Coordination Compounds – NEET UG ChemistryPractice Questions & PYQs

Molecular chirality in octahedral coordination complexes requires the complete absence of any internal plane of symmetry. The tris-oxalate iron complex $\left[\mathrm{Fe}(\mathrm{ox})_3\right]^{3-}$ contains three bidentate ligands twisted in an octahedral geometry that lacks any plane of symmetry, making it chiral and optically active. Conversely, the trans isomer of the diaquabis(oxalato)chromium(III) ion features two water ligands directly opposite each other, which creates an active plane of symmetry slicing through the chromium center and water bonds to render it achiral.

The spectrochemical series arranges ligands in order of their increasing crystal field splitting energy, which dictates the magnitude of d-orbital separation. Carbon monoxide acts as an exceptionally strong field ligand due to extensive back-bonding capabilities, causing massive orbital splitting compared to weaker species. This descending continuum of ligand field strengths is accurately represented by the sequence $\mathrm{CO}>\mathrm{NH}_3>\mathrm{H}_2 \mathrm{O}>\mathrm{Cl}^{-}$.

A $1:3$ electrolyte dissociates in water to yield a single complex cation bearing a triple-positive charge along with three monovalent anions. Hexaamminecobalt(III) chloride dissolves in aqueous solution by completely breaking apart into one bulky complex cation and three individual chloride ions. This ionization behavior defines the compound $\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right] \mathrm{Cl}_3$ as a classic electrolyte of this specific category.

Coordination isomerism arises in salts containing complex ions in both the cation and anion parts when ligands are interchanged between the two coordination spheres. The base formulation of the salt allows various distributions of ammine and chloro ligands between the copper and platinum centers. By systematically swapping one, two, or all corresponding ligands across the ionic boundary, distinct constitutional isomers are generated. Accounting for the parent structure alongside the various ligand-swapped arrangements yields a total of four unique isomeric variations for this combination of complex ions.

Naming coordination entities requires arranging all attached ligands alphabetically by their ligand names, followed by the central metal name with its Roman numeral oxidation state in parentheses, and ending with the external counter anion. The ligands present-ammine, bromo, chloro, and nitro-alphabetize directly into triammine, bromo, chloro, and nitro prefixes. Accounting for the negative charges of the inner ligands and the outer chloride counter ion reveals that the platinum center exhibits an oxidation state of $+4$.

The spin-only magnetic moment is directly proportional to the number of unpaired electrons present in the metal center, which is dictated by the metal's oxidation state and the crystal field strength of the surrounding ligands. Evaluating each species begins by determining the central ion configuration and applying ligand field effects. The complex $\left[\mathrm{Ti}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{3+}$ contains titanium in the plus-three oxidation state with a $3d^1$ electron configuration, yielding one unpaired electron. In $\left[\mathrm{Mn}(\mathrm{CN})_6\right]^{3-}$, manganese exhibits a plus-three oxidation state with a $3d^4$ configuration; the strong-field cyanide ligand forces electron pairing within the lower $t_{2g}$ orbitals, leaving two unpaired electrons. For $\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{3-}$, iron is in the plus-three state with a $3d^5$ configuration, where strong-field pairing results in a single unpaired electron. Meanwhile, $\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}$ features cobalt in the plus-three state with a $3d^6$ configuration, and the strong-field ammonia ligands pair all six electrons to leave zero unpaired electrons. Comparing these values shows that $\left[\mathrm{Mn}(\mathrm{CN})_6\right]^{3-}$ possesses the highest number of unpaired electrons, generating the largest magnetic moment.

An ambidentate ligand is a monodentate species containing two different donor atoms, allowing it to coordinate with a central metal ion through either atom depending on reaction conditions. While ligands like ethane-1,2-diamine and oxalate are didentate with identical pairs of donor atoms, and ethylenediaminetetraacetate functions as a hexadentate ligand utilizing multiple binding sites simultaneously, the thiocyanate ion possesses both a sulfur atom and a nitrogen atom capable of acting independently as electron-pair donors. This dual donor capability enables the thiocyanate species to link through either element, establishing its classification as an ambidentate ligand.

Bidentate ligands contain two separate donor atoms positioned such that both can coordinate simultaneously to the same central metal ion to form a stable chelate ring. Among the given choices, methylamine and acetonitrile contain only one primary donor site, and the bromide ion acts as a simple monodentate halide. In contrast, the oxalate ion $\mathrm{C}_{2}\mathrm{O}_{4}^{2-}$ features two coordinating sites capable of dual donation, making it a classic bidentate ligand.

