/iit-jee-chemistry-questions.html
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Syllabus
Nomenclature of mononuclear coordination compounds, XII 10.1,2,3
Cis-trans and ionisation isomerisms, 10.4
Hybridization and geometries of mononuclear coordination compounds (linear, tetrahedral, square planar and octahedral).10.7
The topics are covered in detail Jauhar's XII book. The section numbers are given beside the topic.
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Coordination compounds are a special class of compounds in which the centgral metal atom is surrounded by ions or molecules beyond their valency.
There are also referred to as coordination complexes or complexes.
Haemoglobin, Chlrophyll, and vitamin B-12 are coordinatio compounds of iron, magnesium and cobalt respectively.
The interesting thing of coordination compound is that these are formed from apparently saturated molecules capable of independent existence.
for example, when acqueous ammonia is addedt o green solution of nickel chloride, NiCl2, the colour changes to purple. The ni^2+ ions almost diappear from the solution. The solution on evaporation yields purple crystals corresponding to the formula [Ni(NH-3)-6]Cl-2. such a compound is called coordinatin compound. When this compound is now dissolved in water, there is hardly any evidence of Ni^2+ ions or NH-3 molecules. It ionizes to give a new species [Ni(NH-3)-6]^2+. the species in the square brackets does not ionise further. It remains as a single entity as an ion.
This is the unique feature of coordination compounds.
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Nomenclature rules
From
http://www.iupac.org/publications/books/principles/principles_of_nomenclature.pdf
IUPAC booklet available for download at the above page id.
A coordination entity is composed of a central atom or atoms to which are attached
other atoms or groups of atoms, which are termed ligands. A central atom occupies a
central position within the coordination entity. The ligands attached to a central
atom define a coordination polyhedron. Each ligand is assumed to be at the vertex of
an appropriate polyhedron. The usual polyhedra are shown in Table 3.3 and they are
also listed in Table 4.4. Note that these are adequate to describe most simple
coordination compounds, but that real molecules do not always fall into these simple
categories. In the presentation of a coordination polyhedron graphically, the lines
defining the polyhedron edges are not indicative of bonds.
However, many ligands do not behave as donors of a single electron pair. Some
ligands donate two or more electron pairs to the same central atom from different
donor atoms. Such ligands are said to be chelating ligands, and they form chelate
rings, closed by the central atom. The phenomenon is termed chelation.
The number of electron pairs donated by a single ligand to a specific central atom
is termed the denticity. Ligands that donate one pair are monodentate, those that
donate two are didentate, those that donate three are tridentate, and so on.
Sometimes ligands with two or more potential donor sites bond to two (or more)
different central atoms rather than to one, forming a bridge between central atoms. It
may not be necessary for the ligand in such a system to be like ethane-l ,2-diamine,
with two distinct potential donor atoms. A donor atom with two or more pairs of
non-bonding electrons in its valence shell can also donate them to different centralatoms. Such ligands, of whatever type, are called bridging ligands. They bond to two
or more central atoms simultaneously. The number of central atoms in a single
coordination entity is denoted by the nuclearity: mononuclear, dinuclear, trinuclear,
etc. Atoms that can bridge include 5, 0 and Cl.
The original concepts of metal—ligand bonding were essentially related to the
dative covalent bond; the development of organometallic chemistry has revealed a
further way in which ligands can supply more than one electron pair to a central
atom. This is exemplified by the classical cases of bis(benzene)chromium and
bis(cyclopentadienyl)iron, trivial name ferrocene. These molecules are characterised
by the bonding of a formally unsaturated system (in the organic chemistry sense, but
expanded to include aromatic systems) to a central atom, usually a metal atom.
4.4.3 Mononuclear coordination compounds
4.4.3.1 Formulae. The central atom is listed first. The formally anionic ligands appear next,listed in alphabetical order of the first symbols of their individual formulae. The neutral ligands follow, also in alphabetical order. Polydentate ligands are included in alphabetical order, the formula to be presented as discussed in Chapter 3. The formula for the entire coordination entity, whether charged or not, is enclosed in square brackets. For coordination formulae, the nesting order of enclosing marks is as given on p. 13. The charge on an ion is indicated in the usual way by use of a right superscript. Oxidation states of particular atoms are indicated by an appropriate roman numeral as a right superscript to the symbol of the atom in question, and not in parentheses on the line. In the formula of a salt containing coordination entities, cation always precedes anion, no charges are indicated and there is no space between the formulae for cation and anion.
