Coordination Chemistry
In the IUPAC nomenclature of coordination compounds, the groups that surround the central atom must be identified in the name. They are listed as prefixes to the name of the central atom along with any appropriate multipliers. Isomerism in Coordination Compounds are mainly of 2 types (A) Stereoisomerism viz: (i) Geometrical isomerism (ii) Optical isomerism and (B) Structural isomerism viz: (i) Linkage isomerism (ii) Coordination isomerism (iii) Ionisation isomerism (iv) Solvate isomerism. Many approaches have been put forth to explain the nature of bonding in coordination compounds viz. Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT) and Molecular Orbital Theory (MOT). The magnetic properties of coordination compounds can be measured by the magnetic susceptibility experiments. The results can be used to obtain information about the number of unpaired electrons and hence, structures adopted by metal complexes.
What is the oxidation state and d orbital occupation of the cis-[CrCl2(en)2]Cl?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → +III & \( t_{2g}^3,\ e_g^0 \)
In an octahedral complex, the ligands split the five d orbitals into a lower set of three, t2g, and an upper set of two, eg, separated by Δo. Electrons enter the lower t2g set first. For d1 to d3 ions each electron goes singly into a t2g orbital, so the arrangement is the same whether the ligands are strong or weak.
Oxidation state: en (ethane-1,2-diamine) is neutral, each \( \ce{Cl^{-}} \) is −1, and there is one \( \ce{Cl^{-}} \) outside the bracket:
x + 2(−1) + 2(0) + (−1) = 0 ⇒ x = +3
d electron count:
Cr (Z = 24): [Ar] 3d5 4s1
\( \ce{Cr^{3+}} \): remove the 4s electron and two 3d electrons → [Ar] 3d3
Filling: the three d electrons occupy the three t2g orbitals singly (Hund’s rule), and the eg set stays empty:
t2g: [↑] [↑] [↑] eg: [ ] [ ] ⇒ \( t_{2g}^3\,e_g^0 \)
So the complex has Cr in the +III state with the configuration t2g3 eg0 (3 unpaired electrons).
Why the other options are incorrect
Option (1) +II & \( t_{2g}^3,\ e_g^0 \) — Incorrect. The charges add up to +3 for Cr, not +2; \( \ce{Cr^{2+}} \) would also be d4, not d3.
Option (2) +III & \( t_{2g}^2,\ e_g^1 \) — Incorrect. The oxidation state is right, but an electron would not go up to the higher eg level while a t2g orbital is still empty.
Option (3) +IV & \( t_{2g}^3,\ e_g^0 \) — Incorrect. The charge balance gives +3; \( \ce{Cr^{4+}} \) would be d2.
NCERT (2026-27 edition):
Crystal Field Theory – “… Let us assign electrons in the d orbitals of metal ion in octahedral coordination entities. Obviously, the single d electron occupies one of the lower energy t2g orbitals. In d2 and d3 coordination entities, the d electrons occupy the t2g orbitals singly in accordance with the Hund’s rule.”