Chemistry · Coordination Compounds · NEET
n is the number of UNPAIRED electrons in the d-orbitals of the metal ion (not the total number of d-electrons). Unpaired means an electron sitting alone in an orbital, with no partner of opposite spin. Example: Cr2+ is 3d4 with all 4 electrons in separate orbitals, so n = 4. Only unpaired electrons make the ion magnetic, so only they go into the formula.
Step 1: Find the oxidation state of the metal (using the charges of ligands and the overall charge). Step 2: Take the neutral atom's configuration. Step 3: Remove electrons for the oxidation state — IMPORTANT: for transition metals you remove the 4s electrons FIRST, then 3d. Example: Fe is [Ar]3d6 4s2. For Fe3+ remove 2 from 4s and 1 from 3d, leaving 3d5. So Fe3+ = d5.
Fill the five d-orbitals using Hund's rule: put one electron in each of the five boxes first (all single), then start pairing. For a WEAK field / free ion: d1=1, d2=2, d3=3, d4=4, d5=5 unpaired; then pairing begins, so d6=4, d7=3, d8=2, d9=1, d10=0. Memorise this staircase (up to 5 then back down) — it saves time in NEET.
It is called spin-only because it counts only the electron SPIN contribution and ignores the orbital motion contribution. For most first-row transition metal ions the orbital part is quenched (nearly zero), so the spin-only value matches experiment well. NEET always uses the spin-only formula unless it clearly asks for something else.
Yes, indirectly. A strong-field ligand (like CN-, CO, NH3) can force electrons to pair up, which LOWERS n and so lowers μ. A weak-field ligand (like F-, Cl-, H2O) leaves electrons unpaired, keeping n and μ high. Same ion, different ligand → different n. Example: Co3+ (d6) is n=0 with NH3 (low spin) but n=4 with F- (high spin).
n=1 → √3 = 1.73 BM. n=2 → √8 = 2.83 BM. n=3 → √15 = 3.87 BM. n=4 → √24 = 4.90 BM. n=5 → √35 = 5.92 BM. NEET options are usually written as these decimals or as √8, √24, √35 etc., so knowing both forms helps you match answers fast.
The calculated spin-only magnetic moment of Cr2+ ion is:
'Spin only' magnetic moment is same for which of the following ions? A. Ti3+ B. Cr2+ C. Mn2+ D. Fe2+ E. Sc3+
Among the species below, the spin-only magnetic moment is highest for: (Ti=22, Mn=25, Fe=26, Co=27)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
μ = √[n(n+2)] Bohr Magnetons (BM), where n is the number of unpaired electrons. This is the standard NCERT formula and is enough for every NEET question on this topic.
The unit is the Bohr Magneton, written as BM. NEET options are always given in BM, for example 4.90 BM or √24 BM.
n = 0 means no unpaired electrons, so the complex is diamagnetic (μ = 0). Any complex with n ≥ 1 (at least one unpaired electron) is paramagnetic.
Always use the metal ION in the correct oxidation state, not the neutral atom. First find the oxidation state, remove 4s electrons before 3d, then count unpaired d-electrons.
Because it combines three skills in one question: finding oxidation state, writing the d-configuration, and applying ligand field strength (high spin vs low spin). It appears almost every year, so mastering it is quick, guaranteed marks.
