Bonding in Coordination Compounds — Prelims Strategy

Prelims Strategy

To effectively tackle NEET questions on Bonding in Coordination Compounds, a systematic approach is crucial. For VBT-based questions (hybridization, geometry, magnetic nature):

1
Determine Oxidation State — First, find the oxidation state of the central metal ion. This is critical for determining its d-electron configuration.
2
Write d-electron Configuration — Based on the oxidation state, write the electron configuration of the metal ion (e.g., $3d^n$).
3
Identify Ligand Strength — Although VBT doesn't explicitly use the spectrochemical series, you need to know if the ligand is strong enough to cause electron pairing. Common strong field ligands are $CN^-$, $CO$, $en$, $NH_3$ (for $3d$ metals, especially $Co^{3+}$). Weak field ligands include halides ($F^-$, $Cl^-$, $Br^-$, $I^-$), $H_2O$, $OH^-$.
4
Determine Hybridization — Based on the coordination number and ligand strength, decide if inner d-orbitals are used ($d^2sp^3$ for octahedral, $dsp^2$ for square planar) or outer d-orbitals ($sp^3d^2$ for octahedral, $sp^3$ for tetrahedral). Strong field ligands generally lead to inner orbital/low spin complexes, while weak field ligands lead to outer orbital/high spin complexes.
5
Predict Geometry and Magnetic Nature — Hybridization directly gives geometry. Count unpaired electrons to determine paramagnetism (unpaired electrons) or diamagnetism (all paired).

For CFT-based questions (color, CFSE, magnetic moment, high/low spin):

1
Determine Oxidation State and d-electron Configuration — Same as for VBT.
2
Identify Geometry — Usually octahedral or tetrahedral for NEET.
3
Apply Spectrochemical Series — Crucially, use the spectrochemical series to determine if the ligand is strong field (large \\Delta) or weak field (small \\Delta).
4
Electron Distribution — For $d^4$ to $d^7$ configurations in octahedral complexes, compare \\Delta with pairing energy (P). If \\Delta > P (strong field), electrons pair up (low spin). If P > \\Delta (weak field), electrons occupy higher energy orbitals unpaired (high spin).
5
Calculate CFSE — Use the formula $CFSE = (-0.4 n_{t_{2g}} + 0.6 n_{e_g})\\Delta_o$ (for octahedral) and account for pairing energy if applicable.
6
Calculate Magnetic Moment — Count the number of unpaired electrons (n) from the electron distribution and use $\\mu = \\sqrt{n(n+2)}\text{ BM}$.
7
Explain Color — Relate the magnitude of \\Delta to the wavelength of light absorbed (E = $hc/\\lambda = \\Delta$) and identify the complementary color.

Trap Options: Be wary of options that swap strong/weak field ligand effects, miscalculate unpaired electrons, or confuse VBT and CFT principles. Practice drawing d-orbital splitting diagrams and electron filling for various d-configurations and ligand strengths.