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The Coordination Compounds (Complexes).

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Complex compounds are the most widespread and diverse class of inorganic substances. They also include many organometallic compounds that combine the previously isolated inorganic chemistry and organic chemistry into a single entity. Many complex compounds – vitamin B12, hemoglobin, chlorophyll and others – play a great role in physiological and biochemical processes.

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The properties and structure of complex compounds are explained most successfully by the coordination theory proposed by Alfred Werner, a Swiss chemist, Nobel Prize winner. He was one of the founders of the concept of complex compounds.

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According to the coordination theory, one of the ions in a molecule of any complex compound generally a positively charged one, occupies a central position and is called a complexing agent or central ion. Around it are arranged, or, as we say, coordinated, a certain number of oppositely charged ions or electrically neutral molecules called ligands (or addends) and forming the inner coordination sphere of the compound. The remaining ions not accommodated in the inner sphere are farther from the central ion and form the outer coordination sphere.

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For example, the coordination formula of the complex salt having the composition PtCl4·2KCl is K2[PtCl6]. Here the inner sphere consist of a central platinum atom in the oxidation number +4 and chloride ions, while the potassium ions are in the outer sphere. The main characteristic of central ion is the coordination number. It shows how many bonds the central ion may forms with ligands.

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The coordination number is not a constant quantity for given complexing agent, but also depends on the nature of the ligand and its electron properties. Even for the same complexing agents and ligands, the coordination number depends on the state of aggregation, the concentration of the components, and the temperature of the solution.

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Central atom charge

Coordinative number

Type of hybrid. Example

+1 2 Sp [Ag(NH3)2]Cl

+2 4,6 sp3, dsp2 K4[Fe(CN)6]

[Cu(NH3)4]Cl2

+3 6,4 dsp2 K3[Fe(CN)6]

+4 8 d2sp3 [Pt(NH3)4Cl4]

The Coordination Number

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Chemical Bonding

29Cu 1s22s22p63s23p64s13d104p0 , the outer layer 4s14p0

Cu+2 3d94s04p0 it has 4 free orbitals (4 bonds).

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The bonds between central ion and ligands are donor-acceptor (dative).

27Co 1s22s22p63s23p64s23d74p0

Co+3 [Ar]3d64s04p0 6 free orbitals remain (6 bonds).

d2sp3 hybridization

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Ligands occupying one site in the inner coordination sphere are called monodentates (examples of monodentate ligands are OH‾, Cl‾, I‾, Br‾, CN‾, H2O, NH3). Ligands that occupy two or several sites in the inner sphere are called bi- and polydentates.

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ClassificationI. According to the inner sphere charge there are:1.The Anionic complexes - Na2[NiCl4]-2 2. The Cationic complexes – [Zn(NH3)4]+2Cl2

3. The Neutralic complexes – Pt(NH3)2Cl2]0

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II. According to the nature of the ligands. 1.Ammines – complexes in which ammonia molecules are the ligands, for instance [Cu(NH3)4]SO4 – tetraamminecupper (II) sulfate; [Co(NH3)6]Cl3 – hexaamminecobalt (III) chloride.2.Aquacomplexes contain water as the ligand: [Co(H2O)6]Cl2 , [Al(H2O)6]Cl3. Hydrated cations in an aqueous solution contain an aquacomplex as the central unit.

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3.Acidocomplexes. In these complexes, anions are the ligands. They include complexes of the double salt type, for example, K2[PtCl4], K4[Fe(CN)6] (they can be represented as the product of the coupling of two salts - PtCl4·2KCl, Fe(CN)2·4KCN, etc.), complex acids – H2[SiF6], H2[CoCl4], hydroxocomplexes – Na2[Zn(OH)4].4. Transition series exist between these classes, which include complexes with different ligands. K[Pt(NH3)Cl3].

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5.Cyclic, or chelate (from the Greek word “chele” – claw) complex compounds contain a bi- or polydentate ligand that grips the central atom like the claws of a crab:

:NH2 – CH2

M :NH2 – CH2

In such complexes, the symbol M stands for a metal atom, and the arrow depicts a donor–acceptor mechanism of covalent bonding.

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Examples of such complexes are the oxalate complex of iron (III) [Fe(C2O4)3]3- and the ethylenediamine complex of platinum (IV) [PtEn3]4+. The group of chelates also includes intracomplex compounds in which the central atom is part of a ring, forming covalent bonds with ligands in various ways – donor–acceptor mechamism of covalent bond and bonds at the expense of unpaired atomic electrons.

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Complexes of this kind are very characteristic of the aminocarboxylic acids. For example: EDTA

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Hemoglobin

Chelate compounds are extremely stable because their central atom is “blocked”, as it were, by the cyclic ligand. Chelates with five- and six- membered rings have the highest stability.

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Naming complexes.The coordination compounds are named in the following way. A. To name a coordination compound, no matter whether the complex ion is the cation or the anion, always name the cation before the anion. (This is just like naming an ionic compound.)

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B. In naming the complex ion:1. Name the ligands first, in alphabetical order, then the metal atom or ion. Note: The metal atom or ion is written before the ligands in the chemical formula. 2. The names of some common ligands are listed in Table 1..

