Coordination Compounds – NEET UG ChemistryPractice Questions & PYQs

Optical isomerism in octahedral coordination complexes requires the complete absence of a plane of symmetry and a center of inversion, rendering the complex chiral. Complexes with the general formula $[MA_3B_3]$, such as $[Co(NH_3)_3Cl_3]^0$, possess internal symmetry elements that allow them to form facial and meridional geometric isomers instead of optical isomers. In contrast, chelating bidentate ligands in systems like $[Co(en)_3]^{3+}$ and cis-$[Co(en)_2Cl_2]^+$ remove these symmetry planes, generating non-superimposable mirror images that exhibit optical activity.

The absorption of visible light by coordination complexes relies on $d-d$ electronic transitions, which require partially filled $d$-orbitals where electrons can be promoted between split energy levels. Metal ions possessing either a completely empty $3d^0$ configuration, such as $\mathrm{Ti}^{4+}$ and $\mathrm{Sc}^{3+}$, or a completely filled $3d^{10}$ configuration, such as $\mathrm{Zn}^{2+}$, lack the necessary orbitals to undergo these transitions and are consequently colourless. In contrast, chromium in $[\mathrm{Cr}(\mathrm{NH}_3)_6]^{3+}$ adopts a $3d^3$ electronic configuration, providing the partially filled orbitals essential for absorbing photons in the visible spectrum.

Transition metal species appear coloured only when they contain partially filled d-orbitals that enable d-d electronic transitions upon absorbing visible light. In the complex $\mathrm{TiF}_{6}^{2-}$, titanium exists in a $+4$ oxidation state with a $3d^0$ configuration, while in $\mathrm{Cu}_{2}\mathrm{Cl}_{2}$, copper exists in a $+1$ oxidation state with a $3d^{10}$ configuration. Because both species feature completely empty or fully filled d-subshells, they cannot undergo d-d transitions, leaving $\mathrm{TiF}_{6}^{2-}$ and $\mathrm{Cu}_{2}\mathrm{Cl}_{2}$ entirely colourless.

The magnitude of crystal field splitting energy in an octahedral field is directly determined by the ligand field strength according to the spectrochemical series, where stronger ligands induce a wider gap between the $t_{2g}$ and $e_g$ orbitals. Across the given complexes, the central cobalt ion remains in a similar oxidation state, meaning the variation in splitting is primarily driven by the nature of the attached ligands. Weak-field ligands like water and oxalate produce relatively small crystal field splittings, while ammine produces a moderately higher splitting. Cyanide is an exceptionally strong-field carbon-donor ligand located near the extreme high-field end of the spectrochemical series, causing maximum electrostatic interaction and resulting in the largest crystal field splitting energy.

Paramagnetism is directly proportional to the number of unpaired electrons present in the central metal ion's $d$-orbitals, which depends on the oxidation state and the crystal field strength of the surrounding ligands. In $[\mathrm{V}(\mathrm{gly})_2(\mathrm{OH})_2(\mathrm{NH}_3)_2]^+$, vanadium is in the $+5$ oxidation state with a $3d^0$ configuration, yielding zero unpaired electrons. For $[\mathrm{Fe}(\mathrm{en})(\mathrm{py})(\mathrm{NH}_3)_2]^{2+}$, iron is in the $+2$ state with a $3d^6$ configuration, but strong-field ligands force complete pairing to leave no unpaired electrons, and titanium in $[\mathrm{Ti}(\mathrm{NH}_3)_6]^{3+}$ adopts a $3d^1$ configuration with only one unpaired electron. Conversely, cobalt in $[\mathrm{Co}(\mathrm{ox})_2(\mathrm{OH})_2]^-$ achieves a $+5$ oxidation state with a $3d^4$ configuration where weak-field oxalate and hydroxide ligands prevent electron pairing, resulting in four unpaired electrons and the highest magnetic moment.

Enantiomorphs are stereoisomers that exist as non-superimposable mirror images, a property characteristic of chiral octahedral complexes lacking a center or plane of symmetry. Symmetric hexammine, tetraammine, and coordination salt combinations generally possess high symmetry elements like planes of symmetry, rendering them achiral and superimposable on their mirror images. In contrast, complexes containing symmetrical bidentate chelating ligands arranged in a cis-configuration, such as $[Co(en)_2Cl_2]^+$, disrupt internal symmetry and yield distinct left-handed and right-handed optical isomers.

Paramagnetic behavior is directly determined by the presence of unpaired electrons, where fewer unpaired electrons result in lower paramagnetism. Water acts as a weak-field ligand, allowing d-electrons to fill the orbitals singly according to Hund's rule before pairing begins. Among the given metal complexes, the nickel system possesses an electronic configuration with the fewest unpaired electrons, thereby exhibiting the minimum paramagnetic behavior.

Coordination compounds exhibit various types of structural and stereoisomerism depending on their composition and ligand connectivity. Ionization isomerism is possible here because the compound can exchange chloride ions inside or outside the coordination sphere to yield different ions in solution. Linkage isomerism arises from the presence of the ambidentate nitrite ligand, which can coordinate through either the nitrogen or oxygen atom, while geometrical isomerism occurs due to the octahedral arrangement of four ammonia ligands and two nitro ligands.

The spin-only magnetic moment is given by the formula $\mu = \sqrt{n(n+2)}$, where $n$ represents the number of unpaired electrons. Substituting the given magnetic moment value into this relationship yields three unpaired electrons, corresponding to a chromium ion with three d-electrons. In an octahedral crystal field, these three electrons singly occupy the lower-energy orbitals designated as xy, yz, and xz.

