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Exercises · 5.24

Q.Write down the IUPAC name for each of the following complexes and indicate the oxidation state, electronic configuration and coordination number. Also give stereochemistry and magnetic moment of the complex:

(i) K[Cr(H2O)2(C2O4)2]⋅3H2OK[Cr(H_2O)_2(C_2O_4)_2] \cdot 3H_2O
(ii) [Co(NH3)5Cl]Cl2[Co(NH_3)_5Cl]Cl_2
(iii) [CrCl3(py)3][CrCl_3(py)_3]
(iv) Cs[FeCl4]Cs[FeCl_4]
(v) K4[Mn(CN)6]K_4[Mn(CN)_6]
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This problem asks for the systematic IUPAC name, oxidation state, electronic configuration, coordination number, stereochemistry, and magnetic moment for five coordination complexes. The key is to determine the metal’s oxidation state from the overall charge and ligand charges, then use crystal field theory to predict the d-electron count, geometry, and magnetic behavior. For each complex, the final answer is given in a compact table below.

Let’s work through each complex one by one. The approach is always the same: find the oxidation state of the metal, then its d-electron configuration, then the coordination number (which tells you geometry), and finally use the ligand field strength to decide whether the complex is high-spin or low-spin — that determines the magnetic moment.


1. Complex (i): K[Cr(H2O)2(C2O4)2]⋅3H2OK[Cr(H_2O)_2(C_2O_4)_2] \cdot 3H_2O

Step 1: Oxidation state of Cr.

The complex ion is [Cr(H2O)2(C2O4)2]−[Cr(H_2O)_2(C_2O_4)_2]^- (since K⁺ is +1, the anion must be –1). Water is neutral, oxalate (C2O42−C_2O_4^{2-}) is –2 each. Let Cr oxidation state be xx:

x+2(0)+2(−2)=−1  ⟹  x−4=−1  ⟹  x=+3x + 2(0) + 2(-2) = -1 \implies x - 4 = -1 \implies x = +3.

So Cr is in +3 state.

Step 2: Electronic configuration.

Cr (atomic number 24): [Ar]3d54s1[Ar] 3d^5 4s^1. For Cr³⁺, remove three electrons (the 4s electron and two 3d electrons): [Ar]3d3[Ar] 3d^3. So d3d^3 configuration.

Step 3: Coordination number.

Water is monodentate (one donor oxygen atom) and oxalate (C2O42−C_2O_4^{2-}) is bidentate (two donor oxygen atoms). So total donor atoms: 2 from the two water molecules + 2 × 2 from the two oxalate ligands = 6. Coordination number = 6. Geometry: octahedral.

Step 4: Stereochemistry.

For an octahedral complex with two monodentate and two bidentate ligands, the possible isomers are cis and trans. Here, the two water molecules can be adjacent (cis) or opposite (trans). The given formula doesn’t specify which, but typically such complexes are cis because oxalate is a chelating ligand that prefers to occupy adjacent sites. We’ll note both possibilities, but the common form is cis.

Step 5: Magnetic moment.

For d3d^3 in an octahedral field, the crystal field splitting puts three electrons in the t2gt_{2g} orbitals (all unpaired). No pairing possible because t2gt_{2g} holds three electrons singly. So number of unpaired electrons n=3n = 3. Magnetic moment μ=n(n+2)=3×5=15≈3.87\mu = \sqrt{n(n+2)} = \sqrt{3 \times 5} = \sqrt{15} \approx 3.87 BM.

Watch out

A common mistake is to think Cr³⁺ is d4d^4 or to forget that oxalate is bidentate — that would mess up the coordination number. Always count donor atoms, not just ligand molecules.

IUPAC name: Potassium diaquadioxalatochromate(III) trihydrate.

(Note: “diaqua” for two water, “dioxalato” for two oxalate ligands — following the same simple-prefix convention NCERT itself uses for oxalato, e.g. “trioxalatochromate(III)” for [Cr(C2O4)3]3– — “chromate(III)” for the metal in –ate form, and “trihydrate” for the three water molecules outside the coordination sphere.)


2. Complex (ii): [Co(NH3)5Cl]Cl2[Co(NH_3)_5Cl]Cl_2

Step 1: Oxidation state of Co.

The complex ion is [Co(NH3)5Cl]2+[Co(NH_3)_5Cl]^{2+} (because two Cl⁻ outside). NH₃ is neutral, Cl⁻ is –1. Let Co be xx:

x+5(0)+(−1)=+2  ⟹  x−1=+2  ⟹  x=+3x + 5(0) + (-1) = +2 \implies x - 1 = +2 \implies x = +3.

