Coordination Compounds: Complete Concepts, Tricks and PYQs for NEET 2027

Coordination compounds are one of the most scoring Inorganic Chemistry chapters in NEET, because most questions follow fixed rules for identifying the coordination sphere, ligand charge, denticity and the oxidation state of the metal. A small slip, such as counting ions wrongly or mixing up primary and secondary valency, is enough to lose marks.

Check the difference between double and complex salts, ligands and denticity, coordination number and nomenclature, and then Werner’s theory, VBT, CFT and metal carbonyls. PW’s NEET resources, including like PW’s NEET notes and PYQs, MCQs and diagrams, can support your revision for each topic.

Origin of Coordination Compounds

Coordination compounds are formed when a metal atom or ion makes sigma coordinate bonds with a fixed number of molecules or ions called ligands. When two aqueous salts are mixed and heated until the water evaporates, an addition compound is formed. On dissolving it in water, it may behave as a double salt or a complex salt.

Important Double Salts

• Mohr’s salt: (NH4)2Fe(SO4)2.6H2O

• Carnallite: KMgCl3.6H2O

• Potash alum: KAl(SO4)2.12H2O

The water molecules in these formulas are crystalline water. Ferrous means iron in the +2 oxidation state, and ferric means iron in the +3 oxidation state.

Complex Salts and Partial Dissociation

In water, K4[Fe(CN)6] gives 4K+ and [Fe(CN)6]4-. The potassium ions separate, but the iron and six cyanide ligands stay together as one complex ion, which is called partial dissociation. The iron ion accepts electron pairs, the cyanide ions donate lone pairs, and six sigma coordinate bonds are formed. A coordinate bond is a type of covalent bond.

Coordination Sphere and Counter Ions

In K4[Fe(CN)6]:

• Coordination sphere: [Fe(CN)6]4-, which contains the central metal ion and its ligands.

• Counter ions: Ions outside the brackets, which separate readily in water.

If the coordination sphere is negative, positive counter ions are present outside it, and if it is positive, negative counter ions are present.

Number of Ions in Solution

• For [Complex]Cl3, the three chloride ions separate and the complex stays as one ion, so the total is 3 + 1 = 4 ions.

• For K4[Fe(CN)6], there are 4 K+ ions and one complex ion, so the total is 5 ions.

Memory tip: counter ions are released immediately in water, and the bracketed complex counts as one ion.

Central Metal Ion and Ligands

The central metal ion (CMI) acts as a Lewis acid because it accepts electron pairs. Effective complex formation needs vacant orbitals, small ionic size, high positive charge and a high charge-to-radius ratio. This is why d-block metals form a large number of complexes. Ligands donate electron pairs to the central metal ion and act as Lewis bases.

Geometry of Coordination Compounds

Octahedral complexes are the most common because of their stable, symmetrical structure, and tetrahedral complexes are also frequent. For coordination number 4, tetrahedral geometry has bonds directed towards the corners of a tetrahedron, while square planar geometry has all four bonds in one plane.

Ligands and Denticity

Denticity is the number of donor atoms of one ligand that form coordinate bonds simultaneously with the same metal. It refers to one ligand, not to the total number of bonds around the metal.

In an octahedral complex, six coordinate bonds are needed. This can be achieved by six monodentate ligands (6 x 1), three bidentate ligands (3 x 2), two tridentate ligands (2 x 3) or one hexadentate ligand (1 x 6).

Important Ligands

• Oxalato: bidentate, with two oxygen donor atoms

• Glycinato: bidentate, with one nitrogen and one oxygen donor atom

• Ethylenediamine: bidentate

• Diethylenetriamine: tridentate

• Triethylenetetramine: tetradentate

• EDTA4-: hexadentate, with two nitrogen and four oxygen donor atoms

Memory tip: EDTA = 2 nitrogen + 4 oxygen = 6 donor atoms, so it is hexadentate.

Ambidentate and Chelating Ligands

• Ambidentate ligands have more than one possible donor atom but coordinate through only one at a time, so their denticity is one. Examples are NO2-, CN- and SCN-. Memory tip: multiple donor choices, but one donation at a time.

• Chelating ligands coordinate through two or more donor atoms and form a ring with the metal. Ethylenediamine, oxalato, glycinato and EDTA are chelating ligands.

• Homoleptic complexes have only one kind of ligand, while heteroleptic complexes have more than one kind.

Oxidation State, Coordination Number and EAN

Three properties are tested most often.

• Coordination number: The total number of donor atoms directly bonded to the metal. For [Cr(C2O4)3]3-, oxalate is bidentate, so the coordination number is 3 x 2 = 6.

• Oxidation number: For [Cr(C2O4)3]3-, each oxalate has charge -2, so x + 3(-2) = -3 and x = +3.

• EAN: Atomic number - oxidation number + electrons donated by ligands. For K4[Fe(CN)6], EAN = 26 - 2 + 12 = 36.

IUPAC Nomenclature of Coordination Compounds

Follow this order: name the cation first, then the ligands, then the metal, and write the oxidation state in Roman numerals.

• Common prefixes are di, tri, tetra, penta and hexa. Ligands in heteroleptic complexes are arranged alphabetically, and numerical prefixes are ignored while alphabetising.

• For anionic complexes, the metal name takes the suffix “-ate”.

