Valence Bond Theory — Revision Notes

⚡ 30-Second Revision

VBT Core — Metal vacant orbitals + Ligand lone pairs $\rightarrow$ Coordinate covalent bond.
Hybridization — Dictates geometry.

* CN=4: $sp^3$ (Tetrahedral), $dsp^2$ (Square Planar) * CN=6: $d^2sp^3$ (Octahedral, Inner), $sp^3d^2$ (Octahedral, Outer)

Ligand Strength

* Strong Field (e.g., CN$^-$, CO, NH$_3$): Forces electron pairing, leads to inner orbital/low spin, fewer unpaired electrons. * Weak Field (e.g., H$_2$O, F$^-$, Cl$^-$): No electron pairing, leads to outer orbital/high spin, more unpaired electrons.

Magnetic Moment — $\mu = \sqrt{n(n+2)}$ BM, where $n$ = number of unpaired electrons.
Diamagnetic — All electrons paired ($n=0$).
Paramagnetic — Unpaired electrons ($n>0$).
Limitations — Cannot explain color, quantitative stability, or origin of ligand strength.

2-Minute Revision

Valence Bond Theory (VBT) explains bonding in coordination compounds by focusing on the overlap of vacant hybrid orbitals of the central metal ion with filled orbitals of ligands. The metal's s, p, and d orbitals undergo hybridization (e.

g., $sp^3$, $dsp^2$, $d^2sp^3$, $sp^3d^2$) to form new, equivalent orbitals that dictate the complex's geometry (tetrahedral, square planar, octahedral). A critical step is determining the metal's oxidation state and its d-electron count.

Ligand strength is crucial: strong field ligands (like CN$^-$, NH$_3$) force d-electron pairing, leading to inner orbital ($d^2sp^3$) or low-spin complexes with fewer unpaired electrons. Weak field ligands (like H$_2$O, Cl$^-$) do not cause pairing, resulting in outer orbital ($sp^3d^2$) or high-spin complexes with more unpaired electrons.

The number of unpaired electrons ($n$) determines the magnetic properties: $n=0$ means diamagnetic, $n>0$ means paramagnetic, with magnetic moment $\mu = \sqrt{n(n+2)}$ BM. Remember VBT's limitations: it doesn't explain complex color or quantitative stability.

5-Minute Revision

Valence Bond Theory (VBT) is a qualitative model for understanding bonding in coordination compounds. It proposes that the central metal ion utilizes its vacant atomic orbitals (s, p, and d) which hybridize to form a set of new, degenerate hybrid orbitals. These hybrid orbitals then overlap with the filled orbitals (containing lone pairs) of the ligands, forming coordinate covalent bonds. The type of hybridization directly determines the geometry of the complex:

Coordination Number 4 — $sp^3$ hybridization leads to a tetrahedral geometry (e.g., [NiCl$_4$]$^{2-}$). $dsp^2$ hybridization leads to a square planar geometry (e.g., [Ni(CN)$_4$]$^{2-}$).
Coordination Number 6 — $d^2sp^3$ hybridization leads to an inner orbital octahedral geometry (e.g., [Co(NH$_3$)$_6$]$^{3+}$). $sp^3d^2$ hybridization leads to an outer orbital octahedral geometry (e.g., [Fe(H$_2$O)$_6$]$^{2+}$).

The key to applying VBT is a systematic approach:

1
Determine Metal Oxidation State — Calculate the charge on the central metal ion.
2
Metal Ion Electronic Configuration — Write the d-electron configuration for the metal ion.
3
Identify Ligand Strength — Classify ligands as strong field (e.g., CN$^-$, CO, NH$_3$, en) or weak field (e.g., H$_2$O, F$^-$, Cl$^-$). Strong field ligands cause electron pairing in the d-orbitals, while weak field ligands do not.
4
Determine Electron Configuration in Complex — Based on ligand strength, fill the d-orbitals. If strong field, pair electrons to make inner d-orbitals available. If weak field, fill singly first (Hund's rule).
5
Identify Hybridization — Select the appropriate vacant orbitals (d, s, p) for hybridization to match the coordination number. This determines the geometry.
6
Calculate Magnetic Moment — Count the number of unpaired electrons ($n$). If $n=0$, the complex is diamagnetic. If $n>0$, it's paramagnetic. Calculate the spin-only magnetic moment using $\mu = \sqrt{n(n+2)}$ Bohr Magnetons (BM).

