Class 12 Chemistry · Chapter 5 NotesCoordination Compounds

Alfred Werner proposed that a metal ion in a coordination compound shows two types of linkages. The primary valence is normally ionisable and is satisfied by negative ions. The secondary valence is non-ionisable, is satisfied by neutral molecules or negative ions, and equals the coordination number, which is fixed for a given metal. The groups bound by secondary linkages occupy definite spatial arrangements called coordination polyhedra. Werner studied cobalt(III) chloride-ammonia compounds: 1 mol CoCl3.6NH3 (yellow) gave 3 mol AgCl, CoCl3.5NH3 (purple) gave 2 mol AgCl, and both forms of CoCl3.4NH3 gave 1 mol AgCl. This showed that six groups remain bonded to cobalt, so the compounds were written as [Co(NH3)6]3+3Cl-, [CoCl(NH3)5]2+2Cl- and [CoCl2(NH3)4]+Cl-. The species inside the square bracket is the coordination entity; ions outside are counter ions. Octahedral, tetrahedral and square planar shapes are common.

• Primary valenceionisable, satisfied by negative ions
• Secondary valencenon-ionisable, equals coordination number
• Coordination entitycentral metal atom or ion bonded to a fixed number of ions or molecules
• Counter ionsions written outside the square bracket

A ligand is an ion or molecule bound to the central atom or ion, which acts as a Lewis acid. A ligand bound through one donor atom is unidentate (Cl-, H2O, NH3); through two donor atoms it is didentate (ethane-1,2-diamine, oxalate); and with several donor atoms it is polydentate. EDTA4- is a hexadentate ligand, binding through two nitrogen and four oxygen atoms. A di- or polydentate ligand that uses two or more donor atoms simultaneously to bind one metal ion is a chelate ligand, and the number of ligating groups is its denticity; chelate complexes are more stable than similar unidentate ones. An ambidentate ligand has two different donor atoms and can bind through either, as NO2- (through N or O) and SCN- (through S or N). The coordination number is the number of ligand donor atoms directly sigma-bonded to the metal; pi bonds are not counted. The coordination sphere is the central atom plus its ligands enclosed in square brackets. The spatial arrangement of directly attached ligand atoms gives the coordination polyhedron, commonly octahedral, square planar or tetrahedral. The oxidation number is the charge the central atom would carry if all ligands were removed with their shared electron pairs, written as a Roman numeral. Homoleptic complexes contain only one kind of donor group; heteroleptic complexes contain more than one kind.

• UnidentateCl-, H2O, NH3; didentate: en, C2O4^2-; hexadentate: EDTA4-
• AmbidentateNO2- and SCN-
• Coordination number counts only sigma-bonded donor atoms
• Homoleptic[Co(NH3)6]3+; heteroleptic: [Co(NH3)4Cl2]+

IUPAC rules give an unambiguous way of writing formulas and names. In a formula, the central atom is listed first, then ligands in alphabetical order regardless of charge; polydentate ligands and abbreviations are also placed alphabetically. The whole coordination entity is enclosed in square brackets, polyatomic ligands and abbreviations in parentheses, with no space between ligands and metal. The charge on a charged entity is shown as a right superscript outside the bracket, number before sign, for example [Co(CN)6]3-. While naming, the cation is named first, then ligands alphabetically before the name of the central atom. Anionic ligand names end in -o; neutral ligands keep their names except aqua for H2O, ammine for NH3, carbonyl for CO and nitrosyl for NO. Prefixes mono, di, tri indicate the number of each ligand; bis, tris, tetrakis are used when the ligand name already contains a numerical prefix, as in dichloridobis(triphenylphosphine)nickel(II). The metal oxidation state is shown by a Roman numeral in parentheses. In complex cations the metal keeps its element name; in complex anions the name ends in -ate, with Latin names used for some metals, such as ferrate for Fe.

• Formulacentral atom first, ligands alphabetical, entity in square brackets
• Namecation first, ligands alphabetical, then metal with oxidation state
• Anionic ligands end in -o; aqua, ammine, carbonyl, nitrosyl for neutral ligands
• Anionic complexmetal name ends in -ate (for example ferrate)

Isomers have the same chemical formula but different arrangements of atoms, so they differ in physical or chemical properties. Stereoisomers have the same bonds but different spatial arrangement, while structural isomers have different bonds. Geometrical isomerism arises in heteroleptic complexes of coordination number 4 and 6. In square planar [MX2L2], the two X ligands may be cis (adjacent) or trans (opposite); [MX2(L-L)2] and octahedral [MX2L4] also show cis and trans forms, and [Ma3b3] shows facial (fac) and meridional (mer) isomers. Tetrahedral complexes do not show geometrical isomerism because the relative positions of unidentate ligands are the same. Optical isomers are non-superimposable mirror images called enantiomers; such molecules are chiral and are labelled dextro (d) and laevo (l) by the direction they rotate plane-polarised light. Optical isomerism is common in octahedral complexes with didentate ligands, such as [Co(en)3]3+, and only the cis isomer of [PtCl2(en)2]2+ is optically active. Structural isomerism includes linkage isomerism (ambidentate ligand binding through different atoms), coordination isomerism (interchange of ligands between cationic and anionic entities), ionisation isomerism (counter ion itself a potential ligand, for example [Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4) and solvate or hydrate isomerism, as in [Cr(H2O)6]Cl3 (violet) and [Cr(H2O)5Cl]Cl2.H2O (grey-green).

