Coordination Compounds - Bonding in Coordination Compounds (VBT and CFT)

Valence Bond Theory (VBT): Coordination compounds involve the overlap of vacant hybrid orbitals of the central metal ion with filled orbitals of the ligands. For coordination number 6, hybridization is either $d^2sp^3$ (Inner orbital complex) or $sp^3d^2$ (Outer orbital complex).

Magnetic Properties in VBT: If a complex contains unpaired electrons, it is paramagnetic. If all electrons are paired, it is diamagnetic. Strong field ligands like $CN^-$ and $CO$ often cause electron pairing.

Crystal Field Theory (CFT): This theory treats the metal-ligand bond as purely electrostatic. The five degenerate $d$-orbitals of the metal ion split into different energy levels when surrounded by a ligand field.

Octahedral Splitting ($Δ_o$): In an octahedral field, the $d$-orbitals split into two sets: lower energy $t_{2g}$ ($d_{xy}, d_{yz}, d_{zx}$) and higher energy $e_g$ ($d_{x^2-y^2}, d_{z^2}$).

Tetrahedral Splitting ($Δ_t$): In a tetrahedral field, the splitting is inverted compared to octahedral. The $e$ orbitals are lower in energy than the $t_2$ orbitals. The splitting energy is smaller: $\Delta_t = \frac{4}{9} \Delta_o$.

Spectrochemical Series: Ligands are arranged in increasing order of their crystal field splitting energy ($\Delta$): $I^- < Br^- < SCN^- < Cl^- < S^{2-} < F^- < OH^- < C_2O_4^{2-} < H_2O < NCS^- < edta^{4-} < NH_3 < en < CN^- < CO$.

Strong Field vs. Weak Field Ligands: For strong field ligands, $\Delta_o > P$ (where $P$ is pairing energy), resulting in low-spin complexes. For weak field ligands, $\Delta_o < P$, resulting in high-spin complexes.