Example: [Cr(NH$_3$)$_6$]$^{3+}$

1
Cr is +3. Cr$^{3+}$ is $3d^3$.
2
NH$_3$ is strong field. For $d^3$, there are already three singly occupied d-orbitals and two vacant d-orbitals. Pairing is not required to make inner d-orbitals available.
3
Configuration: $\uparrow \uparrow \uparrow \_ \_$. Two inner $3d$ orbitals are vacant.
4
Hybridization: $d^2sp^3$. Geometry: Octahedral.
5
Unpaired electrons ($n=3$). Magnetic moment: $\mu = \sqrt{3(3+2)} = \sqrt{15} \approx 3.87$ BM. Paramagnetic.

Limitations: VBT fails to explain the color of complexes, the quantitative stability of complexes, and the origin of ligand field strength. These aspects are better explained by Crystal Field Theory.

Prelims Revision Notes

Valence Bond Theory (VBT) for Coordination Compounds - NEET Revision Notes

1. Core Principle:

Central metal ion (Lewis acid) accepts electron pairs from ligands (Lewis bases).
Coordinate covalent bonds form via overlap of vacant metal orbitals and filled ligand orbitals.

2. Hybridization & Geometry:

Metal orbitals (s, p, d) hybridize to form new, degenerate orbitals.
Coordination Number (CN) 4:

* $sp^3$ hybridization $\rightarrow$ Tetrahedral geometry (e.g., [NiCl$_4$]$^{2-}$). * $dsp^2$ hybridization $\rightarrow$ Square Planar geometry (e.g., [Ni(CN)$_4$]$^{2-}$).

Coordination Number (CN) 6:

* $d^2sp^3$ hybridization $\rightarrow$ Octahedral geometry (Inner orbital/Low spin) (e.g., [Co(NH$_3$)$_6$]$^{3+}$). * $sp^3d^2$ hybridization $\rightarrow$ Octahedral geometry (Outer orbital/High spin) (e.g., [Fe(H$_2$O)$_6$]$^{2+}$).

3. Ligand Field Strength & Electron Pairing:

Strong Field Ligands (SFL) — (e.g., CO, CN$^-$, NO$_2^-$, en, NH$_3$)

* Cause pairing of d-electrons in the metal ion. * Leads to fewer unpaired electrons. * Favors inner orbital complexes ($d^2sp^3$).

Weak Field Ligands (WFL) — (e.g., F$^-$, Cl$^-$ , Br$^-$ , I$^-$ , H$_2$O, OH$^-$)

* Do NOT cause pairing of d-electrons. * Electrons occupy orbitals singly according to Hund's rule. * Leads to more unpaired electrons. * Favors outer orbital complexes ($sp^3d^2$).

4. Magnetic Properties:

Paramagnetic — Contains unpaired electrons ($n > 0$). Attracted to magnetic field.
Diamagnetic — All electrons are paired ($n = 0$). Repelled by magnetic field.
Spin-only Magnetic Moment ($\mu$) — $\mu = \sqrt{n(n+2)}$ Bohr Magnetons (BM).

* $n=1 \implies \mu = 1.73$ BM * $n=2 \implies \mu = 2.83$ BM * $n=3 \implies \mu = 3.87$ BM * $n=4 \implies \mu = 4.90$ BM * $n=5 \implies \mu = 5.92$ BM

5. Steps for VBT Analysis:

1
Determine oxidation state of central metal ion.
2
Write electronic configuration of the metal ion (d-electrons).
3
Identify coordination number and ligand type (SFL/WFL).
4
Fill d-orbitals considering ligand effect (pairing/no pairing).
5
Identify vacant orbitals for hybridization (s, p, d).
6
Determine hybridization and geometry.
7
Count unpaired electrons ($n$) and calculate $\mu$.

6. Limitations of VBT:

Does not explain the color of coordination compounds.
Does not provide quantitative information about stability or reaction rates.
Does not explain the origin of strong/weak field nature of ligands (empirical).
Cannot explain distortions in complexes (e.g., Jahn-Teller effect).
Does not explain why certain complexes are inner/outer orbital for the same metal ion in the same oxidation state.

Vyyuha Quick Recall

Very Bright Teachers Help Graduates Memorize Ligands.

Valence Bond Theory
Hybridization (determines geometry)
Geometry (Tetrahedral, Square Planar, Octahedral)
Magnetic properties (Paramagnetic/Diamagnetic, $\mu = \sqrt{n(n+2)}$)
Ligands (Strong field $\rightarrow$ Pairing; Weak field $\rightarrow$ No pairing)