Chemistry · Coordination Compounds · NEET
It is an experimentally found list of ligands arranged in increasing order of field strength (how much they split the d-orbitals). The full NCERT order is: I- < Br- < SCN- < Cl- < S2- < F- < OH- < C2O4^2- < H2O < NCS- < NH3 < en < CN- < CO < edta^4-. Left side = weak field (small splitting Δ). Right side = strong field (large splitting Δ). For NEET, most questions only compare a few ligands, so remember the common ones in order: Cl- < F- < H2O < NH3 < en < CN- < CO.
Its position in the spectrochemical series decides it. Ligands near the right (CN-, CO, en, NH3) are strong field: they cause a large splitting Δo, so electrons pair up and you get a LOW-SPIN complex. Ligands near the left (I-, Br-, Cl-, F-, H2O) are weak field: they cause a small splitting Δo, so electrons spread out and you get a HIGH-SPIN complex. Do NOT try to guess from charge alone: CN- and CO are strong even though CO has no charge, while F- is weak.
When the d-orbitals split, the lower set (t2g in octahedral) drops by 0.4Δo and the upper set (eg) rises by 0.6Δo. CFSE is the net energy the complex saves by putting electrons into the lower orbitals instead of an unsplit set. Formula (octahedral): CFSE = (-0.4 x number of t2g electrons + 0.6 x number of eg electrons) Δo. A larger negative CFSE means a more stable complex. If any electrons had to pair, you also add the pairing energy term.
Δo is the splitting in an OCTAHEDRAL field (6 ligands); Δt is the splitting in a TETRAHEDRAL field (4 ligands). For the same metal and same ligand, the tetrahedral splitting is much smaller: Δt = (4/9) Δo, roughly Δt ≈ 0.45 Δo. Because Δt is so small, it is almost always LESS than the pairing energy, so tetrahedral complexes are nearly always high-spin. This exact 4/9 ratio was tested directly in a NEET question.
A stronger field ligand gives a larger Δo. The complex absorbs light whose energy equals Δo, using E = hc/λ. Larger Δo means larger absorbed energy, which means SHORTER wavelength absorbed. So going up the spectrochemical series (weak to strong ligand), the wavelength absorbed gets shorter. Example: [Co(H2O)6]3+ (weak, long λ) absorbs longer wavelength than [Co(en)3]3+ (strong, short λ). This inverse link (strong field = short wavelength) is a favourite NEET trap.
Which of the following is the correct order of increasing field strength of ligand to form coordination compound?
The Crystal Field Stabilisation Energy (CFSE) for [CoCl6]4- is 18000 cm^-1. The CFSE for [CoCl4]2- will be:
Correct increasing order for the wavelengths of absorption in the visible region for the complexes of Co3+ is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
In the NCERT spectrochemical series, edta^4- is written last, but for the standard NEET comparison of common ligands, CO is the strongest field ligand, followed by CN-. The usual top end is en < CN- < CO. In the ReNEET 2026 paper, the correct decreasing order was CO > NH3 > H2O > Cl-.
Strong field ligands give a large Δo. Since the lower t2g orbitals drop by 0.4Δo, a bigger Δo means each t2g electron gains more stability. So more electrons in t2g plus a large Δo gives a more negative (more stable) CFSE.
Almost always high spin. Because Δt = (4/9)Δo is small, it is usually less than the pairing energy P, so electrons prefer to stay unpaired and spread out rather than pair up.
A large (more negative) CFSE means the complex is more stable and usually low-spin. It also means a larger Δo, so the complex absorbs higher-energy, shorter-wavelength light.
You should know the common order Cl- < F- < H2O < NH3 < en < CN- < CO. NEET rarely asks the full 15-ligand list; it usually gives you 3-4 ligands to rank, so knowing the common ones and using a memory hook is enough.
