(a) Identification and Reasoning

• Step 1 (Definition of a Ligand): [0.5 Mark] A ligand is defined as an ion or molecule that can donate at least one pair of electrons to the central metal atom/ion to form a coordinate covalent bond.
• Step 2 (Analysis of Species): [0.5 Mark]
  • $\text{OH}^-$: Has lone pairs on Oxygen. (Can act as a ligand)
  • $\text{NH}_4^+$: All valence electrons of Nitrogen are involved in bonding; it has no lone pair to donate. (Cannot act as a ligand)
  • $\text{CH}_3\text{NH}_2$: Has a lone pair on Nitrogen. (Can act as a ligand)
  • $\text{H}_2\text{O}$: Has lone pairs on Oxygen. (Can act as a ligand)
• Step 3 (Final Conclusion): [0.0 Mark] The species that cannot act as a ligand is $\text{NH}_4^+$ because it lacks a lone pair of electrons to donate to the central metal atom.

(b) IUPAC Nomenclature of Linkage Isomer

• Step 1 (Concept of Linkage Isomerism): [0.5 Mark] Linkage isomerism occurs in complexes containing ambidentate ligands (like $\text{NO}_2^-$), which can coordinate through different donor atoms. The linkage isomer of $[\text{Co}(\text{NH}_3)_5(\text{NO}_2)]\text{Cl}_2$ is $[\text{Co}(\text{NH}_3)_5(\text{ONO})]\text{Cl}_2$.
• Step 2 (IUPAC Rules Application): [0.5 Mark]
  • Ligands: Pentaammine (5 $\text{NH}_3$) and nitrito-O ($\text{ONO}$).
  • Metal: Cobalt.
  • Oxidation State: $x + 5(0) + (-1) = +2 \implies x = +3$.
  • Counter ion: Chloride.
• Step 3 (Final Concluded Answer): [0.0 Mark] The IUPAC name is Pentaamminenitrito-O-cobalt(III) chloride.