How to Figure out Hybridization and Geometry in a Complex – Part 1

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

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Question:

Explain why [PdCl4]2- is square planar and [NiCl4]2- is tetrahedral.

Solution infographic:



Click/tap to enlarge the image

Square planar ML4 complexes

Tetrahedral ML4 complexes

DE > P + S

DE < P + S

Usually low-spin

Mostly high-spin

Metal has 4d/5d d8config. (almost all)

irrespective of SFL or WFL

Metal has d8 or d10 config.

ΔE = Stabilization energy, P = Pairing energy, S= Steric repulsions, SFL = Strong-field ligand, WFL = Weak-field ligand

4d/5d orbitals more radially diffuse (i.e., larger as compared to 3d) => Stronger overlap => Larger ΔE & smaller P (less energy to accommodate them in the same orbital).

Gorgeous geometry❓ 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 Contributors. Orbital hybridisation. Wikipedia. https://en.wikipedia.org/wiki/Orbital_hybridisation.‌

(3)    Brown, C.; Ford, M. Higher Level Chemistry, 2nd ed.; Pearson Education: Harlow, Essex, 2014.

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

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