A-LevelChemistryChemical periodicity9701/43 · Oct/Nov 2023 · Q66 Marks

Five ligands are listed in Table 6.1.

(a)(i)Complete Table 6.1 using the words monodentate, bidentate and polydentate only. Each of these three words may be used once, more than once, or not at all. Table 6.1 ligand | type of ligand NH3 | EDTA4- | CN- | H2NCH2CH2NHCH2CH2NH2 | tridentate C2O42- |[2]
(a)(ii)The molecule H2NCH2CH2NHCH2CH2NH2 is a tridentate ligand. Suggest the meaning of tridentate ligand.[1]
(a)(iii)Suggest how H2NCH2CH2NHCH2CH2NH2 acts as a tridentate ligand.[1]
(b)(i)Nickel forms the octahedral complex [Ni(en)2(H2O)2]2+. This complex can exist in three isomeric forms, listed in Table 6.2. One of these forms is a trans isomer, the other forms are two different cis isomers. Table 6.2 isomer | polarity trans isomer | cis isomer 1 | cis isomer 2 |[1]
(b)(ii)Describe the difference between cis isomer 1 and cis isomer 2.[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.

[1] NH3 = monodentate
[1] EDTA4- = polydentate / hexadentate
[1] CN = monodentate
[1] C2O42- = bidentate
any two [1] all four [2]
[1] (ligand that) donates 3 lone pairs to central metal atom / ion OR (ligand that) forms 3 dative bonds to central metal atom / ion
[1] ref to using the electrons / lone pair on (each) N / amine group
[1] trans = non-polar cis isomer 1 = polar cis isomer 2 = polar ALL
[1] optical isomers / non-superimposable mirror images ALLOW enantiomers / they rotate polarised light differently

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