Chemistry · Coordination Compounds · NEET
An inner orbital complex uses the metal's inner (n-1)d orbitals, for example the 3d orbitals of a first-row metal, for hybridisation. This gives d2sp3 hybridisation. An outer orbital complex uses the outer nd orbitals, for example 4d, giving sp3d2 hybridisation. Both are octahedral in shape, but they differ in which d orbitals bond and therefore in how many unpaired electrons remain.
d2sp3 is the INNER orbital complex because the two d orbitals used come BEFORE the s and p (they are the inner 3d orbitals). sp3d2 is the OUTER orbital complex because the two d orbitals come AFTER the s and p (they are the outer 4d orbitals). Easy rule: if 'd' is written first (d2sp3), it is inner; if 'd' is written last (sp3d2), it is outer.
Look at the ligand. Strong-field ligands (CN-, CO, NH3, en) force electrons to pair up in the inner 3d orbitals, freeing them for d2sp3 bonding, so you get an INNER orbital, low-spin complex. Weak-field ligands (F-, Cl-, H2O) do not force pairing, so the metal must use outer 4d orbitals for sp3d2 bonding, giving an OUTER orbital, high-spin complex. This is why for NEET you must remember the spectrochemical series order.
In both, cobalt is Co3+ (a d6 ion) and both are octahedral. NH3 is a strong-field ligand, so all six d electrons pair up in the inner 3d orbitals: t2g6 eg0, zero unpaired electrons, so it is a low-spin d2sp3 inner orbital complex and diamagnetic. F- is a weak-field ligand, so the electrons stay spread out: t2g4 eg2, four unpaired electrons, making it a high-spin sp3d2 outer orbital complex and paramagnetic. Same metal, same shape, different ligand strength.
No. Inner orbital just means the inner 3d orbitals are used (d2sp3). The number of unpaired electrons depends on the d electron count. For example, [Co(NH3)6]3+ (d6) is inner and has 0 unpaired electrons, but [Mn(CN)6]3- (d4) is also inner (d2sp3) yet still keeps 2 unpaired electrons. So an inner orbital complex can still be paramagnetic, just with fewer unpaired electrons than the outer version.
More unpaired electrons means more paramagnetic (larger magnetic moment). Inner orbital complexes force pairing, so they usually have fewer unpaired electrons and are less magnetic or diamagnetic. Outer orbital complexes keep electrons unpaired, so they have more unpaired electrons and are more paramagnetic. To get the actual value you count the unpaired electrons and put them into the spin-only formula.
Statement I: Both [Co(NH3)6]3+ and [CoF6]3- complexes are octahedral but differ in their magnetic behaviour. Statement II: [Co(NH3)6]3+ is diamagnetic whereas [CoF6]3- is paramagnetic. In the light of the above statements, choose the correct answer.
Pick out the correct statement with respect to [Mn(CN)6]3-.
The geometry and magnetic behaviour of the complex [Ni(CO)4] are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is an octahedral complex where the metal uses its inner (n-1)d orbitals, such as 3d, for bonding. This happens with strong-field ligands that pair up the electrons. The hybridisation is d2sp3 and the complex is usually low spin.
sp3d2. Here the two d orbitals used are the OUTER nd orbitals, for example 4d. Outer orbital complexes form with weak-field ligands and are high spin with more unpaired electrons.
Yes, for octahedral complexes inner orbital always means low spin, because pairing has occurred to free the inner d orbitals. Outer orbital always means high spin. But low spin does not always mean zero unpaired electrons; it just means the maximum possible pairing for that ligand.
NEET regularly asks you to compare two octahedral complexes of the same metal and decide which is paramagnetic or diamagnetic, or to give the hybridisation (d2sp3 vs sp3d2). Knowing strong-field vs weak-field ligands lets you answer these directly. It also connects to the spin-only magnetic moment formula, a very common numerical question.
Step 1: find the metal oxidation state and its d-electron count. Step 2: check the ligand. Strong field (CN-, CO, NH3, NO2-, en) means inner orbital, d2sp3, low spin. Weak field (F-, Cl-, Br-, H2O, OH-) means outer orbital, sp3d2, high spin. Step 3: count unpaired electrons and, if asked, use the spin-only formula.