Examples
1. [Co(NH3)6]Cl
2. [PtC14]2
3. [CoC1(NH3)5]C1
6. [Cr"(NCS)4(NH3)2]
4. Na[PtBrC1(N02)(NH3)J
7. [Fe"(CO)4]2
5. [CaC12{OC(NH2)2}2]
The precise form of a formula should be dictated by the needs of the user.
The precise form of a formula should be dictated by the needs of the user. For
example, it is generally recommended that a ligand formula within a coordination
formula be written so that the donor atom comes first, e.g. [TiCl3(NCMe)3], but this
is not mandatory and should not affect the recommended order ofligand citation. It
may also be impossible to put all the donor atoms first, e.g. where two donors are
present in a chelate complex: [Co(NH2CH2CH2NH2)3]3. Whether the ethane-l,2-
diamine is displayed as shown, or simply aggregated as [Co(C2H8N2)3]3t is a matter
of choice. Certainly there is a conflict between this last form and the suggestion that
the donor atoms be written first. The aim should always be clarity, at the expense of
rigid adherence to recommendations.
It is often inconvenient to represent all the ligand formulae in detail. Abbreviations
are often used and are indeed encouraged, with certain provisos. These are: the
abbreviations should all be written in lower case (with minor exceptions, such as Me,
Et and Ph) and preferably not more than four letters; with certain exceptions of wide
currency, abbreviations should be defined in a text when they first appear; in a
formula, the abbreviation should be enclosed in parentheses, and its place in the
citation sequence should be determined by its formula, as discussed above; and
particular attention should be paid to the loss of hydrons from a ligand precursor.
This last proviso is exemplified as follows. Ethylenediaminetetraacetic acid
should be rendered H4edta. The ions derived from it, which are often ligands in
coordination entities, are then (H3edta), (H2edta)2, (Hedta)3 and (edta)4. This
avoids monstrosities such as edta-H2 and edtaH_2 which arise if the parent acid is
represented as edta. A list of recommended abbreviations is presented in Table 4.5.
4.4.3.2 Names. The addition of ligands to a central atom is paralleled in name construction.
The names of the ligands are added to that of the central atom. The ligands are listed
in alphabetical order regardless of ligand type. Numerical prefixes are ignored in this
ordering procedure, unless they are part of the ligand name. Charge number and
oxidation number are used as necessary in the usual way.
Of the two kinds of numerical prefix (see Table 4.2), the simple di-, tn-, tetra-, etc.
are generally recommended. The prefixes bis-, tris-, tetrakis-, etc. are to be used only
with more complex expressions and to avoid ambiguity. They normally require
parentheses around the name they qualify. The nesting order of enclosing marks is as
cited on p. 13. There is normally no elision in instances such as tetraammine and the
two adjacent letters 'a' are pronounced separately.
The names of ligands recommended for general purposes are given in Table 4.6.
The names for anionic ligands end in -o. If the anion name ends in -ite, -ate or -ide, the ligand name is changed to -ito, -ato or -ido. The halogenido names are, by
custom, abbreviated to halo. Note that hydrogen as a ligand is always regarded as
anionic, with the name hydride. The names of neutral and cationic ligands are never
modified. Water and ammonia molecules as ligands take the names aqua and
ammine, respectively. Parentheses are always placed around ligand names, which
themselves contain multiplicative prefixes, and are also used to ensure clarity, but
aqua, ammine, carbonyl (CO) and nitrosyl (NO) do not require them.
The names of all cationic and neutral entities end in the name of the element,
together with the charge (if appropriate) or the oxidation state (if desired). The
names of complex anions require modification, and this is achieved by adding the
termination -ate. All these recommendations are illustrated in the following examples.
