Essential Ligands: JEE Master List
Master the formulas, charges, denticities, and special properties of every crucial ligand required for IUPAC naming, isomerism, and crystal field theory.
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1. Neutral Monodentate Ligands
Ammine
$$\ce{NH3}$$
Charge: 0
Denticity: 1
Donor: N
A strong field ligand in most complexes (except with $+2$ 3d metals like $Ni^{2+}$ where it acts border-line). Notice the double 'm' in naming!
Aqua
$$\ce{H2O}$$
Charge: 0
Denticity: 1
Donor: O
A classic weak field ligand according to the Spectrochemical Series. Forms high-spin complexes.
Carbonyl
$$\ce{CO}$$
Charge: 0
Denticity: 1
Donor: C
The strongest field ligand. Exhibits synergic bonding ($\sigma$-donor and $\pi$-acceptor), heavily stabilizing low oxidation states of metals.
Nitrosyl
$$\ce{NO}$$
Charge: 0 / +1
Denticity: 1
Donor: N
Usually considered $+1$ (Nitrosonium) in classic complexes like the Brown Ring complex, acting as a strong three-electron donor.
Pyridine (py)
$$\ce{C5H5N}$$
Charge: 0
Denticity: 1
Donor: N
A heterocyclic neutral ligand. Acts as a moderately strong field ligand, similar to ammine.
Triphenylphosphine (PPh3)
$$\ce{P(C6H5)3}$$
Charge: 0
Denticity: 1
Donor: P
A bulky, strong field ligand ($\pi$-acceptor) famously used in Wilkinson's catalyst.
2. Anionic Monodentate Ligands
Cyano
$$\ce{CN^-}$$
Charge: -1
Denticity: 1
Donor: C
A very strong field ligand ($\pi$-acceptor). Almost always forces pairing of electrons, resulting in low-spin complexes.
Hydroxido
$$\ce{OH^-}$$
Charge: -1
Denticity: 1
Donor: O
Weak field ligand. Old IUPAC name was 'hydroxo', currently preferred is 'hydroxido'.
Halido (Fluoro, Chloro...)
$$\ce{X^-}\; \text{(F, Cl, Br, I)}$$
Charge: -1
Denticity: 1
Donor: Halogen
The weakest field ligands in the spectrochemical series (especially $I^-$). Almost always form high-spin complexes.
Azido
$$\ce{N3^-}$$
Charge: -1
Denticity: 1
Donor: N
A pseudo-halide. It is a linear, highly symmetrical anion that acts as a weak field ligand.
Amido / Imido / Nitrido
$$\ce{NH2^-} / \ce{NH^{2-}} / \ce{N^{3-}}$$
Charge: -1 / -2 / -3
Denticity: 1
Donor: N
Nitrogen-based anionic ligands resulting from the progressive deprotonation of ammonia.
Superoxido / Peroxido
$$\ce{O2^-} \text{ / } \ce{O2^{2-}}$$
Charge: -1 / -2
Denticity: 1 (or 2)
Donor: O
Reactive oxygen species that can bind to metals. Can act as bridging ligands in binuclear complexes.
3. Ambidentate Ligands (Linkage Isomerism)
Thiocyanato / Isothiocyanato
$$\ce{SCN^-} \text{ / } \ce{NCS^-}$$
Charge: -1
Denticity: 1
Donor: S or N
Can bind through Sulfur (thiocyanato) or Nitrogen (isothiocyanato). Responsible for Linkage Isomerism.
Nitrito-N / Nitrito-O
$$\ce{NO2^-} \text{ / } \ce{ONO^-}$$
Charge: -1
Denticity: 1
Donor: N or O
Nitrito-N ($NO_2$) is a strong field ligand, whereas Nitrito-O ($ONO$) is a weak field ligand.
Cyanato / Isocyanato
$$\ce{OCN^-} \text{ / } \ce{NCO^-}$$
Charge: -1
Denticity: 1
Donor: O or N
Another classical example of an ambidentate pseudo-halide ligand.
Thiosulfato
$$\ce{S2O3^{2-}}$$
Charge: -2
Denticity: 1
Donor: S or O
Often binds through sulfur. Extensively used in photography to dissolve AgBr into a soluble complex.
4. Bidentate & Chelating Ligands
Ethylenediamine (en)
$$\ce{H2N-CH2-CH2-NH2}$$
Charge: 0
Denticity: 2
Donor: 2 N
Symmetrical bidentate ligand. Forms highly stable 5-membered chelate rings (Chelate Effect).
Oxalato (ox)
$$\ce{C2O4^{2-}}$$
Charge: -2
Denticity: 2
Donor: 2 O
Symmetrical anionic bidentate ligand. Often forms optically active complexes when 3 are bound to a metal (e.g., $[Co(ox)_3]^{3-}$).
Glycinato (gly)
$$\ce{NH2-CH2-COO^-}$$
Charge: -1
Denticity: 2
Donor: N & O
Unsymmetrical bidentate ligand. Can exhibit geometrical isomerism (Facial/Meridional or Cis/Trans) even in homoleptic-like complexes.
Dimethylglyoximato (dmg)
$$\ce{DMG^-}$$
Charge: -1
Denticity: 2
Donor: 2 N
Forms the famous rosy red precipitate with $Ni^{2+}$. The complex is stabilized by extra intramolecular hydrogen bonding.
Acetylacetonato (acac)
$$\ce{CH3COCHCOCH3^-}$$
Charge: -1
Denticity: 2
Donor: 2 O
Forms a highly stable, 6-membered planar chelate ring stabilized by extensive resonance.
2,2'-Bipyridine (bpy)
$$\ce{C10H8N2}$$
Charge: 0
Denticity: 2
Donor: 2 N
A strong field bidentate ligand. Forms highly stable, colorful complexes with transition metals like Ru and Fe.
1,10-Phenanthroline (phen)
$$\ce{C12H8N2}$$
Charge: 0
Denticity: 2
Donor: 2 N
A rigid, planar bidentate ligand. Very strong field ligand, commonly used in colorimetric analysis (e.g., Ferroin).
5. Polydentate Ligands
EDTA
$$\text{Ethylenediaminetetraacetato}$$
Charge: -4
Denticity: 6
Donor: 2 N, 4 O
The ultimate hexadentate chelating agent. Used extensively for treating lead poisoning and estimating hardness of water. Can also act as a tetradentate or pentadentate ligand depending on pH.
Diethylenetriamine (dien)
$$\ce{NH2-CH2-CH2-NH-CH2-CH2-NH2}$$
Charge: 0
Denticity: 3
Donor: 3 N
A neutral tridentate ligand. Forms two fused 5-membered chelate rings upon binding.
Triethylenetetramine (trien)
$$\ce{N4C6H18}$$
Charge: 0
Denticity: 4
Donor: 4 N
A linear tetradentate ligand forming three highly stable 5-membered chelate rings around the central metal atom.
6. Organometallic & $\pi$-Donor Ligands
Ethylene ($\eta^2$)
$$\ce{H2C=CH2}$$
Charge: 0
Hapticity: 2
Donor: $\pi$-cloud
Found in Zeise's Salt. Donates $\pi$-electrons to the metal and accepts back-donation into its $\pi^*$ antibonding orbitals.
Cyclopentadienyl (cp)
$$\ce{C5H5^-} \; (\eta^5)$$
Charge: -1
Hapticity: 5
Donor: $\pi$-cloud
Famous for forming "sandwich" compounds like Ferrocene. A highly stabilizing, aromatic planar ligand.
Benzene ($\eta^6$)
$$\ce{C6H6}$$
Charge: 0
Hapticity: 6
Donor: $\pi$-cloud
A neutral aromatic ligand that donates 6 $\pi$-electrons to form stable sandwich complexes like bis(benzene)chromium.
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