How to Compare the Wavelengths of Absorption by the Metal-Ligand Complexes

A coordination complex is a chemical compound made of a central atom/ion (usually metallic) called the coordination center and a surrounding array of bound molecules/ions referred to as ligands or complexing agents.


Coordination complexes typically having transition metals or d-block elements in the periodic table are known as metal-ligand complexes (MLCs).

A spectrochemical series is a list of ligands ordered by ligand-field ‘strength’ (energy).

Spectrochemical series (in the increasing order of the ligand-field strength):

I− < Br− < S2− < SCN− < Cl− < NO3− < N3− < F− < OH− < C2O42− < O2− < H2O < NCS− < CH3CN < EDTA4- < py < NH3 < en < bipy < phen < NO2− < PPh3 < CN− < CO

py = Pyridine, en = Ethylenediamine, bipy = 2,2'-Bipyridine, phen = 1,10-Phenanthroline, PPh3 = Triphenylphosphine

Mnemonic: I Bring Some Sweet Chocolate Nice Nuts For Our Cute Optimistic Happy Naughty Children Effortlessly, Perhaps Not Enough, But Pal, No Problem Caring Cubs

The Planck relation (aka Planck's energy–frequency relation, the Planck–Einstein relation, or Planck equation) is a fundamental equation in quantum mechanics which states that the photon energy is proportional to the photon frequency.

Question:

Arrange the following in the increasing order of the wavelengths of absorption in the visible region: [Co(NH3)6]3+, [Co(en)3]3+, [Co(H2O)6]3+.

Solution:



Click/tap to enlarge the image

E= Energy, h = Planck constant, c = Velocity of light, λ = Wavelength

Do metal-ligand complexes look a bit simple now❓ Let Chemaficionado know in the comments below or at mychemistryhomework@gmail.com❗

References:

(1)    Spectrochemical series. Wikipedia. https://en.wikipedia.org/wiki/Spectrochemical_series.

(2)    Wikipedia. Planck Relation. Wikipedia. https://en.wikipedia.org/wiki/Planck_relation.

‌(3)    Mnemonic: Modified (by Chemaficionado) internet source

(4)    Ncert. Chemistry : Textbook for Class XII - Part.I; National Council Of Educational Research And Training: New Delhi.

(5)    Wikipedia Contributors. Coordination complex. Wikipedia. https://en.wikipedia.org/wiki/Coordination_complex.

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