The geometry of a coordination complex is dictated by the coordination number of the central metal atom and its hybridization, which is heavily influenced by the nature of the ligands and the metal's electronic configuration. For $[\mathrm{Pt}(\mathrm{Cl}_2)(\mathrm{NH}_3)_2]$, platinum resides in the $+2$ oxidation state with a $d^8$ configuration, leading to a coordination number of four and a square planar arrangement that aligns with III.

The complex $[\mathrm{Co}(\mathrm{NH}_3)_6]\mathrm{Cl}_3$ features a cobalt ion coordinated to six ammonia molecules, establishing a coordination number of six that results in an octahedral geometry corresponding to I. Meanwhile, $[\mathrm{NiCl}_4]^{2-}$ contains nickel paired with chloride ions acting as weak field ligands, forming a tetrahedral shape due to a coordination number of four without electron pairing, matching IV. Lastly, $[\mathrm{Fe}(\mathrm{CO})_5]$ contains iron in a zero oxidation state bonded to five carbonyl groups, creating a coordination number of five with a trigonal bipyramidal structure that pairs with II.

Coordination compounds exhibit various forms of isomerism depending on their geometry, ligand types, and bonding behavior. The square planar complex $[\mathrm{Pt}(\mathrm{NH}_3)_2\mathrm{Cl}_2]$ features identical ligands positioned adjacently or oppositely, which directly leads to geometrical isomerism. Meanwhile, the octahedral cation $[\mathrm{Co}(\mathrm{en})_3]^{3+}$ contains three chelating ethylenediamine ligands creating a chiral environment entirely devoid of a plane of symmetry, thereby producing optical isomerism.

The complex $[\mathrm{Co}(\mathrm{NH}_3)_5\mathrm{NO}_2]\mathrm{Cl}_2$ incorporates the ambidentate nitro group capable of coordinating through either nitrogen or oxygen atoms, giving rise to linkage isomerism. Finally, the coordination entity $[\mathrm{Cr}(\mathrm{H}_2\mathrm{O})_6]\mathrm{Cl}_3$ allows for the interchange of water molecules inside the coordination sphere with chloride counterions outside, which characterizes solvate isomerism.

Coordination compound formulas place the central metal atom first, followed by anionic and neutral ligands enclosed inside brackets with numerical prefixes indicating ligand quantities. The prefix dichloro places two chloride ligands inside the coordination sphere, while bis(urea) designates two neutral urea molecules bonded directly to the metal. Because copper carries a $+2$ oxidation state and each chloride contributes a $-1$ charge, the overall complex remains electrically neutral without requiring external counter ions.

Coordination nomenclature builds formulas by placing the central metal atom followed by its ligands inside a square bracket, with counter ions balancing the overall charge outside. Cobalt sits in the $+3$ oxidation state, surrounded by four neutral ammonia molecules, one neutral water molecule, and one negatively charged chloride ion inside the coordination sphere to form $\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_4\left(\mathrm{H}_2 \mathrm{O}\right) \mathrm{Cl}\right] \mathrm{Cl}_2$. The cumulative charge of this complex ion comes out to be positive two because the cobalt contributes plus three and the coordinated chloride contributes minus one, requiring two chloride counter ions to achieve electrical neutrality.

Octahedral coordination complexes featuring two sets of three identical ligands, designated as the $Ma_3b_3$ type, give rise to facial and meridional stereoisomers. Facial isomers place three identical ligands at the corners of one triangular face of the octahedron, whereas meridional isomers arrange them along a meridian passing through the metal center. This specific geometric requirement is uniquely fulfilled by the chromium complex $\left[\mathrm{Cr}(\mathrm{py})_3(\mathrm{Cl})_3\right]$, where pyridine and chloride ligands each appear in equal tripling.

The coordination number is defined by the total number of donor atoms bound directly to the central metal atom, while the oxidation state is the residual charge on the metal after accounting for all ligand charges. In $\mathrm{K}_{3}\mathrm{Cr}(\mathrm{C}_{2} \mathrm{O}_{4})_{3}$, the oxalate ion $\mathrm{C}_{2}\mathrm{O}_{4}^{2-}$ is bidentate, meaning three such ligands provide a total of six donor atoms to the chromium center. Applying the algebraic charge balance equation $3(+1) + x + 3(-2) = 0$ for potassium, chromium, and oxalate respectively reveals that the chromium atom carries an oxidation state of plus three.

