Coordination Compounds MCQs with Answers – Part 2 (Class 12 Chemistry)

Explanation: Each monodentate ligand contributes one donor atom, so six such ligands give coordination number \(6\). Each tridentate ligand contributes three donor atoms. Two tridentate ligands therefore also provide \(2\times3=6\) metal–donor attachments. The number of ligand particles falls from six to two even though the coordination number is unchanged. This reduction in bound ligand count can accompany release of several separate particles during chelate formation.

Explanation: Multiple attachment points can make complete ligand separation less probable, so Row P is supported. Suitably sized rings can form without excessive strain and may contribute to stable chelation, supporting Row R. Release of several separate ligands can increase the number of free particles and favour the process entropically, supporting Row S. Row Q is unsuitable because poor donor spacing can introduce strain or prevent effective simultaneous bonding. Structural compatibility matters as much as the mere presence of several donor atoms.

Explanation: The chelate effect is a strong general trend, but it depends on a suitable geometric and electronic match. If donor atoms are too close together or too far apart, simultaneous binding may strain the ligand or distort the metal environment. An unsuitable donor type may also interact weakly with a particular metal ion. Very small or very large rings can be less favourable than rings of appropriate size. Stability comparisons therefore require comparable bonding conditions rather than an absolute rule based only on denticity.

Explanation: \( \textbf{Reaction amount:} \) The starting complex is present in an amount of \(0.50\,\mathrm{mol}\). \( \textbf{Ligand-release ratio:} \) \[ 1\,\mathrm{mol}\,\mathrm{[ML_6]}\rightarrow6\,\mathrm{mol}\ L \] \( \textbf{Free }L\textbf{ released:} \) \[ 0.50\times6=3.0\,\mathrm{mol} \] \( \textbf{Chelating-ligand ratio:} \) \[ 1\,\mathrm{mol}\,\mathrm{[ML_6]}\text{ consumes }3\,\mathrm{mol}\,\mathrm{en} \] \( \textbf{Amount of } \mathrm{en}\textbf{ consumed:} \) \[ 0.50\times3=1.5\,\mathrm{mol} \] \( \textbf{Coordination-number check:} \) Six monodentate donor atoms are replaced by six nitrogen donor atoms from three \(\mathrm{en}\) ligands. \( \textbf{Particle interpretation:} \) More separate \(L\) particles are released than \(\mathrm{en}\) particles consumed. \( \textbf{Final answer:} \) The process releases \(3.0\,\mathrm{mol}\) of \(L\) and consumes \(1.5\,\mathrm{mol}\) of \(\mathrm{en}\).

Explanation: In Case 1, one chlorido ligand connects two metal centres and therefore acts as a bridging ligand. In Case 2, both nitrogen atoms of the same \(\mathrm{en}\) molecule bind one metal centre. The ligand chain and metal form a chelate ring. Bridging concerns connection between different metal centres, whereas chelation concerns multiple attachment to one metal. The two arrangements cannot be distinguished merely from ligand charge.

Explanation: A hydroxido ligand joining two metals is bridging, so P matches \(2\). Ethane-1,2-diamine binding twice to one metal forms a chelate and matches \(3\). A single ammonia ligand attached through nitrogen is terminal and monodentate, giving R-\(1\). A carbonyl ligand connecting two metal atoms is described as a bridging carbonyl, giving S-\(4\). The same broad ligand family may adopt terminal or bridging modes depending on the actual bonding arrangement.

Explanation: The notation \(\mu\) is commonly used to show that a ligand bridges metal centres. Such a ligand makes direct bonding interactions with more than one metal. This arrangement can produce multinuclear coordination entities. Bridging does not require the ligand to form a ring around one metal, so Statement III is invalid. A ligand may be bridging without being chelating, just as a chelating ligand may involve only one metal centre.

