Coordination Compounds: Numerical Problems Set (1)

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Coordination Compounds: numerical problems set for JEE+NEET

doubts.ai · Accepted Answer

Solution — Step by Step Total charge of complex: +3. NH₃ is a neutral ligand (charge 0). So: Co³⁺ has electronic configuration [Ar] 3d^6. Coordination number = number of donor atoms attached to the central metal. Six NH₃ ligands, each donating one nitrogen lone pair: NH₃ is a strong-field ligand (high in the spectrochemical series). For an octahedral d^6 complex with strong field: - Crystal field splitting o pairing energy P. - Electrons fill t {2g} orbitals first, pairing up before going to e g. - Configuration: t {2g}^6 \, e g^0, all electrons paired. This makes the complex diamagnetic and low-spin. Rules: - Cation first (here the complex is the cation). - Ligands in alphabetical order, with prefixes for multiple identical ligands ("hexa" for six NH₃). - Metal name with oxidation state in parentheses. - For NH₃ as ligand: use "ammine" (two m's). Name: hexaamminecobalt(III) ion , or with counterion (say chloride): hexaamminecobalt(III) chloride. Why This Works Coordination compounds combine three subtle ideas: charge balance (oxidation state), geometry (coordination number), and crystal field theory (electronic configuration and magnetism). The spectrochemical series tells you whether a ligand is "strong-field" (forces low-spin, all paired in t {2g} if possible) or "weak-field" (allows high-spin, electrons spread to e g). For a d^6 octahedral complex: - Strong field: t {2g}^6 e g^0, 0 unpaired, diamagnetic - Weak field: t {2g}^4 e g^2, 4 unpaired, paramagnetic So magnetic measurement is a JEE Main/NEET favourite for distinguishing low-spin from high-spin. Alternative Method Magnetic moment formula: = \sqrt{n(n+2)} BM where n is the number of unpaired electrons. For 0 unpaired: = 0 (diamagnetic). For 4 unpaired: \mu = \sqrt{24} 4.9 	ext{ BM}. NEET often gives and asks you to deduce n — same formula, run backward. Memorise the strong-field ligands at the top of the spectrochemical series: CN⁻ CO NO₂⁻ en NH₃ . And the weak-field ligands at the bottom: I⁻ Common Mistake Three classic slips: 1. Wrong oxidation state. For complexes with charged ligands (e.g., [Fe(CN) 6]^{4-} where CN is -1), forgetting to account for ligand charges gives the wrong metal oxidation state. Always: total charge = metal charge + sum of ligand charges. 2. Confusing coordination number with denticity. Coordination number is the number of donor atoms . For a chelating ligand like ethylenediamine (en), each en counts as 2 donor atoms. So [Co(en) 3]^{3+} has coordination number 6, not 3. 3. Ammine vs amine spelling. "Ammine" (two m's) for NH₃ as a ligand. "Amine" (one m) for an NH₂ functional group. Different molecules, different spelling. Final answer: Co oxidation state +3, coordination number 6, 0 unpaired electrons (low-spin), name: hexaamminecobalt(III) ion.

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