Octahedral complexes with the formula $[\mathrm{Co}(\mathrm{NO}_{2})_{3}(\mathrm{NH}_{3})_{3}]$ containing two sets of three identical ligands adopt distinct spatial arrangements known as facial and meridional isomers. In the facial arrangement, all three nitro ligands occupy adjacent positions at one triangular face of the octahedron while the three ammonia ligands occupy the opposite face. Alternatively, the meridional isomer places each set of three identical ligands along a meridian bisecting the coordination polyhedron, yielding exactly two distinct geometrical configurations.

Square planar coordination compounds featuring two pairs of identical monodentate ligands, such as $[\mathrm{Pt} (\mathrm{NH}_{3})_{2} \mathrm{Cl}_{2}]$, can arrange themselves in two spatial orientations around the central metal atom. When identical ligands occupy adjacent coordination positions relative to each other, the configuration is designated as the cis isomer. Conversely, placing identical ligands directly across from each other across the metal center forms the trans isomer, resulting in a total of two distinct geometrical arrangements.

Precipitation of two moles of silver chloride upon treatment with silver nitrate confirms that both chlorine atoms reside outside the coordination sphere as counter ions. This leaves the cobalt center coordinated to five ammonia molecules and one nitro group inside a cationic complex entity carrying a $+2$ charge. Dissolving this formulation in water releases three total moles of ions per mole of the compound, matching the structural formula $[\mathrm{Co}(\mathrm{NH}_{3})_{5}(\mathrm{NO}_{2})] \mathrm{Cl}_{2}$.

Carbon monoxide acts as a neutral monodentate ligand that coordinates to transition metals exclusively through the carbon atom rather than the oxygen atom. This preference arises because the lone pair of electrons resides primarily on the carbon atom, allowing it to donate electron density into vacant metal d-orbitals. Consequently, in metal carbonyl complexes, the metal atom is bonded directly to the carbon atoms of the carbonyl groups.

Paramagnetism arises from the presence of one or more unpaired electrons in the metal ion's $d$-orbitals, which depends heavily on the oxidation state of nickel and the spectrochemical series strength of the surrounding ligands. In $[\mathrm{NiCl}_4]^{2-}$, nickel assumes a plus-two oxidation state with a $3d^8$ configuration, adopting $sp^3$ hybridization that leaves two unpaired electrons and imparts paramagnetism. Both $\mathrm{Ni}(\mathrm{CO})_4$ and $\mathrm{Ni}(\mathrm{PPh}_3)_4$ contain neutral zero-oxidation-state nickel bonded to strong-field ligands that pair all $d$-electrons, resulting in zero unpaired electrons and diamagnetism. The tetracyanidonickelate(II) ion, $[\mathrm{Ni}(\mathrm{CN})_4]^{2-}$, features a square planar geometry with $dsp^2$ hybridization where strong-field cyanide forces complete electron pairing, making it diamagnetic. In contrast, $[\mathrm{Ni}(\mathrm{H}_2\mathrm{O})_6]^{2-}$ contains nickel in the plus-two state within a weak-field aqua ligand environment, leading to $sp^3d^2$ hybridization and two unpaired electrons that render the complex paramagnetic. Therefore, paramagnetism is observed specifically in the complexes containing chloride and water ligands.

Electrical conductance of a coordination complex in solution is determined by the total number of ions it dissociates into, as well as the ionic charges, because higher ion multiplicity increases charge carrier mobility. Dissolving complex compounds in an aqueous medium causes them to break apart into distinct cationic and anionic constituents, where species releasing a greater quantity of free ions exhibit significantly higher molar conductance. Coordination entities like $[\mathrm{Co} (\mathrm{NH}_3)_6] \mathrm{Cl}_3$ and $[\mathrm{Co} (\mathrm{NH}_3)_5 \mathrm{Cl}] \mathrm{Cl}$ yield multiple ions per formula unit, resulting in substantial electrolytic conductivity. Conversely, neutral coordination compounds such as $[\mathrm{Co} (\mathrm{NH}_3)_4 \mathrm{Cl}_2]$ do not ionize into free cations and anions in solution. Lacking mobile ions to transport an electrical current, this neutral species displays the minimum conductance among the available choices.