# Unit 1: Bonding in coordination compounds (CH314) ## Transition Series: Configuration and Periodic Trends - **d-block elements**: 3d (Sc-Zn), 4d (Y-Cd), 5d (La, Hf-Hg). General configuration $(n-1)d^{1-10}ns^{0-2}$. Exceptions: Cr $3d^54s^1$, Cu $3d^{10}4s^1$, Mo $4d^55s^1$, Ag $4d^{10}5s^1$, Pt $5d^96s^1$, Au $5d^{10}6s^1$. - **General trends**: variable oxidation states, coloured ions, paramagnetism, complex formation, catalytic activity, high melting points, interstitial compounds. - **Comparison of series**: - **Atomic/ionic size**: 4d larger than 3d, but 5d similar to 4d because of the **lanthanide contraction**. - **Oxidation states**: higher states are more stable in 4d and 5d (Mo(VI), W(VI), Os(VIII)) than in 3d (Fe(VI) rare). - **Metal-metal bonding** is stronger in 4d and 5d; they form clusters. - **Ligand field splitting** $\Delta$ is larger (3d < 4d < 5d), so 4d and 5d complexes are mostly **low spin**. - Higher ionisation enthalpies and densities for 5d; 3d metals more reactive. - Magnetic moments of 4d and 5d are lower than spin-only predicted due to spin-orbit coupling. ## IUPAC Nomenclature - Ligands in alphabetical order, then metal with oxidation state; anionic complexes end in -ate. Examples: $[Ni(CO)_4]$ tetracarbonylnickel(0); $K_4[Fe(CN)_6]$ potassium hexacyanidoferrate(II); $[Co(NH_3)_4Cl_2]Cl$ tetraamminedichloridocobalt(III) chloride; $Na_3[Fe(CN)_6]$ ...hexacyanidoferrate(III). ## Valence Bond Theory (VBT) - Ligand lone pair donates to empty hybrid orbitals of the metal; geometry follows hybridisation; **magnetism** follows number of unpaired electrons. | Complex | Metal | Hybridisation | Geometry | Magnetism | |---|---|---|---|---| | $[Ni(CN)_4]^{2-}$ | Ni(II) $d^8$ | $dsp^2$ (CN- pairs the 3d electrons) | Square planar | Diamagnetic | | $[Ni(CO)_4]$ | Ni(0) $3d^{10}$ (or $3d^84s^2$) | $sp^3$ | Tetrahedral | Diamagnetic | | $[NiCl_4]^{2-}$ | Ni(II) $d^8$ | $sp^3$ | Tetrahedral | Paramagnetic (2 unpaired, 2.83 BM) | | $[Fe(CN)_6]^{3-}$ | Fe(III) $d^5$ | $d^2sp^3$ | Octahedral (low spin) | Paramagnetic, 1 unpaired (1.73 BM) | | $[Fe(CN)_6]^{4-}$ | Fe(II) $d^6$ | $d^2sp^3$ | Octahedral (low spin) | Diamagnetic | - Limitations: cannot explain colour, spin states quantitatively, magnetic moments with orbital contribution, or the varying stability. ## Crystal Field Theory (Elementary) - Ligands as **point charges**; interaction is purely electrostatic. The five degenerate d orbitals split in a ligand field. - **Octahedral**: $t_{2g}$ ($-0.4\Delta_o$) and $e_g$ ($+0.6\Delta_o$). - **Tetrahedral**: $e$ ($-0.6\Delta_t$) below $t_2$ ($+0.4\Delta_t$), $\Delta_t = \frac49\Delta_o$. - **Square planar**: $d_{x^2-y^2} > d_{xy} > d_{z^2} > d_{xz}, d_{yz}$. - **Splitting of $d^n$ configurations in octahedral field**: $d^1$ to $d^3$ give $t_{2g}^n$. $d^4$ to $d^7$: choose between **high spin** ($\Delta_o < P$; $t_{2g}^3e_g^1$ for $d^4$) or **low spin** ($\Delta_o > P$; $t_{2g}^4$). $d^8$: $t_{2g}^6e_g^2$; $d^9$: $t_{2g}^6e_g^3$; $d^{10}$: $t_{2g}^6e_g^4$. **[Figure: d-orbital splitting in octahedral, tetrahedral and square planar fields]** Three energy-level diagrams with relative orbital positions. ## Factors Affecting 10 Dq ($\Delta_o$) 1. **Nature of ligand** (spectrochemical series): $I^- < Br^- < Cl^- < F^- < OH^- < H_2O < NH_3 < en < NO_2^- < CN^- < CO$. 2. **Oxidation state** of metal: higher charge gives higher $\Delta$. 3. **Position in the series**: 3d < 4d < 5d. 4. **Geometry** (number of ligands) and **metal-ligand distance** ($\Delta \propto 1/r^5$). ## CFSE, Pairing Energy and Magnetic Moment - **CFSE** (octahedral) $= [-0.4 n(t_{2g}) + 0.6 n(e_g)]\Delta_o + xP$ (x is extra pairs). - **Pairing energy** $P$: energy cost of putting two electrons in one orbital (coulombic repulsion + loss of exchange energy). High spin if $\Delta_o < P$. - **Examples**: $d^6$ low spin: CFSE $= -2.4\Delta_o + 2P$; $d^6$ high spin: $-0.4\Delta_o + P$. - **Magnetic moment** from CFT: $\mu_s = \sqrt{n(n+2)}$ BM; $[Fe(H_2O)_6]^{2+}$ high spin $d^6$ has 4 unpaired electrons, 4.90 BM; $[Fe(CN)_6]^{4-}$ has zero. - High spin vs low spin: $d^5$ high spin 5.92 BM, low spin 1.73 BM. ## Jahn-Teller Distortion - A non-linear system in a **degenerate electronic ground state** distorts to remove the degeneracy. Strong when $e_g$ is unsymmetrically occupied ($d^9$, high spin $d^4$, low spin $d^7$); weak when $t_{2g}$ is unsymmetrical. - **Static**: distortion is permanent and observable, e.g. $[Cu(H_2O)_6]^{2+}$ with four short and two long Cu-O bonds. - **Dynamic**: the distortion direction changes rapidly between equivalent axes (vibronic), so the average structure looks regular on the experimental timescale (e.g. at high temperature or in some Cu(II) complexes in EPR). ## Likely Exam Questions 1. Compare the properties of 3d, 4d and 5d series elements. 2. Explain with VBT the hybridisation and magnetism of $[Ni(CN)_4]^{2-}$ and $[NiCl_4]^{2-}$. 3. Draw d-orbital splitting for octahedral, tetrahedral and square planar fields. 4. Define CFSE and calculate for $d^6$ high spin and low spin. 5. What factors affect 10 Dq? (2 marks) 6. Explain static and dynamic Jahn-Teller distortion.