Explanation: \( \textbf{Terminal ligands per metal:} \) Each metal has two terminal chlorido ligands. \( \textbf{Total terminal ligands:} \) \[ 2\times2=4 \] \( \textbf{Bonds from terminal ligands:} \) Each terminal ligand forms one metal–chlorine bond. \[ 4\times1=4 \] \( \textbf{Number of bridging ligands:} \) Two chlorido ligands bridge the two metals. \( \textbf{Bonds per bridge:} \) Each bridging chloride forms one bond to each metal, giving two bonds. \[ 2\times2=4 \] \( \textbf{Total metal–chlorine bonds:} \) \[ 4+4=8 \] \( \textbf{Final answer:} \) The entity contains \(8\) metal–chlorine bonds; counting each bridge as only one bond would miss its attachment to the second metal.

Explanation: \( \textbf{Terminal contribution:} \) Each metal receives two donor-atom attachments from its two terminal ligands. \[ \mathrm{CN_{terminal}}=2 \] \( \textbf{Bridging contribution:} \) Both bridging ligands also bond to each metal. \[ \mathrm{CN_{bridging}}=2 \] \( \textbf{Coordination number per metal:} \) \[ \mathrm{CN}=2+2 \] \[ \mathrm{CN}=4 \] \( \textbf{Counting rule:} \) A bridging ligand is counted once for each direct donor-atom attachment to the particular metal being considered. \( \textbf{Dimer distinction:} \) The total number of bonds in the whole dimer is larger than the coordination number of one metal. \( \textbf{Final answer:} \) Each metal has coordination number \(4\), even though the two bridges are shared between the metal centres.

Explanation: Coordination number counts the donor atoms that form direct bonds with the central metal. A monodentate ligand contributes one, a bidentate ligand contributes two, and a tridentate ligand contributes three when all stated donor sites are attached. The total number of atoms in a ligand is irrelevant to this count. Counter ions outside the brackets do not contribute because they are not directly coordinated. Ligand charge is used in oxidation-state calculations rather than in determining coordination number.

Explanation: One \(\mathrm{en}\) ligand contributes two nitrogen donor atoms. One oxalato ligand contributes two oxygen donor atoms. The two ammonia ligands contribute one donor atom each, giving two more attachments. The coordination number is therefore \(2+2+2=6\). The number of ligand particles is only \(1+1+2=4\), showing why ligand count and coordination number need not be equal.

Explanation: \( \textbf{External counter ion:} \) One \(\mathrm{Cl^-}\) lies outside the brackets. \( \textbf{Complex charge:} \) The bracketed entity must therefore carry charge \(1+\). \( \textbf{Ligand charges:} \) Four \(\mathrm{NH_3}\) ligands are neutral, while two coordinated \(\mathrm{Cl^-}\) ligands contribute \(-2\). \( \textbf{Let chromium oxidation state be:} \) \(x\). \( \textbf{Charge equation:} \) \[ x+4(0)+2(-1)=+1 \] \( \textbf{Solve for chromium:} \) \[ x-2=+1 \] \[ x=+3 \] \( \textbf{Coordination-number count:} \) All six ligands are monodentate. \[ \mathrm{CN}=4(1)+2(1)=6 \] \( \textbf{Final answer:} \) The values are \(+3\), \(1+\), and \(6\); oxidation state, complex charge, and coordination number arise from different calculations.

Explanation: Statement I gives the correct definition of coordination number. Statement II correctly describes oxidation-state determination from the metal, ligand charges, and net complex charge. Statement III is invalid because coordination number is a bond-counting quantity, whereas complex charge is an algebraic electrical quantity. Statement IV is valid for complexes containing multidentate ligands. In \(\mathrm{[M(en)_3]^{n+}}\), three ligand particles produce coordination number \(6\).