So Co is +3.

Step 2: Electronic configuration.

Co (atomic number 27): [Ar]3d74s2[Ar] 3d^7 4s^2. For Co³⁺, remove three electrons: [Ar]3d6[Ar] 3d^6. So d6d^6 configuration.

Step 3: Coordination number.

Five NH₃ (monodentate) + one Cl⁻ (monodentate) = 6 donor atoms. Coordination number = 6, octahedral.

Step 4: Stereochemistry.

With five identical ligands and one different, the only isomerism possible is the position of Cl — but since all NH₃ are equivalent, there’s no cis/trans isomerism here. The complex is simply octahedral with one Cl ligand.

Step 5: Magnetic moment.

NH₃ is a strong field ligand (high in spectrochemical series), so for d6d^6 in an octahedral field, strong field leads to low-spin configuration: all six electrons pair up in t2gt_{2g} (t2g6t_{2g}^6), leaving ege_g empty. Number of unpaired electrons n=0n = 0. Magnetic moment μ=0\mu = 0 BM (diamagnetic).

Tip

For d6d^6 octahedral complexes, the magnetic moment is a quick giveaway: if the complex is low-spin (strong field), μ=0\mu = 0; if high-spin (weak field), μ=4×6≈4.90\mu = \sqrt{4 \times 6} \approx 4.90 BM. Here, NH₃ is strong enough to cause pairing.

IUPAC name: Pentaamminechloridocobalt(III) chloride.

(Note: “pentaammine” for five NH₃, “chlorido” for Cl ligand, “cobalt(III)” for the metal, and “chloride” for the counterions.)


3. Complex (iii): [CrCl3(py)3][CrCl_3(py)_3]

Step 1: Oxidation state of Cr.

The complex is neutral. py (pyridine) is neutral, Cl⁻ is –1 each. Let Cr be xx:

x+3(−1)+3(0)=0  ⟹  x−3=0  ⟹  x=+3x + 3(-1) + 3(0) = 0 \implies x - 3 = 0 \implies x = +3.

So Cr is +3 again.

Step 2: Electronic configuration.

Cr³⁺: [Ar]3d3[Ar] 3d^3, same as in (i).

Step 3: Coordination number.

Three Cl⁻ (monodentate) + three py (monodentate) = 6 donor atoms. Coordination number = 6, octahedral.

Step 4: Stereochemistry.

Here we have three identical Cl and three identical py ligands. This gives two possible geometric isomers: facial (fac) where three Cl occupy one face of the octahedron, and meridional (mer) where three Cl lie in a plane. Both are possible, but the complex is often isolated as the fac isomer because it is more stable. We’ll note both.

Step 5: Magnetic moment.

Again d3d^3, so n=3n = 3, μ=15≈3.87\mu = \sqrt{15} \approx 3.87 BM. (pyridine is a moderate field ligand, but for d3d^3 there’s no high-spin/low-spin choice — always three unpaired electrons.)

IUPAC name: Trichloridotris(pyridine)chromium(III).

(Or: fac-trichloridotris(pyridine)chromium(III) if specifying stereochemistry.)


4. Complex (iv): Cs[FeCl4]Cs[FeCl_4]

Step 1: Oxidation state of Fe.

The complex ion is [FeCl4]−[FeCl_4]^- (Cs⁺ is +1, so anion is –1). Cl⁻ is –1 each. Let Fe be xx:

x+4(−1)=−1  ⟹  x−4=−1  ⟹  x=+3x + 4(-1) = -1 \implies x - 4 = -1 \implies x = +3.

So Fe is +3.

Step 2: Electronic configuration.

Fe (atomic number 26): [Ar]3d64s2[Ar] 3d^6 4s^2. For Fe³⁺, remove three electrons: [Ar]3d5[Ar] 3d^5. So d5d^5 configuration.

Step 3: Coordination number.

Four Cl⁻ ligands, each monodentate. Coordination number = 4. Geometry: tetrahedral (since Cl⁻ is a weak field ligand and Fe³⁺ is d5d^5, tetrahedral is favored over square planar).

Step 4: Stereochemistry.

Tetrahedral complexes with four identical ligands have no geometric isomerism. So no stereoisomers.

Step 5: Magnetic moment.

For tetrahedral d5d^5, the crystal field splitting is smaller than octahedral, and the configuration is always high-spin (because pairing energy is larger than splitting). The five electrons occupy all five d-orbitals singly: e2t23e^2 t_2^3. Number of unpaired electrons n=5n = 5. Magnetic moment μ=5×7=35≈5.92\mu = \sqrt{5 \times 7} = \sqrt{35} \approx 5.92 BM. …

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