Chapter 5: Coordination Compounds | Summary

A coordination entity consists of a central metal atom or ion bonded to a fixed number of ligands. The central atom or ion acts as a Lewis acid. Ligands are the ions or molecules bound to the central atom; they can be unidentate (one donor atom), didentate (two donor atoms), polydentate (many donor atoms), chelate (di/polydentate using multiple donors simultaneously), or ambidentate (two different donor atoms, either of which can coordinate, e.g., NO2- and SCN-). EDTA4- is an important hexadentate ligand binding through two nitrogen and four oxygen atoms.

The coordination number is defined as the number of ligand donor atoms directly bonded to the central atom via sigma bonds. The coordination sphere consists of the central atom and its ligands enclosed in square brackets; ions outside are counter ions. The coordination polyhedron describes the spatial arrangement of ligands; common shapes are octahedral, tetrahedral and square planar. Homoleptic complexes have only one type of ligand, while heteroleptic complexes have more than one type. The oxidation number of the central atom is the charge it would carry if all ligands were removed with their shared electron pairs.

IUPAC nomenclature for coordination compounds follows additive nomenclature. In writing formulas, the central atom is listed first, followed by ligands in alphabetical order regardless of charge; the entire coordination entity is enclosed in square brackets. In naming, the cation is named before the anion; ligands are listed alphabetically before the metal name; anionic ligands end in -o (chlorido, oxalato), while neutral ligands use special names: aqua (H2O), ammine (NH3), carbonyl (CO), nitrosyl (NO). Prefixes mono, di, tri are used for simple ligands; bis, tris, tetrakis for ligands whose names already contain a numerical prefix.

The oxidation state of the metal is indicated by a Roman numeral in parentheses. If the complex ion is a cation, the metal is named as the element; if it is an anion, the name ends in -ate (cobaltate for Co, ferrate for Fe). For example, [Cr(NH3)3(H2O)3]Cl3 is triamminetriaquachromium(III) chloride; [Co(H2NCH2CH2NH2)3]2(SO4)3 is tris(ethane-1,2-diamine)cobalt(III) sulphate.

Isomers have the same chemical formula but differ in arrangement of atoms. Coordination compounds show two principal types: stereoisomerism and structural isomerism. Stereoisomers have the same bonds but different spatial arrangements. Geometric isomerism (cis-trans) arises in heteroleptic complexes: square planar [MX2L2] can show cis (ligands X adjacent) or trans (X opposite) isomers; octahedral [MX2L4] can similarly show cis/trans isomers. Octahedral [Ma3b3] complexes show facial (fac, three same ligands on one face) and meridional (mer, three same ligands on a meridian) isomers. Tetrahedral complexes do not show geometric isomerism.

Optical isomers are non-superimposable mirror images (enantiomers); molecules showing this are chiral. The two forms are dextro (d) and laevo (l) based on rotation of plane-polarised light. Optical isomerism is common in octahedral complexes with didentate ligands. Structural isomerism includes: linkage isomerism (ambidentate ligands binding through different atoms, e.g., -NO2 vs -ONO); coordination isomerism (interchange of ligands between cationic and anionic metal entities); ionisation isomerism (counter ion and ligand interchange, e.g., [Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4); and solvate/hydrate isomerism (solvent molecule inside or outside coordination sphere, e.g., [Cr(H2O)6]Cl3 violet and [Cr(H2O)5Cl]Cl2.H2O grey-green).

Valence Bond Theory (VBT) explains bonding by hybridisation of metal orbitals to accommodate ligand electron pairs. The metal uses (n-1)d, ns, np or ns, np, nd orbitals: sp3 gives tetrahedral geometry, dsp2 gives square planar, sp3d gives trigonal bipyramidal, d2sp3 (inner d orbitals) and sp3d2 (outer d orbitals) both give octahedral geometry. Inner orbital complexes (d2sp3) are low spin/spin-paired while outer orbital complexes (sp3d2) are high spin/spin-free.

