Complex ions are usually linked with transition metals. But every time magnesium sulphate dissolves, it forms one too. Group 2 complexes are weaker and fewer, yet the paper asks about them, and the reason is a single idea: charge density.
Unit 17 of the PECTAA Chemistry 12 book is Group 2 Elements. Section 17.6 covers complex ion formation. This note is for FSc Part 2 (2nd year, Class 12) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.
What to know
Complex ion. An ion in which a central metal ion is bonded to a number of surrounding electron-donating species.
- The central metal ion is a Lewis acid: it accepts electron pairs.
- The molecules or ions with lone pairs, such as H₂O, NH₃, Cl⁻ and OH⁻, are Lewis bases: they donate electron pairs.
- The bonds are coordinate bonds. Their number is the coordination number. Common values are 2, 4 and 6.
Group 2 versus transition metals. Complex formation is more typical of transition metals, which have d-orbitals available for bonding. Group 2 elements also form complexes, especially those at the top of the group, but far less readily.
What decides it: size and charge. All Group 2 ions carry a +2 charge, which attracts electron-pair donors. Smaller ions have higher charge density (charge-to-size ratio) and attract donors more strongly. So complex formation is strongest at the top of the group and weakens going down.
Hydrated ions are complexes. When Group 2 salts dissolve, the metal ions are surrounded by water molecules acting as electron-pair donors.
- Be²⁺, the smallest ion with the highest charge density, forms [Be(H₂O)₄]²⁺, coordination number 4.
- Mg²⁺ is larger and usually forms the hexahydrate [Mg(H₂O)₆]²⁺, coordination number 6. The book's figure shows it with a sulphate ion.
- Ca²⁺, Sr²⁺ and Ba²⁺ also form hydrated ions, mostly with coordination number 6, but the interaction gets weaker down the group.
Ring complexes. Some donors have more than one electron pair to donate. They can form several coordinate bonds to the same metal ion, making a ring-like structure. Such complexes are especially stable for the smaller Group 2 ions.
The EDTA example. EDTA (ethylenediaminetetraacetic acid) donates six electron pairs. With calcium:
Ca²⁺(aq) + EDTA⁴⁻(aq) → [Ca(EDTA)]²⁻(aq)
Uses given in the book:
- Water softening: EDTA removes Ca²⁺ and Mg²⁺ ions from hard water.
- Medicine: the Ca-EDTA complex is used to remove toxic metal ions from the body, and EDTA can sometimes remove excess calcium.
Trend at a glance.
| Ion | Size in the group | Charge density | Hydrated ion in the book | Complex strength |
|---|---|---|---|---|
| Be²⁺ | smallest | highest | [Be(H₂O)₄]²⁺ | strongest |
| Mg²⁺ | small | high | [Mg(H₂O)₆]²⁺ | strong |
| Ca²⁺, Sr²⁺, Ba²⁺ | larger, increasing | lower, decreasing | mostly coordination number 6 | weaker down the group |
Read the table from top to bottom: as the ion grows, its hold on the lone pairs of water and other donors loosens, so the complexes become less stable.
Key points
- Metal ion = Lewis acid; ligand = Lewis base.
- Higher charge density → stronger complexes. Be²⁺ and Mg²⁺ form them most readily.
- Coordination number: 4 for Be²⁺ (small), 6 for Mg²⁺ and the larger ions.
Worked examples
Example 1. Write the formula and coordination number of hydrated beryllium ion. [Be(H₂O)₄]²⁺; coordination number 4.
Example 2. Why does Be²⁺ form complexes more readily than Ba²⁺? Both have a +2 charge, but Be²⁺ is much smaller, so its charge density is higher. It attracts the lone pairs of donors more strongly.
Example 3. Check the charge on the Ca-EDTA complex. Ca²⁺ is +2 and EDTA⁴⁻ is −4, so the complex is (+2) + (−4) = −2, written [Ca(EDTA)]²⁻.
Example 4. In [Mg(H₂O)₆]²⁺, which species is the Lewis acid and which the Lewis base? Mg²⁺ is the Lewis acid (accepts electron pairs). Each H₂O is a Lewis base (donates a lone pair from oxygen).
Common mistakes in the exam
- Reversing Lewis acid and base. The metal ion accepts pairs; the ligands donate them.
- Giving Be²⁺ a coordination number of 6. The book gives [Be(H₂O)₄]²⁺.
- Saying Group 2 forms complexes as easily as transition metals. Group 2 does so far less readily.
- Wrong charge on [Ca(EDTA)]²⁻. Add the charges: +2 and −4 give −2.
- Saying complex formation gets stronger down the group. It gets weaker as ions get larger.
Exam technique
- Define complex ion, Lewis acid, Lewis base and coordination number in one line each.
- Link every trend to charge density.
- Write the Ca-EDTA equation with state symbols and charges, then give one use.
- Check your board's notice for the topics in your paper.
Practice questions
Name two Lewis bases given in the book. Answer: Any two of H₂O, NH₃, Cl⁻, OH⁻.
What are the common coordination numbers? Answer: 2, 4 and 6.
Which Group 2 ion forms a hexahydrate complex in the book's example? Answer: Mg²⁺, as [Mg(H₂O)₆]²⁺.
How many electron pairs does EDTA donate? Answer: Six.
Give one use of EDTA from the book. Answer: Softening hard water by removing Ca²⁺ and Mg²⁺, or removing toxic metal ions from the body.
Why are ring complexes such as Ca-EDTA especially stable? Answer: One donor forms several coordinate bonds to the same metal ion, making a ring-like structure that holds the ion firmly.
Which Group 2 ions does EDTA remove from hard water? Answer: Ca²⁺ and Mg²⁺.
Ready to practise this? Continue in 2nd Year Chemistry.