Werner’s Theory Of Coordination Compounds

Alfred Werner's theory, proposed in 1898, explains the structure and bonding in coordination compounds. Key postulates include:

• Metals exhibit two types of valencies: primary (ionizable, satisfied by negative ions) and secondary (non-ionizable, satisfied by neutral molecules or negative ions).
• The secondary valency equals the coordination number and defines the spatial arrangement of ligands around the central metal atom, leading to specific geometries (e.g., octahedral, tetrahedral, square planar).
• The groups within the square brackets form a coordination entity (complex) that doesn't dissociate, while ions outside are counter ions.

Werner's work explained the differing properties of compounds with the same empirical formula (e.g., cobalt-ammonia complexes) based on the coordination number and the nature of ligand bonding.

Definitions Of Some Important Terms Pertaining To Coordination Compounds

Key terms in coordination chemistry include:

• Coordination Entity: A central metal atom/ion bonded to a fixed number of ions or molecules (ligands), enclosed in square brackets (e.g., [Co(NH₃)₆]³⁺).
• Central Atom/Ion: The atom or ion to which ligands are directly bonded, typically a metal ion (Lewis acid).
• Ligands: Ions or molecules bound to the central metal atom/ion. They can be unidentate (one donor atom, e.g., Cl⁻, H₂O, NH₃), didentate (two donor atoms, e.g., ethane-1,2-diamine), or polydentate (multiple donor atoms, e.g., EDTA). Ambidentate ligands have multiple donor atoms but bind through only one at a time (e.g., NO₂⁻, SCN⁻).
• Coordination Number (CN): The number of ligand donor atoms directly bonded to the central metal ion. For polydentate ligands, each donor atom counts. CN is determined by sigma bonds only.
• Coordination Sphere: The central metal atom/ion and its attached ligands within the square brackets.
• Coordination Polyhedron: The spatial arrangement of ligand atoms directly attached to the central metal atom/ion (e.g., octahedral, tetrahedral, square planar).
• Oxidation Number: The charge of the central metal atom/ion, indicated by a Roman numeral in parentheses (e.g., Cobalt(III)).
• Homoleptic Complexes: Complexes with only one type of ligand (e.g., [Co(NH₃)₆]³⁺).
• Heteroleptic Complexes: Complexes with more than one type of ligand (e.g., [Co(NH₃)₄Cl₂]⁺).

Nomenclature Of Coordination Compounds

IUPAC rules provide a systematic way to name coordination compounds:

Formulas Of Mononuclear Coordination Entities

The central metal atom is written first, followed by ligands in alphabetical order, enclosed in square brackets. Ligands are named based on their chemical nature (anionic ligands end in -o, e.g., chloro, cyano; neutral ligands usually retain their name, e.g., aqua, ammine, carbonyl). Numerical prefixes (di-, tri-, bis-, tris-) indicate the number of ligands. The overall charge of the complex ion is shown as a superscript.

Naming Of Mononuclear Coordination Compounds

The cation is named first, followed by the ligands in alphabetical order. The oxidation state of the metal is given in Roman numerals in parentheses. If the complex ion is an anion, the metal's name ends with '-ate' (e.g., ferrate, cobaltate).

Isomerism In Coordination Compounds

Coordination compounds exhibit isomerism, where compounds have the same formula but different structures or spatial arrangements.

Geometric Isomerism

Occurs in heteroleptic complexes due to different spatial arrangements of ligands. In square planar complexes of type [MX₂L₂], cis- (adjacent) and trans- (opposite) isomers exist. In octahedral complexes of type [MX₂L₄], cis- and trans- isomers are also possible. Complexes of type [Ma₃b₃] can exhibit facial (fac) and meridional (mer) isomers.

Optical Isomerism

Arises when a complex and its mirror image are non-superimposable (chiral). These are dextro (d) and laevo (l) forms, differing in their rotation of plane-polarized light. Common in octahedral complexes with didentate ligands (e.g., [Co(en)₃]³⁺) and some cis-isomers of square planar complexes.

Linkage Isomerism

Occurs when a complex contains an ambidentate ligand (can bind via different donor atoms), such as NO₂⁻ (nitro–N or nitrito–O) or SCN⁻ (thiocyanato–S or isothiocyanato–N).

Coordination Isomerism

Arises from the interchange of ligands between cationic and anionic coordination entities having different central metal atoms.