[Co(NH3)6]3+ uses d2sp3 hybridisation (inner orbital) and is diamagnetic; [CoF6]3- uses sp3d2 (outer orbital) and is paramagnetic with four unpaired electrons. [NiCl4]2- is tetrahedral (sp3) with two unpaired electrons (paramagnetic); [Ni(CO)4] is tetrahedral but diamagnetic (Ni in zero oxidation state, no unpaired electrons); [Ni(CN)4]2- is square planar (dsp2) and diamagnetic. Limitations of VBT include inability to explain colour, quantitative magnetic data, thermodynamic stability, or distinguish between weak and strong ligands.

Crystal Field Theory (CFT) is an electrostatic model treating ligands as point charges (anions) or point dipoles (neutral molecules). In a free metal ion, all five d orbitals are degenerate. In an octahedral complex, the dx2-y2 and dz2 orbitals (eg set) point towards ligands and are raised in energy by 3/5 Do, while dxy, dyz and dxz (t2g set) are lowered by 2/5 Do. In tetrahedral complexes, splitting is inverted and Dt = 4/9 Do; low spin configurations are rarely observed. The crystal field splitting energy Do depends on the nature of the ligand.

Ligands are arranged in the spectrochemical series in order of increasing field strength: I- < Br- < SCN- < Cl- < S2- < F- < OH- < C2O42- < H2O < NCS- < edta4- < NH3 < en < CN- < CO. Weak field ligands (Do < P, where P is pairing energy) form high spin complexes; strong field ligands (Do > P) form low spin complexes. The colour of coordination compounds is attributed to d-d transitions: [Ti(H2O)6]3+ absorbs blue-green light (498 nm) and appears violet. In the absence of ligands, d orbital splitting does not occur and the substance is colourless. Limitations of CFT include its failure to account for the fact that anionic ligands produce weak fields and its neglect of covalent character of metal-ligand bonding.

Homoleptic carbonyls are formed by most transition metals. Ni(CO)4 is tetrahedral, Fe(CO)5 is trigonal bipyramidal, Cr(CO)6 is octahedral, Mn2(CO)10 consists of two square pyramidal units joined by a Mn-Mn bond, and Co2(CO)8 has a Co-Co bond bridged by two CO groups.

The metal-carbon bond in metal carbonyls has both sigma and pi character. The M-C sigma bond is formed by donation of a lone pair from the carbonyl carbon into a vacant metal orbital. The M-C pi bond is formed by back-donation of a filled d orbital electron pair from the metal into the vacant antibonding pi* orbital of CO. This synergic bonding between metal and CO ligand mutually strengthens the metal-carbon bond.

Coordination compounds are vital in biological systems: chlorophyll (Mg complex) enables photosynthesis; haemoglobin (Fe complex) carries oxygen; vitamin B12 or cyanocobalamine (Co complex) prevents pernicious anaemia; enzymes like carboxypeptidase A and carbonic anhydrase contain coordinated metal ions. In analytical chemistry, EDTA, DMG (dimethylglyoxime), and related chelating reagents are used for detection and estimation of metal ions; water hardness is estimated by titration with Na2EDTA.

In metallurgy, gold is extracted as [Au(CN)2]- using cyanide; impure nickel is purified via [Ni(CO)4] formation and decomposition. In industry, the Wilkinson catalyst [(Ph3P)3RhCl] is used for alkene hydrogenation. Electroplating with silver and gold uses [Ag(CN)2]- and [Au(CN)2]-; black and white photography uses [Ag(S2O3)2]3- as a fixing agent. Chelate therapy in medicine includes D-penicillamine for copper/iron removal, EDTA for lead poisoning treatment, and cis-platin for tumour inhibition.

Coordination compounds represent a fascinating intersection of chemical bonding, structural chemistry and reactivity. Werner's pioneering theory laid the foundation, later refined by Valence Bond Theory and Crystal Field Theory, each offering complementary explanations for bonding, geometry, colour and magnetic behaviour. The spectrochemical series and concepts of high spin/low spin complexes provide powerful tools for predicting the properties of coordination entities.

For board examinations, this chapter demands a thorough understanding of nomenclature rules, isomerism types with examples, hybridisation schemes in VBT, CFT splitting diagrams and the spectrochemical series. Applications of coordination compounds in biological systems, medicine, metallurgy and analytical chemistry are frequently examined. Mastery of the key terms and the ability to write formulas and IUPAC names are essential skills.