Coordination Compounds
- Introduction
Complex compounds or coordination compounds are those molecular compounds which retain their identity in solid as well as in solution are known as complex compounds.
Example,
K [Fe(CN)]+H₂O 4K(aq) + [Fe(CN)]+(aq).
(2)MOLECULAR OR ADDITION COMPOUNDS
When a solution containing two or more simple stable compounds in molecular proportions is allowed to evaporate, it produces crystals of new substances known as molecular or addition compounds.
Example:
KCI+MgCl2 + 6H2O
KCLMgCl2.6H2O
(Camallite) [Cu(NH3)4] SO4 (Tetrammine copper (II) sulphate) CuSO4+4NH3 →
(3) Types of Molecular compounds
3.1. Double Salt
A double salt is a substance formed by combining two different salts that crystallize as a single substance but ionize as two distinct salts when dissolved inwater. These salts lose their identity in solution, which means that when dissolved in water, they test positive for all of the ions present in the salt. eg. Mohr’s salt, potash alum.
Example:
FeSO4 (NH4)2SO4.6H2O→ Fe2+ (aq)+ 6H2O+2NH4 (aq) + 2SO4 (aq)
3.2.Coordination Compounds
A coordination compound is a molecular compound formed by the combination of two or more simple molecular compounds that retains its identity both solid and dissolved.
Example:
[Cu(NH3)4]SO4 [Cu(NH3)4]2+ + SO42-
(4)COORDINATION COMPOUNDS:
A ligand, a central atom, a complex ion, a cation, or an anion make up a coordination compound. In general, the complex ion is written in a square box, and the ion (cation or anion) is written outside the complex ion.
eg:
[Co(NH3)6]Cl3
[Complex ion] anion
General formula: A [ML] [ML] By where M is the central metal atom/ion, L is the ligand, A is the cation and B is the anion.
(4)COORDINATION ENTITY:
It is the fixed central metal atom or ion that is bonded to a specific number of ions or molecules. Six ammonia molecules, for example, are surrounded by three chloride ions in \lceil Co(NH_{3}){6}\rceil Cl{3} , a coordination entity.Ligands
The ions or molecules bound to the central atom/ion in the coordination entity are called ligands.
Types of Ligands
(i) Unidentate, a ligand which is bound to a metal ion through a single donor atom.
e.g., H₂O, NH, CO, CI, NH, etc.
(ii) Didentate, a ligand which is bound to a metal ion through a two donor atoms.
e.g..
COO
CH-NH
CH-NH,
COO
ethylene diamine(iii) Polydentate, a ligand which is bound to a metal ion through a several donor atoms.
e.g., ethylene diamine tetraacetate ion [EDTA].
(iv) Ambidentate ligands, which can ligate through two different atoms.
e.g.,-NO-ONO, – SCN-NCS etc.
(v) Chelate ligands, these may be a di- or polydentate ligand which form closed ring with central metal ion. Closed ring is known as chelate ring. Number of more chelate ring in complex, complex will be more stable. The number of such ligating groups is called the denticity.
Coordination sphere: –
[Mix]”
کا A/T alphabet order
Outside region:-
lonisation sphere
Free ion
Cation
Naming of ligands
-O-suffix provided to the name of anionic ligands.
-ium suffix provided to the name of cationic ligands.
Anionic ligands ending with-ide are named by replacing -ide with suffix -o or replacing -e by-o.
Ligands whose names end in ite or ate become -ito or -ato, i.e., by replacing the ending -e with -o.Coordination Number
Total no of e pair accepted by CMA
- [Ni(dmg),]
- [Pt(trien)]Cl
- [Fe(EDTA)]
4
6
SAVE
The number of atoms in a ligand that directly bonded to the central metal atom or ion by coordinate bond is called coordination number of metal atom or ion.
- [Co(en) Ox]Cl
Some important points
6
- Generally C.N of monovalent cation is two and four except
Ex. [Cu(CN)] [Cu(CN)] CN-4
- Generally C.N of bivalent cation is four and six except.
