Coordination Compounds – NEET UG ChemistryPractice Questions & PYQs

Paramagnetism arises from the presence of unpaired electrons within the metal ion's valence shell, whereas strong-field ligands or zero-oxidation state carbonyl complexes typically force complete electron pairing. Neutral metal carbonyl complexes such as chromium hexacarbonyl and iron pentacarbonyl achieve a stable noble gas configuration with zero unpaired electrons due to strong back-bonding. In contrast, chromium(III) coordinated to six ammonia molecules retains three unpaired $d$ electrons, conferring clear paramagnetic behavior.

Electroplating requires a slow and controlled deposition of metal ions to ensure a smooth, uniform, and strongly adhering metallic coating on the object. When a reactive metal like copper is immersed in a simple silver salt solution, spontaneous redox displacement occurs because copper is more electropositive than silver, resulting in a flaky and non-uniform layer. Utilizing a stable coordination compound keeps the concentration of free silver ions exceptionally low, which prevents this unwanted spontaneous displacement by copper and allows a smooth electroplate to form during electrolysis.

The reaction between copper sulfate and potassium cyanide initially precipitates white cuprous cyanide along with the release of cyanogen gas. When potassium cyanide is added in excess, this insoluble precipitate undergoes a complexation reaction to redissolve completely. This dissolution is driven by the coordination of cyanide ligands around the cuprous ion, yielding the soluble potassium cuprocyanide complex represented by the formula $\mathrm{K}_{3}[\mathrm{Cu}(\mathrm{CN})_{4}]$.

Coordination compound formulas are written by placing the central metal atom first, followed by neutral ligands and anionic ligands inside square brackets, with counter ions placed outside. Cobalt with an oxidation state of $+3$ coordinates to one chloro ligand, two aqua ligands, and three ammine ligands inside the coordination sphere. To satisfy the remaining ionic charge requirements of the $+2$ complex cation, two chloride ions must reside outside the bracket as counter ions.

The coordination number of a central metal atom is determined by the total number of coordinate covalent bonds formed by surrounding ligands. Oxalate ion ($\mathrm{C_2O_4^{2-}}$) is a symmetrical bidentate ligand, meaning each molecule donates two pairs of electrons to the metal center through two separate donor atoms. With three bidentate oxalate ligands bound to the central nickel ion, the total number of donor atoms attached is twice three, resulting in a coordination number of six.

Coordination complexes containing ambidentate ligands and appropriate geometrical symmetries can exhibit multiple types of structural and stereoisomerism simultaneously. The tetraamminedithiocyanatochromium(III) ion contains the ambidentate ligand thiocyanate ($\mathrm{SCN^{-}}$), which allows for linkage isomerism alongside cis-trans geometrical isomerism. Because of this combination of structural variability and spatial arrangements, it achieves the highest total count of distinct isomers among the given choices.

The ionic conductivity of an electrolyte solution depends directly on the total number of free ions furnished per formula unit upon complete dissociation in water. Potassium hexacyanoferrate(II) dissociates into four potassium cations and one large coordination anion, yielding a total of five independent charge carriers. In contrast, the other given coordination entities produce fewer ions or remain entirely unionized as neutral molecules, carrying significantly less charge through the solution. This maximum count of dissociated particles grants the potassium-based complex superior charge mobility and the highest overall conductivity among the choices.

Coordination number four complexes involving carbonyl ligands typically undergo $sp^3$ hybridization, resulting in a tetrahedral geometry where all electrons are fully paired due to the strong-field nature of the ligand. Nickel in tetracarbonylnickel possesses a zero oxidation state with a $3d^{10}4s^2$ electronic configuration that rearranges to a completely filled $3d^{10}$ shell upon complexation. Since there are no unpaired electrons present in this completely filled configuration, the complex is inherently diamagnetic rather than paramagnetic. Therefore, describing tetracarbonylnickel as paramagnetic contains a direct contradiction regarding its magnetic behavior.

Metal complexes involving carbonyl ligands feature synergistic bonding where ligand-to-metal donation forms a standard $\sigma$-bond while metal-to-ligand back-donation forms a dative $\pi$-bond. In the pentaamminecarbonylcobalt(III) ion, the ammonia molecule coordinates purely via a $\sigma$-bond through its lone pair, whereas the carbonyl ligands establish both $\sigma$-donation and $\pi$-backbonding interactions with the metal center. This dual bonding mode across different ligands present in the coordination sphere satisfies the criteria for both $\sigma$ and $\pi$-bonding within the same complex.

Isomerism in coordination compounds encompasses both stereoisomerism, which includes geometrical and optical variants, and structural variations depending on the nature of biomass, ligands, and the coordination sphere. A complex containing five distinct monodentate ligands around a central metal atom can exhibit geometrical isomerism due to different relative spatial positions of the constituent groups. Furthermore, the presence of asymmetrical ligand combinations introduces chirality into specific spatial arrangements, resulting in non-superimposable mirror-image pairs. Combining these geometric configurations with their respective optical counterparts yields a total count of four distinct isomeric forms for this specific multi-ligand composition.

Coordination number five transition metal complexes typically adopt a trigonal bipyramidal geometry to minimize steric crowding and ligand-ligand repulsions around the central metal atom. Iron pentacarbonyl features a central iron atom utilizing a $dsp^3$ hybridization scheme to accommodate the five surrounding carbonyl ligands in this specific spatial arrangement. Three of the carbonyl groups lie along an equatorial plane with mutually symmetrical bond angles, while the remaining two occupy axial positions perpendicular to this plane. This structural configuration satisfies the electronic and spatial requirements of a system with five electron pairs around the metal center.