• Stereoisomerismgeometrical and optical
• Structural isomerismlinkage, coordination, ionisation and solvate
• cis and trans isomers; fac and mer isomers for [Ma3b3]
• Enantiomersd and l forms; chiral molecules rotate plane-polarised light

According to Valence Bond Theory, the metal atom or ion uses its (n-1)d, ns, np or ns, np, nd orbitals for hybridisation, giving equivalent orbitals of definite geometry. Coordination number 4 gives sp3 (tetrahedral) or dsp2 (square planar); coordination number 5 gives sp3d (trigonal bipyramidal); coordination number 6 gives sp3d2 or d2sp3 (octahedral). In [Co(NH3)6]3+, cobalt is +3 with a 3d6 configuration; the inner 3d orbital is used, so it is an inner orbital, low spin, spin paired and diamagnetic complex. [CoF6]3- uses outer 4d orbitals (sp3d2) and is an outer orbital, high spin, spin free complex. [NiCl4]2- is tetrahedral and paramagnetic with two unpaired electrons, while [Ni(CN)4]2- is square planar (dsp2) and diamagnetic. Magnetic susceptibility measurements give the number of unpaired electrons. For d4 and d5 ions, [Mn(CN)6]3- has two unpaired electrons while [MnCl6]3- has four; [Fe(CN)6]3- has one unpaired electron while [FeF6]3- has five; [CoF6]3- is paramagnetic with four unpaired electrons while [Co(C2O4)3]3- is diamagnetic. The limitations of VBT are that it uses many assumptions, gives no quantitative interpretation of magnetic data, does not explain colour, does not give quantitative thermodynamic or kinetic stability, cannot make exact predictions for tetrahedral versus square planar 4-coordinate complexes, and does not distinguish weak from strong ligands.

• Hybridisationsp3 tetrahedral, dsp2 square planar, sp3d2 and d2sp3 octahedral
• Inner orbital or low spind2sp3; outer orbital or high spin: sp3d2
• Magnetic moment reveals the number of unpaired electrons
• VBT does not explain colour or give quantitative magnetic data

Crystal Field Theory treats the metal-ligand bond as purely ionic, with ligands as point charges or point dipoles. In an isolated metal atom the five d orbitals are degenerate; an asymmetrical field of ligands lifts this degeneracy. In an octahedral field, the dx2-y2 and dz2 orbitals point towards the ligands, experience more repulsion and form the higher energy eg set, while dxy, dyz and dxz form the lower energy t2g set. The energy separation is the crystal field splitting, delta-o; the eg orbitals rise by (3/5)delta-o and the t2g orbitals fall by (2/5)delta-o. Ligands arranged in order of increasing field strength form the spectrochemical series: I- < Br- < SCN- < Cl- < S2- < F- < OH- < C2O4^2- < H2O < NCS- < edta4- < NH3 < en < CN- < CO. If delta-o is less than the pairing energy P, the fourth d electron enters an eg orbital (t2g3 eg1) giving a high spin complex with weak field ligands; if delta-o is greater than P, it pairs in a t2g orbital (t2g4 eg0) giving a low spin complex with strong field ligands. In tetrahedral fields the splitting is inverted and smaller, delta-t = (4/9)delta-o, so low spin configurations are rare. Colour arises from d-d transitions: [Ti(H2O)6]3+ absorbs blue-green light and appears violet, and the colour observed is complementary to the light absorbed. Anhydrous CuSO4 is white while CuSO4.5H2O is blue. Limitations of CFT include that anionic ligands should exert the greatest splitting by the point charge model but actually lie at the low end of the spectrochemical series, and it ignores covalent character. In metal carbonyls the M-C sigma bond forms by donation of the carbonyl carbon lone pair to a vacant metal orbital, and the M-C pi bond forms by donation from a filled metal d orbital into the vacant antibonding pi* orbital of CO; this synergic effect strengthens the bond.

• Octahedral splittingt2g lower, eg higher, separation delta-o
• delta-o < P gives high spin; delta-o > P gives low spin
• Spectrochemical series orders ligands by field strength
• Colour is due to d-d transitions; observed colour is complementary to absorbed light
• Metal carbonyls show synergic sigma and pi bonding

Coordination compounds occur widely in minerals, plants and animals and are used in analytical chemistry, metallurgy, biological systems, industry and medicine. Colour reactions with chelating ligands such as EDTA, dimethylglyoxime, alpha-nitroso-beta-naphthol and cupron form the basis of detection and estimation of metal ions. Hardness of water is estimated by titration with Na2EDTA, since Ca2+ and Mg2+ form stable complexes with EDTA. Gold combines with cyanide in the presence of oxygen and water to form [Au(CN)2]-, from which gold is separated by adding zinc; impure nickel is converted to [Ni(CO)4] and then decomposed to give pure nickel. Chlorophyll is a magnesium complex, haemoglobin an iron complex and vitamin B12 (cyanocobalamine) a cobalt complex; enzymes such as carboxypeptidase A and carbonic anhydrase contain coordinated metal ions. Wilkinson catalyst, [(Ph3P)3RhCl], is used for hydrogenation of alkenes. Electroplating with silver and gold uses [Ag(CN)2]- and [Au(CN)2]-; fixing of developed film uses hypo to dissolve undecomposed AgBr as [Ag(S2O3)2]3-. In chelate therapy, D-penicillamine and desferrioxime B remove excess copper and iron, EDTA treats lead poisoning, and platinum compounds such as cis-platin inhibit tumour growth.

• AnalysisEDTA, DMG and other chelating reagents; water hardness by Na2EDTA titration
• Metallurgy[Au(CN)2]- for gold, [Ni(CO)4] for nickel purification
• Biologychlorophyll (Mg), haemoglobin (Fe), vitamin B12 (Co)
• MedicineEDTA for lead poisoning, cis-platin for tumours