Explanation: \( \textbf{Required coordination number:} \) The entity is six-coordinate. \( \textbf{Contribution from } \mathrm{dien}\textbf{:} \) One tridentate ligand contributes three donor atoms. \[ \mathrm{CN_{dien}}=3 \] \( \textbf{Contribution required from ammonia:} \) \[ 6-3=3 \] \( \textbf{Denticity of ammonia:} \) Each \(\mathrm{NH_3}\) is monodentate. \( \textbf{Number of ammonia ligands:} \) \[ x=\frac{3}{1}=3 \] \( \textbf{Total ligand-particle count:} \) \[ 1\mathrm{\ dien}+3\mathrm{\ NH_3}=4 \] \( \textbf{Final answer:} \) The entity contains three ammonia ligands and four ligand particles in total, while six donor atoms surround the metal.

Explanation: Two monodentate ammonia ligands give coordination number \(2\) in Row P. Four monodentate chlorido ligands give coordination number \(4\) in Row R. A hexadentate \(\mathrm{EDTA^{4-}}\) ligand contributes six donor atoms, supporting Row S. In Row Q, each \(\mathrm{en}\) ligand is bidentate, so three such ligands give \(3\times2=6\), not \(3\). The incorrect assignment results from counting ligand molecules instead of donor atoms.

Explanation: Six monodentate ligands provide \(6\times1=6\) donor atoms. Three bidentate ligands provide \(3\times2=6\), and two tridentate ligands provide \(2\times3=6\). Their ligand-particle counts differ substantially, but their coordination numbers are equal. No conclusion about charge or oxidation state follows from denticity alone. Equal coordination number describes equal numbers of metal–donor attachments, not identical chemical composition.

Explanation: \( \textbf{Amount of complex:} \) The sample contains \(0.25\,\mathrm{mol}\) of coordination entities. \( \textbf{Ligands per entity:} \) Each entity contains three \(\mathrm{en}\) ligands. \( \textbf{Denticity of } \mathrm{en}\textbf{:} \) Each ligand forms two metal–nitrogen bonds. \( \textbf{Bonds per coordination entity:} \) \[ 3\times2=6 \] \( \textbf{Mole relation:} \) \[ 1\,\mathrm{mol}\,\mathrm{[M(en)_3]^{n+}}\rightarrow6\,\mathrm{mol}\,\mathrm{M-N\ bonds} \] \( \textbf{Substitution:} \) \[ 0.25\times6=1.50 \] \( \textbf{Bond amount:} \) \[ n(\mathrm{M-N\ bonds})=1.50\,\mathrm{mol} \] \( \textbf{Final answer:} \) The sample contains \(1.50\,\mathrm{mol}\) of metal–nitrogen coordinate bonds; multiplying only by three would count ligands rather than donor attachments.

Explanation: \( \textbf{Octahedral coordination:} \) The entity has coordination number \(6\). \( \textbf{Contribution from oxalato ligands:} \) Each oxalato ligand is bidentate. \[ 2\times2=4 \] \( \textbf{Remaining coordination positions:} \) \[ 6-4=2 \] \( \textbf{Contribution from aqua ligands:} \) Each \(\mathrm{H_2O}\) ligand is monodentate. \( \textbf{Number required:} \) \[ x=\frac{2}{1}=2 \] \( \textbf{Resulting ligand set:} \) Two oxalato and two aqua ligands provide six donor atoms. \( \textbf{Final answer:} \) The value of \(x\) is \(2\); four ligand particles are sufficient to occupy six octahedral coordination positions.

Explanation: A square-planar arrangement places the metal and four donor atoms in the same plane. Adjacent positions subtend angles of \(90^\circ\), while opposite positions subtend \(180^\circ\). A tetrahedral arrangement also has coordination number \(4\), but its donor atoms do not all lie in one plane. Linear geometry contains only two donor positions. Octahedral geometry contains six positions, including two perpendicular to the square plane.

Explanation: \(\mathrm{[NiCl_4]^{2-}}\) is tetrahedral in the usual coordination-chemistry model. Chlorido is a weak-field ligand and does not produce the pairing associated with the square-planar arrangement in this case. The cyanido ligand is strong-field, and \(\mathrm{[Ni(CN)_4]^{2-}}\) is square planar. Both entities have four directly bonded donor atoms. Their contrast shows that coordination number must be combined with metal configuration and ligand-field effects when predicting geometry.