Chapter 134 of 2363 min read

Valence bond and crystal field theory

Why this is asked: Tetrahedral crystal-field splitting is always smaller than octahedral for the same metal and ligands (roughly 4/9 the size) — small enough that tetrahedral complexes are almost always high-spin regardless of the ligand, unlike octahedral complexes where a strong-field ligand like CN⁻ or CO forces low-spin pairing. VBT explains a complex's geometry and magnetic moment through hybridisation, but not why it's coloured or why one ligand counts as "strong field" — that gap is exactly what crystal field theory fills.

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Test yourself

•The correct order of the wavelength of light absorbed by the following complexes is: A. $[Co (N H_{3})_{6}]^{3 +}$ · B. $[Co (CN)_{6}]^{3 -}$ · C. $[Cu (H_{2} O)_{4}]^{2 +}$ · D. $[Ti (H_{2} O)_{6}]^{3 +}$ Choose the correct answer from the options given below:
•Which of the following are paramagnetic? A. $[NiC l_{4}]^{2 -}$ · B. $Ni (CO)_{4}$ · C. $[Ni (CN)_{4}]^{2 -}$ · D. $[Ni (H_{2} O)_{6}]^{2 +}$ · E. $Ni (PP h_{3})_{4}$ Choose the correct answer from the options given below:
•Given below are two statements: Statement I: Both $[Co (N H_{3})_{6}]^{3 +}$ and $[Co F_{6}]^{3 -}$ complexes are octahedral but differ in their magnetic behaviour. Statement II: $[Co (N H_{3})_{6}]^{3 +}$ is diamagnetic whereas $[Co F_{6}]^{3 -}$ is paramagnetic. In the light of the above statements, choose the correct answer from the options given below:

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