Pt, Pd only 4
- Generally C.N. of trivalent cation is six except some exception4. C.N of tetravalent cation is 6.
- C.N of CMA depends upon charge of CMA, six of CMA size of ligands and concentration of ligand.
Effective Atomic Number (EAN)
Total number of e of CMA after accepting e pair from ligands
ΕΝΑ = 2-(0.5) + 2 x C.N.
- K. [Fe(CN)]26-(+2)+2×636 [K]r O.S. +2, C.N. = 6
- K, [Fe(CN)26-(+3)+2×635 [Kr] O.S. +3, C.N. = 6
- [Fe(n-CH), 26-(+2)+2×6 = 36 [Kr] O.S. +2, C.N. = 6
Sidgwick rule
If EAN of CMA in metal carbonyl is equal to Atomic number of nearest inert gas then the stability of metal is high(a) [Mn(CO)] [Mn(CO),] Stability EAN-35 EAN-36
(b) [V(CO)] reduction [V(CO)] More stable EAN = 35 EAN = 36
(c) [Fe(CO) neither oxidising nor reducing Sidgwick rule is appliable only for metal
carbonyl.Oxidation Number of Central Metal Atom
It is defined as the charge that the central metal ion would have if all ligands and electron pairs were removed. It is computed as follows:
Example:
K, [Fe(CN)]→4K +[Fe(CN)] Charge on the complex ion is -4. Let charge on Fe be x. Now, the charge on cyanide ions is -1. ⇒x+6x(-1)=-4 x=+2 Hence, the oxidation number of Fe is +2 (II).Homoleptic and Heteroleptic Complexes
Homoleptic complexes are those in which the central atom is coordinated with only one type of ligand, such as [Co(NH,),]” Hetroleptic complexes are those in which the central atom is coordinated with more than one type of ligand, such as [Co(NH), Cl,].Nomenclature of Coordination Compounds
Cationic Complex [Cr(NH),(H₂O),]CI
triamminetriaquachromium (III) chloride
(i) Prefixes mono, di, tri, etc, are used to indicate the number of the individual ligands and ligands are named in an alphabetical order.
(ii) Central metal atom and oxidation state indicated by Roman numeral in parenthesis.
(iii) Name of ionisable anion.
Anionic Complex K[Fe(CN)]
Potassium hexacyanoferrate (III)
(i) Name of ionisable metal and oxidation state
(ii) Name of ligand in an alphabetical order
(iii) Central metal atom + ate and oxidation state
Neutral Complex [Pt(NH), CI(NO₂)]
Diammine chloronitrito-N-platinum (II)
(i) Name of ligands in an alphabetical order
(ii) Central metal atom and oxidation stateIsomerism in Coordination Compounds
Stereo isomerism and structural isomerism are the two principal types of isomerisms which are known among coordination compounds.
Stereo Isomerism
It occurs due to different arrangements of ligands around central metal atom. It is of two types: geometrical isomerism and optical isomerism.
Geometrical Isomerism
It arises in heteroleptic complexes due to different possible geometric arrangements of the ligands. Important examples of this behavior are found in square planar and octahedral complexes.
(i) Square planar complex of formula [MX,L,] (X and L are unidentate), the two ligands X may be arranged adjacent to each other in a cis isomer, or opposite to each other in a trans isomer.
e.g., [Pt(NH),CI]
(ii) Square planar complex of the type [MABXL] (where A, B, X, L are unidentates) shows three isomers two cis and one trans. Such isomerism is not possible for tetrahedral geometry. e.g., [Pt(NH)(Br)(Cl)(Py)]
(iii) Octahedral complexes of formula [MX,L], in which the two ligands X may be oriented cis or trans to each other. e.g., [Co(NH), CI]
(iv) Octahedral complexes of formula |MX, A, where X are unidentates and A are didentate and form cis and trans isomers. e.g., [CoCl₂(en),]
(v) Octahedral coordination entities of the type Mab, like [Co(NH), (NO₂), ]. If three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral face, we have the facial (fac) isomer. When the positions are around the meridian of the octahedron, we get the meriodional (mer) isomer.
