RI Transition Elements - Answers to Self-Check Questions

-8- Suggested Answers to Transition Elements Self-check questions 1(a) A transition element is a d-block element which can form one or more stable ions with a partially filled d subshell. Note: The definition should be in terms of partially filled d subshell (not orbitals). Partially filled d orbitals refer specifically to the d5 electronic configuration, where all the five d orbitals are partially/singly filled. 1(b) A complex ion is a charged species which consists of a central metal ion linked to one or more surrounding ions or molecules (called ligands) by dative covalent bonds. 1(c) A ligand is an ion or a molecule which contains at least one atom bearing a lone pair of electrons which can be donated into a low-lying vacant orbita l of a central metal atom or ion forming a co- ordinate bond (or dative covalent bond), resulting in the formation of a complex. 1(d) In a complex, the co-ordination number of the central metal ion (or atom) is the total number of co-ordinate bonds that the central metal ion (or atom) has formed with ligands. 2a(i) Cu < Ni < V < Fe < Cr 2a(ii) Zn2+ < Ti2+ < V2+ < Co3+ < Mn2+ 2b(i) 1s2 2s2 2p6 3s2 3p6 3d5 4s1 2b(ii) 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d2 2c central metal oxidation state electronic configuration ion or atom of the transition metal atom/ion (i) TiCl2 Ti2+ +2 1s2 2s2 2p6 3s2 3p6 3d2 (ii) [V(H2O)6]3+ V3+ +3 1s2 2s2 2p6 3s2 3p6 3d2 (iii) [Cr(NH3)6]Cl3 Cr3+ +3 1s2 2s2 2p6 3s2 3p6 3d3 (iv) [Fe(SCN)(H2O)5]2+ Fe 3+ +3 1s2 2s2 2p6 3s2 3p6 3d5 (v) [Fe(C N)6]4– Fe2+ +2 1s2 2s2 2p6 3s2 3p6 3d6 (vi) [Ni(CO)4] Ni 0 1s2 2s2 2p6 3s2 3p6 3d8 4s2 3 Answer: D 4 Answer: D 5 (a) Haemoglobin (denoted by Hb) acts as a transporter of oxygen in the human body. It carries out this function by having the oxygen molecule acting as a ligand forming a dative covalent bond to Fe(II) in its haem group. The resulting compound formed is called oxyhae moglobin and it travels in the blood to places in the human body which requires oxygen. The H 2O ligand replaces the O2 ligand when the oxygen is released to the oxygen-starved sites in the human body. Hb-O2 + H2O ⇌ Hb-OH2 + O2 (b) I n haemoglobin, a H2O molecule acts as a ligand and forms a dative covalent bond with Fe(II) in the haem group. When converted into oxyhaemoglobin, a ligand exchange reaction occurs wi th the H2O ligand being replaced by the O2 ligand. There is no redox reaction and hence no change in the oxidation number of Fe(II) in the haem group. (c) T he interaction between CO and haemoglobin is that of a co-ordinate bond formed between the carbon atom of CO and the iron( II) in the haemoglobin. The CO acts as a monodentate ligand and donates the lone pair of electrons on the C atom into a vacant low-lying orbital in Fe( II) to form a co-ordinate bond. CO is so much more toxic than CO 2 probably because the CO-haemoglobin complex is much more stable than the CO2-haemoglobin complex. In other words, the CO ligand is much