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Coordination Compounds: one-page cheat sheet

NEET Chemistry · Unit 12 · Inorganic Chemistry

ScoreChemby CSJ Sir
Weightage: 3–4 questions · 12–16 marks

1 · Werner's theory

Primaryionisable; equals the oxidation state

Secondarynon-ionisable; equals the coordination number

Complex	AgCl	Ions
[Co(NH $_{3}$ ) $_{6}$ ]Cl $_{3}$	3	4
[Co(NH $_{3}$ ) $_{5}$ Cl]Cl $_{2}$	2	3
[Co(NH $_{3}$ ) $_{4}$ Cl $_{2}$ ]Cl	1	2
[Co(NH $_{3}$ ) $_{3}$ Cl $_{3}$ ]	0	0

Only Cl $^{-}$ outside the bracket precipitates. A double salt (Mohr's salt) gives all its ions in water; a complex keeps its coordination entity.

2 · Ligands

UnidentateCl

^{-}

, H

_{2}

O, NH

_{3}

, CN

^{-}

Didentateen, oxalate (ox), glycinate (gly)

Polydentatedien (3); EDTA

^{4 -}

(6: 2 N + 4 O)

AmbidentateNO

_{2}^{-}

(N or O); SCN

^{-}

(S or N)

Homolepticone kind of ligand: [Co(NH

_{3}

)

_{6}

]

^{3 +}

Heterolepticmore than one kind: [Co(NH

_{3}

)

_{4}

_{2}

]

^{+}

Chelate rings of 5 or 6 members make a complex more stable (chelate effect).

3 · IUPAC naming

Ordercation first, then anion; ligands alphabetical, then metal

Anionic ligandends in -ido: chlorido, cyanido; neutral: ammine, aqua, carbonyl

Multipliersdi, tri; bis, tris for complex ligands

Anionic complexmetal ends in -ate: ferrate, cuprate, argentate, aurate

Oxidation no.Roman numeral in brackets

[Ag(H $_{2}$ O) $_{2}$ ][Ag(CN) $_{2}$ ] is diaquasilver(I) dicyanidoargentate(I).

4 · Isomerism

Type	Example
Ionisation	[Co(NH $_{3}$ ) $_{5}$ Br]SO $_{4}$ / [Co(NH $_{3}$ ) $_{5}$ SO $_{4}$ ]Br
Hydrate	[Cr(H $_{2}$ O) $_{6}$ ]Cl $_{3}$ / [CrCl(H $_{2}$ O) $_{5}$ ]Cl $_{2}$ ·H $_{2}$ O
Coordination	[Co(NH $_{3}$ ) $_{6}$ ][Cr(CN) $_{6}$ ] / [Cr(NH $_{3}$ ) $_{6}$ ][Co(CN) $_{6}$ ]
Linkage	[Co(NH $_{3}$ ) $_{5}$ NO $_{2}$ ]Cl $_{2}$ / [Co(NH $_{3}$ ) $_{5}$ ONO]Cl $_{2}$
Geometrical	cis/trans (MA $_{2}$ B $_{2}$ square planar); fac/mer (MA $_{3}$ B $_{3}$ )
Optical	[Co(en) $_{3}$ ] $^{3 +}$ , cis-[CoCl $_{2}$ (en) $_{2}$ ] $^{+}$

trans-[CoCl $_{2}$ (en) $_{2}$ ] $^{+}$ has a mirror plane: optically inactive. Tetrahedral complexes show no geometrical isomerism.

5 · Oxidation state & coordination number

Complex	O.S.	C.N.
K $_{4}$ [Fe(CN) $_{6}$ ]	+2	6
[Co(en) $_{3}$ ]Cl $_{3}$	+3	6
[Pt(NH $_{3}$ ) $_{2}$ Cl $_{2}$ ]	+2	4
[Cu(NH $_{3}$ ) $_{4}$ ]SO $_{4}$	+2	4

C.N. counts donor atoms, so each en adds 2.

6 · Valence bond theory

Complex	Hybrid.	Shape	Unp. $e^{-}$
[Co(NH $_{3}$ ) $_{6}$ ] $^{3 +}$	$d^{2} s p^{3}$	octahedral	0
[CoF $_{6}$ ] $^{3 -}$	$s p^{3} d^{2}$	octahedral	4
[Ni(CN) $_{4}$ ] $^{2 -}$	$d s p^{2}$	sq. planar	0
[NiCl $_{4}$ ] $^{2 -}$	$s p^{3}$	tetrahedral	2
[Ni(CO) $_{4}$ ]	$s p^{3}$	tetrahedral	0
[Fe(CO) $_{5}$ ]	$d s p^{3}$	trig. bipyramidal	0

7 · Crystal field theory

Octahedral

t_{2 g}

- 0.4 Δ_{o}

;

e_{g}

+ 0.6 Δ_{o}

Δ_{t} = \frac{4}{9} Δ_{o}, CFSE = [- 0.4 n (t_{2 g}) + 0.6 n (e_{g})] Δ_{o} + m P

Δ_{o} > P

low spin (strong field), electrons pair in

t_{2 g}

Δ_{o} < P

high spin (weak field)

$Δ_{t}$ is always small, so tetrahedral complexes are nearly always high spin.

I^{-} < Br^{-} < SCN^{-} < Cl^{-} < F^{-} < OH^{-} < ox^{2 -}

ox^{2 -} < H_{2} O < NCS^{-} < NH_{3} < en < CN^{-} < CO

8 · Colour

Origin

d

–

d

transition; the colour seen is complementary to the light absorbed

^{3 +}

aqua[Ti(H

_{2}

_{6}

]

^{3 +}

absorbs blue-green, appears violet

^{2 +}

aqua[Cu(H

_{2}

_{4}

]

^{2 +}

absorbs red, appears blue

Colourless

d^{0}

and

d^{10}

ions (Sc

^{3 +}

, Zn

^{2 +}

)

9 · Carbonyls & biology

σ

bondCO lone pair

\to

empty metal orbital

π

back-bondfilled metal

d

\to

π^{*}

M–C bond order rises; C–O bond order falls.

Complex	Metal / use
Chlorophyll	Mg $^{2 +}$
Haemoglobin	Fe $^{2 +}$
Vitamin B $_{12}$	Co $^{3 +}$
cis-Platin	Pt; anticancer
Wilkinson's catalyst	Rh; hydrogenation
EDTA	removes Pb poisoning

Seen in NEET 2025

Which of the four Co(III) ammine chlorides conducts least in water?

[Co(NH $_{3}$ ) $_{3}$ Cl $_{3}$ ] is neutral and gives no ions.

Seen in NEET 2020

Increasing field strength:

SCN^{-} < F^{-} < C_{2} O_{4}^{2 -} < CN^{-}

Six traps to remember

Only chloride outside the coordination sphere is precipitated by AgNO $_{3}$ .
An anionic complex takes -ate (often a Latin stem: ferrate, cuprate, argentate).
[NiCl $_{4}$ ] $^{2 -}$ is tetrahedral and paramagnetic; [Ni(CN) $_{4}$ ] $^{2 -}$ is square planar and diamagnetic.
$Δ_{t} = \frac{4}{9} Δ_{o}$ , so tetrahedral complexes are almost always high spin.
cis-[CoCl $_{2}$ (en) $_{2}$ ] $^{+}$ is optically active; the trans isomer is not.
In metal carbonyls, back-donation weakens C–O and strengthens M–C.

Coordination Compounds: free one-page cheat sheet