� For anionic ligands end in "-o"; for anions that end in "-ide"(e.g. chloride), "-ate" (e.g. sulfate, nitrate), and "-ite" (e.g. nirite), change the endings as follows: -ate -ide ato; -ito.-o; -ite

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For neutral ligands, the common name of the molecule is used e.g. H2NCH2CH2NH2 (ethylenediamine). Important exceptions: water is called ‘aqua’, ammonia is called ‘ammine’, carbon monoxide is called ‘carbonyl’, and the N2 and O2 are called ‘dinitrogen’ and ‘dioxygen’.

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Anionic Ligands

Names Neutral Ligands

Names

Br- Bromo NH3 Ammine F- Fluoro H2O Aqua O2- Oxo NO Nitrosyl OH- Hydroxo CO Carbonyl CN- Cyano O2 Dioxygen C2O4

2- Oxalato N2 Dinitrogen CO3

2- Carbonato C5H5N Pyridine CH3COO- Acetato H2NCH2CH2NH2 Ethylenediamine

Table 1. Names of Some Common Ligands

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3. Greek prefixes are used to designate the number of each type of ligand in the complex ion, e.g. di-, tri- and tetra-. If the ligand already contains a Greek prefix (e.g. ethylenediamine) or if it is polydentate ligands (ie. can attach at more than one binding site) the prefixes bis-, tris-, tetrakis-, pentakis-, are used instead. The numerical prefixes are listed in Table 2.

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Number

Prefix Number

Prefix Number

Prefix

1 mono 5 penta (pentakis)

9 nona (ennea)

2 di (bis) 6 hexa (hexakis)

10 deca

3 tri (tris) 7 hepta 11 undeca

4 tetra (tetrakis)

8 octa 12 dodeca

  

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4. After naming the ligands, name the central metal. If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is call cobalt and Pt is called platinum. (See examples 1-4). If the complex ion is an anion, the name of the metal ends with the suffix –ate. (See examples 5 and 6.). For example, Co in a complex anion is called cobaltate and Pt is called platinate. For some metals, the Latin names are used in the complex anions e.g. Fe is called ferrate (not ironate).

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Name of Metal Name in an Anionic Complex

Iron Ferrate

Copper Cuprate

Lead Plumbate

Silver Argenate

Gold Aurate

Tin Stannate

Table 3: Name of Metals in Anionic Complexes

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5. Following the name of the metal, the oxidation state of the metal in the complex is given as a Roman numeral in parentheses. C. To name a neutral complex molecule, follow the rules of naming a complex cation. Remember: Name the (possibly complex) cation BEFORE the (possibly complex) anion.For historic reasons, some coordination compounds are called by their common names. For example, Fe(CN)6

3 and Fe(CN)64 are named ferricyanide and

ferrocyanide respectively, and Fe(CO)5 is called iron carbonyl.

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[Pt(NH3)5Cl]Br3 pentaamminechloroplatinum(IV) bromide[Co(H2NCH2CH2NH2)3]2(SO4)3 tris(ethylenediamine)cobalt(III) sulfate.K4[Fe(CN)6] potassium hexacyanoferrate(II) Na2[NiCl4] sodium tetrachloronickelate(II) Pt(NH3)2Cl4 diamminetetrachloroplatinum(IV) Fe(CO)5 pentacarbonyliron(0)

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Isomerism Structural isomerismStructural isomerism occurs when the bonds are themselves different. Linkage isomerism is only one of several types of structural isomerism in coordination complexes (as well as other classes of chemical compounds).

StereoisomerismStereoisomerism occurs with the same bonds in different orientations relative to one another.

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Cis-trans isomerism. When two ligands are mutually adjacent they are said to be cis, when opposite each other, trans.

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Λ-[Fe(ox)3]3− Δ-[Fe(ox)3]3− Λ-cis-[CoCl2(en)2]+ Δ-cis-[CoCl2(en)2]+

Optical isomerism.Optical isomerism occurs when the mirror image of a compound is not superimposable with the original compound. It is so called because such isomers are optically active, that is, they rotate the plane of polarized light.

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Stability of Complex Compounds in Solutions. Constant of Instability.The aqueous silver forms a complex ion with ammonia by reacting with NH3 in steps:

Ag+(aq) + NH3 (aq) [Ag(NH3)]+

(aq)

Ag(NH3)+(aq) + NH3 (aq) [Ag(NH3)2]+

(aq)

When we add these equations, we get overall equation for the formation of the complex ion Ag(NH3)2

+.

Ag+(aq) + 2NH3 (aq) [Ag(NH3)2]+

(aq)

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The formation constant, or stability constant, Kf, of a complex ion is the equilibrium constant for the formation of the complex ion from aqueous metal ion and the ligands. Thus, the formation constant of [Ag(NH3)2]+ is:

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The value of Kf for [Ag(NH3)2]+ is 1.7×107.The ionization constant (Ki) or (Kd) for a complex ion is the inverse value of Kf . The equation for the dissociation of [Ag(NH3)2]+ is:[Ag(NH3)2]+

(aq) Ag+(aq) + 2NH3 (aq)

and its equlibrium constant is

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Chemical Properties.I. