Chemistry · Ch 9 — Coordination Compounds

Octahedral complexes

9.9.2

Octahedral complexes

Two fully worked octahedral examples illustrate how the free ion's spin state changes which orbitals get used in hybridisation. (a) [Co(NH3)6]3+, LOW SPIN: cobalt's oxidation state here is +3, giving Co3+ a free-ion valence configuration of 3d6 (4s0 4p0). Six NH3 ligands require six vacant metal orbitals for bonding. Because this complex is LOW SPIN, the six 3d6 electrons pair up FIRST, before any hybridisation happens, filling exactly three of the five 3d orbitals completely (paired) and leaving the other TWO 3d orbitals genuinely empty. Those two now-vacant 3d orbitals combine with the metal's 4s orbital and all three 4p orbitals to give six d2sp3 hybrid orbitals ('inner orbital' hybridisation, since it uses the lower (n-1)d set) -- an octahedral geometry. Because every one of cobalt's six d electrons ends up paired, [Co(NH3)6]3+ is diamagnetic. (b) [CoF6]3-, HIGH SPIN: cobalt's oxidation state is again +3, so the free-ion configuration is again 3d6. Six F- ligands (a weak-field ligand, low in the spectrochemical series) again require six vacant orbitals, but because this complex is HIGH SPIN, NO pairing of the 3d6 electrons happens before hybridisation -- they stay arranged exactly as in the free ion (four orbitals singly occupied, one orbital doubly occupied, per Hund's rule: four unpaired electrons). With no 3d orbitals freed up this way, hybridisation instead has to draw on one 4s orbital, all three 4p orbitals, AND two of the empty 4d orbitals, giving sp3d2 hybridisation ('outer orbital', since it uses the higher nd set) -- again an octahedral geometry, but reached by a completely different route. Because four of cobalt's six d electrons remain unpaired in this arrangement, [CoF6]3- is paramagnetic with four unpaired electrons -- in sharp contrast to the di …

Figure 9.9.2aOctahedral structure of [Co(NH3)6]3+ with six ammine ligands coordinated to cobalt(III).

Fig. 9.9.2a — Octahedral structure of [Co(NH3)6]3+ with six ammine ligands coordinated to cobalt(III).

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

[Co(NH3)6]3+ — octahedral. Six ammine ligands donate their nitrogen lone pairs to cobalt(III), one along each of the six octahedral directions. This is the geometry the d2sp3 hybridisation of the box diagrams below predict …

Figure 9.9.2bValence bond orbital box diagrams for low-spin [Co(NH3)6]3+: Co3+ 3d6 pairs up, two 3d + one 4s + three 4p orbitals hybridise d2sp3 and six NH3 lone pairs (red) fill them - diamagnetic, inner-orbital complex.

Fig. 9.9.2b — Valence bond orbital box diagrams for low-spin [Co(NH3)6]3+: Co3+ 3d6 pairs up, two 3d + one 4s + three 4p orbitals hybridise d2sp3 and six NH3 lone pairs (red) fill them - diamagnetic, inner-orbital complex.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

Valence bond orbital box diagrams for low-spin [Co(NH3)6]3+: Co3+ 3d6 pairs up, two 3d + one 4s + three 4p orbitals hybridise d2sp3 and six NH3 lone pairs (red) fill them - dia …

Figure 9.9.2cOctahedral structure of [CoF6]3- with six fluoride ligands around cobalt(III).

Fig. 9.9.2c — Octahedral structure of [CoF6]3- with six fluoride ligands around cobalt(III).

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

[CoF6]3- — octahedral. Six fluoride ligands surround cobalt(III) at the corners of an octahedron. The same geometry as the hexaammine complex, but built from sp3d2 (outer-orbital) hybrids — which is why this com …

Figure 9.9.2dValence bond orbital box diagrams for high-spin [CoF6]3-: the 3d6 configuration stays unpaired, sp3d2 hybridisation uses outer 4d orbitals and six fluoride lone pairs (red) - four unpaired electrons, paramagnetic.

Fig. 9.9.2d — Valence bond orbital box diagrams for high-spin [CoF6]3-: the 3d6 configuration stays unpaired, sp3d2 hybridisation uses outer 4d orbitals and six fluoride lone pairs (red) - four unpaired electrons, paramagnetic.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

Valence bond orbital box diagrams for high-spin [CoF6]3-: the 3d6 configuration stays unpaired, sp3d2 hybridisation uses outer 4d orbitals and six fluoride lone pairs (red) - four u …

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