Bonding — VBT and Crystal Field Theory
Hybridisation-based bonding, crystal field splitting, colour and magnetic properties. (Chemistry › Coordination Compounds, NEET UG syllabus.)
What is Bonding — VBT and Crystal Field Theory?
A theory explaining the formation of covalent bonds in coordination compounds through the overlap of hybridised metal orbitals with ligand orbitals.
Key formula / rule: Spin-only magnetic moment
Key points
- Explain the bonding in coordination compounds using Valence Bond Theory, including hybridisation, geometry, and qualitative magnetic properties.
- Explain the bonding in coordination compounds using Crystal Field Theory, including d-orbital splitting, CFSE, colour, and quantitative magnetic properties.
- Differentiate between strong field and weak field ligands and their effect on electron pairing.
- Predict the geometry and magnetic behaviour of coordination complexes using VBT.
Common exam trap
Confusing the principles and applications of VBT and CFT.
Definitions
- Term
Valence Bond Theory (VBT)
- Meaning
A theory explaining the formation of covalent bonds in coordination compounds through the overlap of hybridised metal orbitals with ligand orbitals.
- Term
Crystal Field Theory (CFT)
- Meaning
A theory that describes the breaking of degeneracies of electron orbital states, usually d or f orbitals, due to the static electric field produced by a surrounding array of point charges (ligands).
- Term
Crystal Field Splitting Energy (CFSE)
- Meaning
The energy difference between the two sets of d-orbitals (e.g., t2g and eg in octahedral complexes) after splitting in a ligand field.
- Term
Spectrochemical Series
- Meaning
An experimentally determined series of ligands arranged in order of their increasing ability to cause crystal field splitting.
- Term
Inner Orbital Complex
- Meaning
A coordination complex where the metal ion uses inner d-orbitals (e.g., (n-1)d) for hybridisation, typically formed with strong field ligands, resulting in low spin.
- Term
Outer Orbital Complex
- Meaning
A coordination complex where the metal ion uses outer d-orbitals (e.g., nd) for hybridisation, typically formed with weak field ligands, resulting in high spin.
- Term
Pairing Energy (P)
- Meaning
The energy required to force two electrons into the same orbital against their mutual repulsion.
Learning objectives
Explain the bonding in coordination compounds using Valence Bond Theory, including hybridisation, geometry, and qualitative magnetic properties.
Explain the bonding in coordination compounds using Crystal Field Theory, including d-orbital splitting, CFSE, colour, and quantitative magnetic properties.
Differentiate between strong field and weak field ligands and their effect on electron pairing.
Predict the geometry and magnetic behaviour of coordination complexes using VBT.
Predict the magnetic behaviour and colour of coordination complexes using CFT.
Apply the spectrochemical series to determine the relative strength of ligands.
Identify the limitations of both VBT and CFT.
Formulae
- Name
Spin-only magnetic moment
- Note
n = number of unpaired electrons, BM = Bohr Magneton
- Expression
μ = √n(n+2) BM
- Name
Relationship between Octahedral and Tetrahedral CFSE
- Note
Δt is Crystal Field Splitting Energy for tetrahedral, Δo for octahedral
- Expression
Δt ≈ (4/9)Δo
- Name
CFSE for Octahedral Complexes
- Note
x = electrons in t2g, y = electrons in eg. P is pairing energy.
- Expression
CFSE = [-0.4x + 0.6y]Δo + P (if pairing occurs)
- Name
CFSE for Tetrahedral Complexes
- Note
x = electrons in e, y = electrons in t2. P is pairing energy.
- Expression
CFSE = [-0.6x + 0.4y]Δt + P (if pairing occurs)
Prerequisites
Basic knowledge of atomic structure and electronic configuration.
Understanding of orbital shapes (s, p, d).
Concept of coordination number and ligands.
Basic understanding of magnetism (paramagnetism, diamagnetism).
Nomenclature of coordination compounds.
Common mistakes
Confusing the principles and applications of VBT and CFT.
Incorrectly determining hybridisation and geometry based on ligand strength (e.g., assuming dsp2 for all CN=4 complexes).
Misinterpreting the d-orbital splitting diagrams for octahedral vs. tetrahedral fields (e.g., which orbitals are higher/lower in energy).
Incorrectly applying the spectrochemical series to determine whether a complex is high spin or low spin.
Failing to relate CFSE and pairing energy (P) to electron distribution in d-orbitals.
Incorrectly calculating the number of unpaired electrons or magnetic moment.
Not understanding that colour arises from d-d transitions, not just the presence of d-electrons.
Keywords
Valence Bond Theory
Crystal Field Theory
Hybridisation
d-orbital splitting
Crystal Field Splitting Energy (CFSE)
Spectrochemical Series
Ligands
Octahedral
Tetrahedral
Square Planar
Magnetic Moment
Colour
Inner Orbital Complex
Outer Orbital Complex
High Spin
Low Spin
Pairing Energy
d-d transition
Practice preview
What is the hybridisation of the central metal ion in the complex [Ni(CN)4]2-?…
easy
Which of the following statements correctly describes the complexes [Fe(CN)6]4- and [FeF6]3-?…
medium
Calculate the spin-only magnetic moment for the complex [CoF6]3-.…
easy