(a) Draw the structures of S₂N₂, S₄N₂, S₁₁N₂, S₅N₆ and I₂Cl₆. 10 marks (b) What are silicones ? Mention some of their uses. How…

(a)

• S₂N₂: four-membered planar ring with alternating S and N atoms: S—N—S—N (closing to the first S). The ring is square-planar.
• S₄N₂: six-membered ring with the sequence S—N—S—N—S—S (closing to the first S), i.e. an N=S=N unit linked through an S—S—S chain. It contains two S—S bonds and four S—N bonds; the ring is puckered.
• S₅N₆: bicyclic cage. It consists of an eight-membered S₄N₄ cradle (as in S₄N₄) in which two sulfur atoms are bridged by an –N—S—N– unit, giving a 'molecular basket'.
• S₁₁N₂: bicyclic molecule. Two three-coordinate, nearly planar nitrogen atoms are the bridgeheads; they are joined by a single sulfur atom (an N—S—N unit) and by two S₅ chains, so the molecule is S(NS₅)₂-like with three bridges between the two N atoms (total S = 1 + 5 + 5 = 11, N = 2). S₁₅N₂ is the analogue with two S₇ chains.
• I₂Cl₆: dimeric iodine trichloride. Two iodine atoms are bridged by two chlorine atoms; each iodine is also bonded to two terminal chlorine atoms. The core is I(μ-Cl)₂I, with each I bearing two terminal Cl, so the molecule is planar.

(b)

Silicones are polymeric organosilicon compounds containing repeating Si—O—Si linkages, with organic groups such as CH₃, C₆H₅, etc., attached to silicon. Their general formula for the common methyl silicone is (CH₃)₃Si—[O—Si(CH₃)₂]ₙ—O—Si(CH₃)₃. They are chemically inert, thermally stable, water-repellent, and excellent electrical insulators. Uses include silicone oils and greases as lubricants, antifoaming agents, water-proofing coatings, sealants, adhesives, electrical insulation, medical implants, catheters, cosmetics, and heat-resistant rubber.

Hexamethyldisiloxane is prepared by hydrolysis and condensation of trimethylchlorosilane: (CH₃)₃SiCl + H₂O → (CH₃)₃SiOH + HCl 2 (CH₃)₃SiOH → (CH₃)₃Si—O—Si(CH₃)₃ + H₂O Net: 2 (CH₃)₃SiCl + H₂O → (CH₃)₃Si—O—Si(CH₃)₃ + 2 HCl. Its structure is (CH₃)₃Si—O—Si(CH₃)₃, i.e. two trimethylsilyl groups joined through an oxygen atom.

If (CH₃)₃SiCl is mixed with (CH₃)₂SiCl₂ and hydrolysed, both undergo hydrolysis: (CH₃)₂SiCl₂ + 2 H₂O → (CH₃)₂Si(OH)₂ + 2 HCl (CH₃)₃SiCl + H₂O → (CH₃)₃SiOH + HCl. The difunctional silanediol condenses to give linear polysiloxane chains, while the monofunctional (CH₃)₃SiOH caps the chain ends. Hence a linear silicone oil/gum is formed: n (CH₃)₂Si(OH)₂ + 2 (CH₃)₃SiOH → (CH₃)₃Si—[O—Si(CH₃)₂]ₙ—O—Si(CH₃)₃ + (n+1) H₂O. The (CH₃)₃SiCl therefore acts as a chain stopper; its proportion controls the chain length and molecular mass of the silicone.

(c)

Four lanthanides can be easily separated from a lanthanide mixture by the valency change method: Ce, Eu, Sm, and Yb.

Justification: Most lanthanides exist stably as M³⁺ ions. Separation by valency change requires formation of a stable +4 or +2 state in aqueous solution, followed by precipitation or extraction. Ce can be oxidised to Ce⁴⁺ and separated as Ce(IO₃)₄, Ce(OH)₄, or Ce(SO₄)₂. Eu, Sm, and Yb can be reduced to Eu²⁺, Sm²⁺, and Yb²⁺ respectively and precipitated as their sulfates or separated by ion exchange. The remaining lanthanides stay as M³⁺ and are not easily separated by this simple method. Although Pr⁴⁺ and Tb⁴⁺ exist in some solid oxides/fluorides, they are not sufficiently stable in aqueous solution for easy valency-change separation. Hence the answer is four.

(d)(i)

Pr³⁺ has the outer configuration [Xe]4f². For 4f², Hund’s rules give S = 1 and L = 5, so the ground term is ³H₄; hence J = 4. Given g = 4/5, the magnetic moment from L–S coupling is μ = g√[J(J+1)] BM = (4/5)√[4(4+1)] BM = (4/5)√20 BM = (4/5)(2√5) BM = (8√5)/5 BM ≈ 3.58 BM.

(d)(ii)

Tb³⁺ has the outer configuration [Xe]4f⁸. For 4f⁸, the ground term is ⁷F₆, so S = 3, L = 3, and J = 6. Given g = 3/2, μ = g√[J(J+1)] BM = (3/2)√[6(6+1)] BM = (3/2)√42 BM = (3√42)/2 BM ≈ 9.72 BM.

(e)(i)

2[Co(CN)₅]³⁻ + MeI → [Co(CN)₅(CH₃)]³⁻ + [Co(CN)₅I]³⁻. Category: oxidative addition. In [Co(CN)₅]³⁻, cobalt is Co(II) (d⁷, 17e). In both products cobalt is Co(III) (d⁶, 18e). Thus two Co(II) centres are oxidised to Co(III), one receiving CH₃ and the other receiving I from MeI. The net change is oxidative addition of MeI across two metal centres, often proceeding through a radical pathway.

(e)(ii)

[Ru(CO)₃(PPh₃)₂] + MeI → [Ru(CO)₂(PPh₃)₂(CH₃)I] + CO. Because the 18-electron Ru(0) complex has no vacant site, the reaction starts by nucleophilic (Sₙ2) attack of the electron-rich metal on the methyl carbon, giving the cation [Ru(CO)₃(PPh₃)₂(CH₃)]⁺ I⁻; iodide then displaces one CO to give the neutral product. Category: oxidative addition accompanied by CO dissociation. The starting complex is Ru(0) (d⁸, 18e). In the product, Ru is Ru(II) (d⁶, 18e): 2 CO = 4e, 2 PPh₃ = 4e, CH₃ = 2e, I = 2e, plus d⁶ = 6e, total 18e. MeI adds as CH₃ and I ligands, while one CO is eliminated to maintain the 18-electron count. The oxidation state of Ru increases from 0 to +2, so the reaction is an oxidative addition.