A-LevelChemistryChemical bonding9701/42 · May/June 2020 · Q39 Marks

Complete the electronic configuration of an isolated gaseous nickel(II) ion, Ni²⁺. 3d [Ar] 1 4s

(a)Complete the electronic configuration of an isolated gaseous nickel(II) ion, Ni²⁺. 3d [Ar] 1 4s[1]
(b)Explain the origin of colour in transition element complexes.[4]
(c)(i)Explain why when aqueous NH₃ is added dropwise to [Ni(H₂O)₆]²⁺ a green precipitate is formed.[1]
(c)(ii)Explain why when a large excess of aqueous NH₃ is added to [Ni(H₂O)₆]²⁺, the green precipitate dissolves and a blue solution is formed.[1]
(d)The complex ion [NiBr₂(CN)₂]²⁻ shows stereoisomerism. Draw diagrams to show the two isomers of [NiBr₂(CN)₂]²⁻. Name the type of stereoisomerism. isomer 1 type of stereoisomerism isomer 2[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[1] 3d (Ni²⁺) 个个个个个 (i.e. all five d orbitals filled with one electron, then paired up to make 8 electrons - so, 3d⁸. The diagram shows 3d⁸, with the first 3 orbitals filled with pairs, and the last two with singles) OR 3d⁸ (4s 0)
[4] M1 d orbitals split into two levels/ lower and upper orbitals M2 electron(s) promoted / excited to a higher d-orbital M3 frequency of light absorbed M4 observed colour is complement of light absorbed
[1] (addition of NH₃) increases [OH⁻] (due to ionisation of NH₃ in water) and shifts equilibrium 1 to the right (forming Ni(OH)₂)
[1] (a large excess of NH₃) shifts eqm 2 to the right (forming [Ni(NH₃)₆]²⁺) AND the [Ni²⁺]/[[Ni(H₂O)₆]²⁺] decreases and eqm 1 shifts to the left (causing the ppt to dissolve)
[2] M1 two correct structures cis & trans for [NiBr₂(CN)₂]²⁻ NC. Br Ni NC Br M2 type of stereoisomerism: cis-trans/geometric

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