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Examples
1. Dichloro(diphenylphosphine)(thiourea)platinum(ii)
2. K4[Fe(CN)6]
3. [Co(NH3)6]C13
4. [CoCl(NH3)5}Cl2
5. [CoC1(N02)(NH3)4]Cl
6. [PtCl(NH2CH3)(NH3)2]Cl
7. [CuC12{OC(NH2)2}2]
8. K2[PdC14]
9. K[OsCl5N]
10. Na[PtBrC1(N02)(NH3)]
11. [Fe(CNCH3)6]Br2
12. [Ru(HSO3)2(NH3)4]
13. [Co(H20)2(NH3)4]C13
14. [PtC12(C5H5N)(NH3)]
15. Ba[BrF4]2
16. K[CrF4O]
17. [Ni(H20)2(NH3)4]S04
potassium hexacyanoferrate(ii)
potassium hexacyanoferrate(4—)
tetrapotassium hexacyanoferrate
hexaamminecobalt(iii) chloride
pentaamminechlorocobalt(2+) chloride
tetraamminechloronitrito-N-cobalt(iii) chloride
diamminechloro(methylamine)platinum(ii)
chloride
dichlorobis(urea)copper(ii)
potassium tetrachloropalladate(ii)
potassium pentachloronitridoosmate(2—)
sodium amminebromochloronitrito-
N-platinate( 1—)
hexakis(methyl isocyanide)iron(ii) bromide
tetraamminebis(hydrogensulfito)ruthenium(ii)
tetraamminediaquacobalt(iii) chloride
amminedichloro(pyridine)platinum(ii)
barium tetrafluorobromate(iii)
potassium tetrafiuorooxochromate(v)
tetraamminediaquanickel(ii) sulfate
***Table to be reformatted
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Designation of donor atom. In some cases, it may not be evident which atom in a
ligand is the donor. This is exemplified by the nitrito ligand in Examples 5 and 10,
p. 59. This can conceivably bind through an 0 or N atom. In simple cases, the donor
atom can be indicated by italicised element symbols placed after the specific ligand
name and separated from it by a hyphen, as demonstrated in those particular
examples. More complex examples will be dealt with below. With polydentate
ligands, this device may still be serviceable. Thus, dithiooxalate ion may be attached through S or 0, and formulations such as dithiooxalato-S,S' and dithiooxalato-0,0'should suffice. It could be necessary to use superscripts to the donor atom symbols if these need to be distinguished because there is more than one atom of the same kind to choose from.
Complicated examples are more easily dealt with using the kappa convention,
and this is particularly useful where a donor atom is part of a group that does not
carry a locant according to organic rules. The two oxygen atoms in a carboxylato
group demonstrate this. The designator i is a locant placed after that portion of the
ligand name that denotes the particular function in which the ligating atom is found.
The ligating atoms are represented by superscript numerals, letters or primes affixed
to the donor element symbols, which follow i without a space. A right superscript to
i denotes the number of identically bound ligating atoms.
Inclusion of structural information. The names described so far detail ligands and
central atoms, but give no information on stereochemistry. The coordination number
and shape of the coordination polyhedron may be denoted, if desired, by a
polyhedral symbol. These are listed in Table 4.4. Such a symbol is used as an affix in
parentheses, and immediately precedes the name, separated from it by a hyphen.
This device is not often used.
Geometrical descriptors, such as cis, trans, mer (from meridional) and fac (from
facial), have found wide usage in coordination nomenclature. The meaning is
unequivocal only in simple cases, particularly square planar for the first two and
octahedral for the others
More complex devices have been developed that are capable of dealing with all
cases. The reader is referred to the Nomenclature of Inorganic Chemistry, Chapter
10.
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Ionisation Isomers
Ionisation isomers:
Molecular structural formula is same. But different isomers give different ions in solution.
one isomer [PtBr(NH3)3]NO2 -> gives NO2- anions in solution
another isomer [Pt(NH3)3(NO2)]Br -> gives Br- anions in solution
Notice that both anions are necessary to balance the charge of the complex, and that they differ in that one ion is directly attached to the central metal but the other is not.
Geometric Isomers or Cis-Trans isomers
Geometric isomers are two or more coordination compounds which contain the same number and types of atoms, and bonds (i.e., the connectivity between atoms is the same), but which have different spatial arrangements of the atoms.
Not all coordination compounds have geometric isomers.
For example, in the square planar molecule, Pt(NH3)2Cl2, the two ammonia ligands (or the two chloride ligands) can be adjacent to one another or opposite one another.
Note that these two structures contain the same number and kinds of atoms and bonds but are non-superimposable. The isomer in which like ligands are adjacent to one another is called the cis isomer. The isomer in which like ligands are opposite one another is called the trans isomer.
For the common structures which contain two or more different ligands, geometric isomers are possible only with square planar and octahedral structures (i.e., geometric isomers cannot exist for linear and tetrahedral structures).
cis-[Co(NH3)4Cl2]+
Note that the two chloride ligands are adjacent to one another in this octahedral complex ion. In aqueous solution, this complex ion has a violet color.
trans-[Co(NH3)4Cl2]+
Note that the two chloride ligands are opposite one another in this complex ion. In aqueous solution, this complex ion has a green color.