Optical Isomerism
It arises when mirror images cannot be superimposed on one another. These mirror images are called as enantiomers. The two forms are called dextro (d)
and laevo (1).
Optical isomerism is common in octahedral complexes but at least one didentate ligand should be present.
e.g., [Co(en),], [PtCl, (en), etc.
Structural Isomerism
In structural isomerism, isomers have different bonding pattern. Different types of structural isomerism is as follows:(i) Linkage isomerism, arises in a coordination compound containing ambidentate ligand.
e.g., [Co(NH), (NO₂)]CI,
[Co(NH), (ONO)]Cl₂
(ii) Coordination isomerism, arises from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex. e.g
., [Co(NH)][Cr(CN)] [Cr(NH)][Co(CN)]
(iii) Ionisation isomerism, when the ionisable anion exchange with anion ligand.
e.g.. [CO(NH),SO ]Br and [Co(NH) Br]SO
(iv) Solvate isomerism, is also known as ‘hydrate isomerism’. In this case water is involved as a solvent.
e.g.. [Cr(H₂O)]Cl,, [Cr(H₂O),Cl, H₂O, [Cr(H₂O) Cl,]C1.2H₂OVALENCE BOND THEORY
Valence Bond Theory (VBT) can explain the bonding in coordination compounds because the d orbitals of the majority of transition metal complexes are incomplete. Valence bond considers orbital hybridization because penultimate d-orbitals are close in energy to s and p-orbitals of the outermost shell, allowing for various types of hybridization.
The following assumption is made by VBT:
(i) The central metal ion has a number of empty orbitals that can accept electrons donated by the ligands. The coordination number of the metal ion for the specific complex is equal to the number of empty d-orbitals.
(ii) Strong bonds are formed when the metal orbitals and ligand orbitals overlap. The greater the extent of overlapping, the more stable the complex. Different orbitals (s, p, or d) hybridize to form a set of equivalent hybridized orbitals that participate in ligand bonding.
(iii) Each ligand contributes two electrons to the central metal ion/atom.
(iv) The inner orbitals contain non-bonding metal electrons that do not participate in chemical bonding.
(v) A complex is paramagnetic if it contains unpaired electrons. The complex is diamagnetic if it does not contain an unpaired electron.
(vi) Under the influence of a strong ligand (CN, CO), electrons can be forced to pair up, thereby violating Hund’s rule of multiplicity.Limitations of VBT
- The change in ligand and metal ion properties could not be explained.
- The valence bond theory is silent on why some complexes are more labile than others.
- The VBT does not explain the existence of inner and outer orbital complexes satisfactorily.
- The VBT was unable to explain the color of complexes.CRYSTAL FIELD THEORY
The valence bond theory is less widely accepted than the Crystal Field Theory. It is assumed that the attraction between a complex’s central metal and its ligands is purely electrostatic. The following assumptions are made in the crystal field.
- Ligands are considered point charges.
- Metal orbitals and ligand orbitals have no interaction.
- In the free atom, all of the d orbitals on the metal have the same energy (that is, they are degenerate). However, when a complex is formed, the ligands destroy the degeneracy of these orbitals, resulting in different energies for the orbitals.Werner’s Theory
(i) In complex compounds, metal atom exhibit two types of valencies primary valency and secondary valency.
(ii) Primary valencies are satisfied by anions only while secondary valencies are satisfied by ligands. Primary valency depends upon oxidation number of central metal atom while secondary valency represents coordination number of central metal atom.
(iii) Primary valencies are ionisable and are non-directional while secondary valencies are non-ionisable and directional. Therefore, geometry of complex is decided by secondary valencies