Target Audience: These notes on Co-ordination Compounds
are meant for college freshmen, or high school students in Grades 11 or 12.
They might be of use to Indian students preparing for the ISC or CBSE Class 11 and 12 Examinations, IIT JEE (main and advanced), AIEEE; students from across the world preparing for their A Level Examinations, IB (International Baccalaureate) or AP Chemistry.
This compilation of notes has been prepared by Aman Kaushal of the Indian Institute of Information Technology Gwalior
Coordination compounds may be defined as a compound that result from the combination of apparently saturated molecules of different species. Coordination compound containing one or more coordinate bonds, which is a link between a pair of electrons in which both electrons are donated by one of the atoms.
→ [Ag(CN)2]- ion is formed by the union of one Ag+ ion with two CN- ions.
Two main Components which are responsible in formation of Coordination Compound:-
1. An Acceptor:-
(i) It can accept a pair of electrons from the donor atom.
(ii) It is usually a metal.
eg:- [Fe(CN)6]4-, Fe++ ion is a acceptor. It acts as a Lewis acid.
2. A Donor:-
(i) It can donate a pair of electrons.
(ii) They are Electron rich.
eg:- It may be neutral( NH3,, H2O) or negatively Charged Species ( CN-, Cl-). It acts as a Lewis Base.
These Donor atoms are called Ligands. Sometimes, positively charged species are not found.
⇨Ligands is said to be Unidentate if it has only one pair of electrons that it can donate
eg:- Cl- , Br-, H2O
⇨ Ligand is said to be bidentate if it has two electron pairs available for donation.
⇨ Ligand is said to be Ambidentate if it can be coordinated to the metal or metal ion through either side.
eg:- nitrite ion(nitro -NO2, Nitrito -ONO-)
1. The total number of ligands attached to a central ion is said to be the coordination number of that ion
eg:- Coordination number of Ni2+ in [Ni(NH3)6]Cl2 is 6.
2. The molecules or ions bonded directly to the central ion constitute written with square brackets.
3. Oxidation state of central metal ion is the charge carried by a complex ion and is the algebraic sum of charges carried by central ion and ligands coordinated to it.
eg:- [Co(NH3)Cl3], Charge on central metal atom x + (0*3)+(-1*3)=0
so, x= +3
• The positive ion (cation) comes first, followed by the name within the coordination sphere, followed by the negative ion (anion).
– These ions are not in the coordination sphere.
– Diamminesilver(I)chloride and potassium hexacyanoferrate (III).
• The inner coordination sphere is enclosed in brackets in the formula. Within this sphere, the ligands are named before the metal, but in formulas the metal ion is written first.
– Tetraamminecopper(II) sulfate and hexaamminecobalt(III) chloride.
• Ligands are named in alphabetical order (name of ligand, not prefix)
– [Co(NH3)4Cl2]+ and [Pt(NH3)BrCl(CH3NH2)]+2
• Anionic ligands are given an ‘o’ suffix. Neutral ligands retain the usual name.
– Coordinated water is called ‘aqua’.
– Chloro, Cl-
– Sulfato, SO42-
• The calculated oxidation number of the metal ion is placed as a Roman numeral in parentheses after the name of the coordination sphere.
– [Pt(NH3)4]+2 and [Pt (Cl)4]-2
– A suffix ‘ate’ is added to the metal ion if the charge is negative.
• The prefixes cis- and trans- designate adjacent and opposite geometric location, respectively.
– trans-diamminedichloroplatinum(III) and
– cis- tetraamminedichlorocobalt(III)
• Bridging ligands between two metal ions have the prefix ‘m’.
– m-amido-m-hydroxo bis(tetraamminecobalt)(IV)
Performed systematic studies to understand bonding in coordination compounds.
- Organic bonding theory and simple ideas of ionic charges were not sufficient.
Two types of bonding
1. Primary – positive charge of the metal ion is balanced by negative ions in the compound.
- 2. Secondary – molecules or ion (ligands) are attached directly to the metal ion.
It is a series in which ligands are arranged in order of increasing magnitude of crystal field splitting.
I-< Br-< Cl-< NO3-< F-< OH- < OX2-< H2O < Py < en < dipy < NO2- < CN- <CO
1. Structural Isomerism
----> Hydrate Isomerism
eg:- [Co(H2O)6]Cl3 & [Co(H2O)4Cl2]Cl.2H2O
-----> Ionisation Isomerism
eg:- [Co(NH3)5SO4]Br & [Co(NH3)5Br]SO4
-----> Linkage Isomerism
eg:- [Co(NH3)5NO2]Cl & [Co(NH3)5 ONO]Cl
-----> Coordination Isomerism
eg:-> [Cr(NH3)6][Co(CN)6] & [Cr(CN)6][Co(NH3)6]
------> Geometrical Isomerism
Complex with general formula; [MA2X2]+/-n, [MA2XY]+/-n, [ MABX2] +/-n, [MABXY]+/-n, [M(AB)2]+/-n
-------> Optical Isomerism
Square Planar complexes do not Show optical Isomerism.
Tetrahedral complexes with formula MABCD or M(AB)2 show optical isomerism