Coordination Chemistry

Q1. (4 marks) Postulates (1 mark each):

1. Metals show two types of valencies — primary (ionizable) and secondary (non-ionizable).
2. Primary valency = oxidation state, satisfied by anions.
3. Secondary valency = coordination number, satisfied by ligands (fixed number).
4. Secondary valencies are directional, giving definite geometry (e.g. octahedral for CN 6).

Q2. (3 marks) (1 mark each) (a) Ambidentate ligand: monodentate ligand that can bind through either of two different atoms, e.g. $NO_2^-$ (via N or O), $SCN^-$. (b) Chelating ligand: polydentate ligand forming a ring with the metal, e.g. ethylenediamine (en). (c) Denticity: the number of donor atoms of a ligand that attach to the central metal, e.g. EDTA has denticity 6.

Q3. (4 marks) (1 mark each) (a) Pentaamminechloridocobalt(III) chloride (b) Potassium hexacyanidoferrate(II) (c) Diamminedichloridoplatinum(II) (d) Hexaaquachromium(III) chloride

Q4. (4 marks) EAN = Z − (oxidation state) + (2 × CN) [electrons donated] (a) $[Fe(CN)_6]^{4-}$: Fe is +2 → electrons = 26 − 2 = 24; + 6×2 = 12 → EAN = 36 (2 marks) (b) $[Co(NH_3)_6]^{3+}$: Co is +3 → 27 − 3 = 24; + 12 → EAN = 36 (2 marks) Both attain Kr configuration.

Q5. (4 marks)

• $Ni^{2+}$ = $d^8$ (1 mark).
• $CN^-$ is a strong field ligand → pairs electrons; one 3d orbital freed (1 mark).
• Hybridization: $dsp^2$ → square planar geometry (1 mark).
• All electrons paired → diamagnetic ($\mu = 0$) (1 mark).

Q6. (4 marks) (a) $\mu = \sqrt{n(n+2)}$ BM, where $n$ = number of unpaired electrons (1 mark). (b) $Mn^{2+}$ = $d^5$ high spin → $n = 5$ (1 mark). $\mu = \sqrt{5(5+2)} = \sqrt{35} = 5.92$ BM (2 marks).

Q7. (4 marks) (a) CFSE: the net decrease in energy of the d-electrons in a complex relative to the hypothetical spherical (unsplit) field, due to their preferential occupation of the lower-energy set of d orbitals (1 mark). (b) $d^6$ low spin: $t_{2g}^6 e_g^0$. CFSE $= [6(-0.4) + 0(+0.6)]\Delta_o = -2.4\,\Delta_o$ (3 marks).

Q8. (3 marks)

• Square planar $[Pt(NH_3)_2Cl_2]$ shows cis and trans geometrical isomers (2 marks).
• The cis isomer is cisplatin, an anticancer drug (1 mark).

Q9. (3 marks) (a) Chelate effect: the extra stability of complexes containing chelating (polydentate) ligands compared to those with equivalent monodentate ligands (1 mark). (b) $[Ni(en)_3]^{2+}$ contains 3 bidentate chelate rings → greater positive entropy change (more particles released) → higher stability constant than $[Ni(NH_3)_6]^{2+}$ (2 marks).

Q10. (7 marks) (a) e.g. $I^- < Br^- < Cl^- < F^- < OH^- < H_2O < NH_3 < en < NO_2^- < CN^- < CO$ (any correct order of 4) (2 marks). (b) Jahn–Teller distortion: a geometric distortion (elongation/compression) that removes electronic degeneracy in the ground state to lower energy; shown by $Cu^{2+}$ ($d^9$) (or high-spin $Mn^{3+}$) (2 marks). (c) $Ti^{3+}$ is $d^1$ → one d electron can undergo a d–d transition, absorbing visible light → coloured. $Sc^{3+}$ is $d^0$ → no d electrons, no d–d transition → colourless (3 marks).

[
  {"claim":"EAN of [Fe(CN)6]4- = 36","code":"Z=26; ox=2; CN=6; result = (Z-ox + 2*CN)==36"},
  {"claim":"EAN of [Co(NH3)6]3+ = 36","code":"Z=27; ox=3; CN=6; result = (Z-ox + 2*CN)==36"},
  {"claim":"Spin-only moment for d5 (n=5) is sqrt(35) ~ 5.92 BM","code":"n=5; mu=sqrt(n*(n+2)); result = abs(float(mu)-5.916)<0.01"},
  {"claim":"CFSE of d6 low spin = -2.4 Delta_o","code":"cfse = 6*Rational(-2,5)+0*Rational(3,5); result = cfse==Rational(-12,5)"}
]