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Material about all isomers - In syllabus only ionic and cis-trans are specially mentioned
http://www.chem.purdue.edu/gchelp/cchem/whatis2.html
Coordination Isomers
Coordination isomers are two or more coordination compounds in which the composition within the coordination sphere (i.e., the metal atom plus the ligands that are bonded to it) is different (i.e., the connectivity between atoms is different).
Not all coordination compounds have coordination isomers.
Coordination isomers have different physical and chemical properties.
Example
[Cr(NH3)5(OSO3)]Br
Note that the sulfate group is bonded to the Cr atom (via an O atom) and is within the coordination sphere. Note also the octahedral structure. The bromide counterion is needed to maintain charge neutrality with the complex ion (i.e., [Cr(NH3)5(OSO3)]+) and is not shown in the structure.
[Cr(NH3)5Br]SO4
Note that the bromine atom is bonded to the Cr atom and is within the coordination sphere. Note also the octahedral structure. The sulfate counterion is not shown in the structure.
Linkage Isomers
Linkage isomers are two or more coordination compounds in which the donor atom of at least one of the ligands is different (i.e., the connectivity between atoms is different).
This type of isomerism can only exist when the compound contains a ligand that can bond to the metal atom in two (or more) different ways. Some ligands that can form linkage isomers are shown below.
Not all coordination compounds have linkage isomers.
Linkage isomers have different physical and chemical properties.
[Co(NH3)4(NO2)Cl]+
Note that the N atom of the nitrite group is bonded to the Co atom. The nitrite group is written as "NO2" in the molecular formula (rather than "ONO") with the N atom nearest to the Co symbol to indicate that the N atom (rather than an O atom) is the donor atom. Note also the octahedral structure.
[Co(NH3)4(ONO)Cl]+
Note that one of the O atoms of the nitrite group is bonded to the Co atom. The nitrite group is written as "ONO" in the molecular formula (rather than "NO2") with the O atom nearest to the Co symbol to indicate that the O atom is the donor atom. Note also the octahedral structure.
Geometric Isomers
Geometric isomers are two or more coordination compounds which contain the same number and types of atoms, and bonds (i.e., the connectivity between atoms is the same), but which have different spatial arrangements of the atoms.
Not all coordination compounds have geometric isomers.
For example, in the square planar molecule, Pt(NH3)2Cl2, the two ammonia ligands (or the two chloride ligands) can be adjacent to one another or opposite one another.
Note that these two structures contain the same number and kinds of atoms and bonds but are non-superimposable. The isomer in which like ligands are adjacent to one another is called the cis isomer. The isomer in which like ligands are opposite one another is called the trans isomer.
For the common structures which contain two or more different ligands, geometric isomers are possible only with square planar and octahedral structures (i.e., geometric isomers cannot exist for linear and tetrahedral structures).
cis-[Co(NH3)4Cl2]+
Note that the two chloride ligands are adjacent to one another in this octahedral complex ion. In aqueous solution, this complex ion has a violet color.
trans-[Co(NH3)4Cl2]+
Note that the two chloride ligands are opposite one another in this complex ion. In aqueous solution, this complex ion has a green color.
Optical Isomers
Optical isomers are two compounds which contain the same number and kinds of atoms, and bonds (i.e., the connectivity between atoms is the same), and different spatial arrangements of the atoms, but which have non-superimposable mirror images. Each non-superimposable mirror image structure is called an enantiomer. Molecules or ions that exist as optical isomers are called chiral.
Not all coordination compounds have optical isomers.
The Two Enantiomers of CHBrClF
Note that the molecule on the right is the reflection of the molecule on the left (through the mirror plane indicated by the black vertical line). These two structures are non-superimposable and are, therefore, different compounds.
Pure samples of enantiomers have identical physical properties (e.g., boiling point, density, freezing point). Chiral molecules and ions have different chemical properties only when they are in chiral environments.
Optical isomers get their name because the plane of plane-polarized light that is passed through a sample of a pure enantiomer is rotated. The plane is rotated in the opposite direction but with the same magnitude when plane-polarized light is passed through a pure sample containing the other enantiomer of a pair.
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web sites
http://www.chem.purdue.edu/gchelp/cchem/whatis2.html
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