• If a ligand already contains a numerical prefix, use bis, tris or tetrakis, for example bis(ethylenediamine).

• For ambidentate ligands, show the donor atom, such as nitrito-N and nitrito-O.

• NH3 is spelt “ammine” with a double m, while amine derivatives use a single m.

Examples: K4[Fe(CN)6] is potassium hexacyanidoferrate(II), and Ni(CO)4 is tetracarbonylnickel(0).

Werner’s Theory

Werner proposed two types of valencies.

Primary valencies are shown by dotted lines and secondary valencies by solid lines. Silver nitrate gives an AgCl precipitate only with ionisable chloride. For [Co(NH3)6]Cl3, [Co(NH3)5Cl]Cl2 and [Co(NH3)4Cl2]Cl, the moles of AgCl formed are 3, 2 and 1 respectively.

VBT and CFT

Valence Bond Theory (VBT)

VBT explains bonding through overlap between vacant hybrid orbitals of the metal and filled donor orbitals of the ligands. The steps are: oxidation state, d configuration, ligand strength, pairing, hybridisation, geometry and magnetism.

• d2sp3: inner-orbital octahedral

• sp3d2: outer-orbital octahedral

• sp3: tetrahedral

• dsp2: square planar

Strong ligands such as CN- cause pairing and may form low-spin, inner-orbital complexes. Weak ligands such as F- generally do not cause pairing and form high-spin, outer-orbital complexes.

Crystal Field Theory (CFT)

• In an octahedral field, the d orbitals split into a lower-energy t2g set and a higher-energy eg set.

• Strong ligands give large splitting and low-spin configurations, and weak ligands give small splitting and high-spin configurations.

• In an octahedral complex, eg0 generally indicates an inner-orbital d2sp3 complex, while occupation of eg generally indicates an outer-orbital sp3d2 complex.

• In a tetrahedral field, the splitting is reversed, with the e set lower and the t2 set higher. The splitting is smaller than in octahedral complexes, so tetrahedral complexes are generally high spin.

Synergic Bonding in Metal Carbonyls

In metal carbonyls, CO donates an electron pair to the metal through a sigma bond, and the metal returns electron density to the pi-antibonding orbital of CO. This is called synergic bonding, and it means the metal-carbon bond has both sigma and pi character.

Back-donation causes:

• A decrease in C-O bond order

• An increase in C-O bond length

• An increase in metal-carbon bond strength

• A decrease in metal-carbon bond length

Greater electron density on the metal causes greater back-donation.

Important Questions for NEET 2027

Q1. d block metals form more no of complexes because

(1) They have high E.A.
(2) They can show variable O.S.
(3) They have small size and high charge.
(4) They have vacant ‘d’ orbitals.

Q2. [NEET-1995] The coordination number and oxidation state of Cr in K3[Cr(C2O4)3] are respectively:

(1) 3 and +3
(2) 3 and 0
(3) 6 and +3
(4) 4 and +2

Q3. In Fe(CO)5 Metal-Carbon bond have:

(1) π character
(2) σ character
(3) Both σ & π character
(4) There is no bond between metal & carbon

Q4. Which of following pairs of name and formula of complexes, is not correct?

(1) Tetraammine copper (II) sulphate ........ [Cu(NH3)4]SO4
(2) Diamminesilver (I) chloride ....... [Ag(NH3)2]Cl
(3) Potassium hexacyanidoferrate (III) ....... K4[Fe(CN)6]
(4) Potassiumamminepentachloridoplatinate (IV) ......... K[PtCl5(NH3)]

Q5. Primary valence of compounds PtCl4.5NH3, PtCl4.4NH3 and PtCl4.3NH3 are respectively:

(1) 5, 4, 3
(2) 3, 4, 5
(3) 4, 4, 4
(4) 5, 5, 3

Key Tips for NEET 2027 Coordination Compounds Revision

These tips help you avoid the most common slips in this chapter.

• Count ions carefully. Counter ions outside the brackets are separate, and the bracketed complex counts as one ion.

• Separate denticity from coordination number. Denticity belongs to one ligand, while coordination number is the total number of donor atoms bonded to the metal.

• Use the oxidation-state method. Write the charge of the complex, subtract the total ligand charge and solve for the metal.

• Learn ligand strength and spin together. Strong ligands such as CN- lead to pairing and low spin, and weak ligands such as F- lead to high spin. Tetrahedral complexes are generally high spin.

• Practise nomenclature daily. Revise prefixes, alphabetical order, the “-ate” suffix and bis/tris from a formula sheet until they become automatic.

• Revise in layers. Use mind maps for quick recall of the whole chapter, then solve MCQs and previous year questions to test application.

Coordination compounds become straightforward once you treat the chapter as a chain: identify the coordination sphere, count donor atoms, find the oxidation state, then apply Werner’s theory, VBT and CFT to predict geometry and magnetism. Revise nomenclature and ligand strength regularly, and practise PYQs so that the commonly tested patterns become second nature. Short, repeated revision in the final weeks is enough to make this chapter reliable.

Related Resource

Coordination Compounds NEET Notes

Coordination Compounds NEET Formulae

Coordination Compounds NEET PYQs

Coordination Compounds MCQs

Coordination Chemistry NEET Mindmaps