Ionisation Isomerism

Occurs when the counter ion in a complex salt is also a potential ligand and can exchange places with a ligand within the coordination sphere.

Solvate Isomerism

Similar to ionisation isomerism, but involves solvent molecules (e.g., water in hydrate isomerism) that can be coordinated to the metal or exist as free solvent molecules in the crystal lattice.

Bonding In Coordination Compounds

Several theories explain bonding in coordination compounds:

Valence Bond Theory

VBT explains the formation, structure, and magnetic properties of complexes. It proposes that metal atoms/ions use hybrid orbitals (involving $(n-1)d$, $ns$, $np$, or $nd$ orbitals) to form coordinate bonds with ligands by accepting electron pairs. The geometry (tetrahedral, square planar, octahedral) depends on the type of hybridization ($sp^3$, $dsp^2$, $sp^3d^2$, $d^2sp^3$). Magnetic properties can be predicted based on the number of unpaired electrons, distinguishing between inner orbital (low spin, $d^2sp^3$) and outer orbital (high spin, $sp^3d^2$) complexes.

Magnetic Properties Of Coordination Compounds

Magnetic properties arise from unpaired electrons. Paramagnetism is due to unpaired electrons, and the magnetic moment is calculated using the spin-only formula ($μ = \sqrt{n(n+2)}$ BM). VBT helps explain magnetism by predicting the number of unpaired electrons based on hybridization and ligand field strength (strong field ligands cause pairing, weak field ligands do not).

Limitations Of Valence Bond Theory

VBT has limitations: it makes assumptions, doesn't quantitatively explain magnetic data, fails to explain the color of complexes, doesn't provide quantitative interpretations of stability, and struggles with exact predictions for tetrahedral and square planar geometries.

Crystal Field Theory

CFT is an electrostatic model treating metal-ligand bonds as purely ionic. It explains that ligands cause splitting of degenerate d orbitals into different energy levels (e.g., $t_{2g}$ and $e_g$ sets in octahedral fields). The splitting energy ($Δ_o$ or $Δ_t$) depends on the metal ion and the ligand field strength. Ligands are arranged in a spectrochemical series ($I^- < Br^- < Cl^- < F^- < OH^- < C_2O_4^{2-} < H_2O < NCS^- < NH_3 < en < CN^- < CO$). If $Δ_o > P$ (pairing energy), low-spin complexes form; if $Δ_o < P$, high-spin complexes form.

Color: The color of coordination compounds arises from d-d transitions, where an electron absorbs energy from visible light to move from a lower energy d orbital to a higher energy one. The observed color is complementary to the absorbed color. Ligand field strength influences the absorbed wavelength and thus the observed color.

Limitations Of Crystal Field Theory

CFT assumes ionic bonding and point charges for ligands, which is not always accurate. It doesn't fully account for the covalent character of metal-ligand bonds or quantitatively predict stability based on covalent interactions. It also doesn't explain the spectrochemical series quantitatively.

Bonding In Metal Carbonyls

Metal carbonyls are compounds containing metal atoms bonded to carbonyl (CO) ligands. The bonding involves two synergistic interactions: a sigma ($σ$) bond formed by donation of electrons from CO's lone pair to the metal's vacant orbital, and a pi ($π$) bond formed by donation of electrons from a filled metal d orbital to the vacant antibonding $π^*$ orbital of CO. This synergic effect strengthens the metal-CO bond.

Importance And Applications Of Coordination Compounds

Coordination compounds are vital in various fields:

• Analysis: Used as qualitative and quantitative reagents (e.g., EDTA for metal ion estimation, DMG for Ni detection).
• Water Hardness: EDTA titration determines $Ca^{2+}$ and $Mg^{2+}$ levels.
• Metallurgy: Used in metal extraction (e.g., gold with cyanide) and purification (e.g., nickel via nickel tetracarbonyl).
• Biological Systems: Essential components like chlorophyll (Mg), hemoglobin (Fe), and Vitamin B₁₂ (Co), and enzymes like carbonic anhydrase.
• Catalysis: Crucial in industrial processes (e.g., Wilkinson's catalyst RhCl(PPh₃)₃ for hydrogenation).
• Electroplating: Smooth and even plating (Ag, Au) from cyanide complexes.
• Photography: AgBr complexes with hypo ($S_2O_3^{2-}$) are used for fixing.
• Medicine: Chelation therapy (EDTA for lead poisoning, D-penicillamine for copper) and anticancer drugs (cis-platin).

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