Optical activity in coordination complexes requires the molecule and its mirror image to be non-superimposable, a condition frequently met by chiral octahedral geometries lacking a plane of symmetry. Square planar complexes never show optical isomerism due to their inherent molecular plane, and trans-octahedral arrangements possess internal planes of symmetry that make them achiral. The cis-isomer containing two symmetrical bidentate ethylenediamine ligands and two monodentate chloride ligands lacks any internal plane of symmetry, rendering its mirror image non-superimposable and optically active.

Inner orbital complexes utilize the inner d-orbitals for hybridization, which typically occurs in the presence of strong-field ligands that force the pairing of electrons to clear out empty orbitals. Cobalt in the trivalent oxidation state has a d6 electronic configuration, and when confronted with strong ammonia ligands, all six electrons pair up in the lower energy set. This leaves two vacant d-orbitals for d2sp3 hybridization while leaving zero unpaired electrons, making the complex both an inner orbital system and completely diamagnetic.

Optical isomerism in coordination compounds requires the absence of both a plane of symmetry and a center of inversion, rendering the molecule chiral. Octahedral complexes containing three symmetrical bidentate ligands, such as ethylenediamine, lack these symmetry elements entirely because their non-superimposable mirror images form distinct enantiomers. Among the given options, only the tris-chelate complex $[\mathrm{Co}(\mathrm{en})_{3}]^{3+}$ possesses this specific structural arrangement, allowing it to exist in optical isomers.

Manganese has an atomic number of $25$ with a neutral outer electronic configuration of $3d^5 4s^2$. Upon losing two electrons to form the $+2$ oxidation state in the complex, the metal ion retains a $3d^5$ configuration. Since water acts as a weak field ligand, it produces a small crystal field splitting energy that is insufficient to overcome the electron pairing energy. Consequently, the five $d$-electrons remain unpaired to maximize exchange energy, resulting in a high-spin complex with five unpaired electrons.

Nickel has a neutral ground-state configuration of $[\mathrm{Ar}] 3d^8 4s^2$, and its hybridization is determined by the strength and pairing ability of the surrounding ligands. In tetracarbonylnickel(0), carbon monoxide is a strong field ligand that forces the $4s$ electrons into the $3d$ orbital, leaving vacant $4s$ and $4p$ orbitals for a tetrahedral $sp^3$ arrangement. For tetracyanonickelate(II), nickel is in the $+2$ oxidation state with a $3d^8$ configuration where strong cyanide ions cause electron pairing to free a $3d$ orbital, resulting in a square planar $dsp^2$ hybridization. Conversely, tetrachloronickelate(II) contains weak chloride ligands that cannot force electron pairing, forcing the complex to adopt a tetrahedral $sp^3$ geometry instead.

The classification of certain ions as strong field ligands stems from their identity as pseudohalides, which possess electronic characteristics similar to traditional halide ions. Because of this pseudohalide nature, these species engage in robust synergistic bonding by donating electron density through a $\sigma$-bond while simultaneously accepting electron density via $\pi$-back-bonding from the central metal atom. This dual bonding mode creates a large crystal field splitting energy, distinguishing them from weak field ligands.

Octahedral coordination geometry requires metal hybrid orbitals that point directly along the cartesian axes where incoming ligands approach. This maximal overlap is achieved by employing the $d_{x^{2}-y^{2}}$ and $d_{z^{2}}$ orbitals, which directly target the axial positions along the $x$, $y$, and $z$ coordinates. The remaining three $d$ orbitals possess lobes directed between the axes and are therefore excluded from participating in this specific hybridization scheme.

Organometallic compounds inherently require a direct covalent bond connecting a metal atom directly to a carbon atom. In metal alkoxides, however, the organic moieties are bonded to the metal center through intervening oxygen heteroatoms rather than carbon linkages. Therefore, compounds like aluminium ethoxide lack any direct metal-carbon bonding, placing them outside the organometallic classification.

The cis isomer of $[\mathrm{Pt}(\mathrm{NH}_{3})_{2} \mathrm{Cl}_{2}]$ serves as an effective chemotherapeutic agent utilized in the treatment of various malignant tumors. Following administration into the bloodstream, its labile chloride groups dissociate, allowing the central platinum atom to coordinate with the nitrogen atoms found in guanosine bases of DNA. By forming bridges between separate guanosine units, this coordination complex disrupts the standard replication cycle of the genetic material.

Organometallic compounds containing multiple bonds or aromatic ring systems typically bond with transition metals through delocalized $\pi$ electrons interacting with vacant metal orbitals. Tetramethyltin consists exclusively of single covalent bonds between carbon and metal centers without any unsaturated carbon systems or delocalized $\pi$ electronic clouds. Therefore, it functions strictly as a $\sigma$-bonded organometallic species rather than a $\pi$-bonded complex.

Cobalt in its elemental state possesses an outer electronic configuration of $3d^{7}4s^{2}$, which transitions to a $3d^{6}$ configuration upon forming a $+3$ ion in the complex. Because the fluoride ion acts as a weak field ligand, it produces a small crystal field splitting energy that is insufficient to force electron pairing in the $3d$ orbitals. Consequently, the six $d$ electrons distribute themselves across the $t_{2g}$ and $e_{g}$ sets according to Hund's rule, leaving a total of four unpaired electrons.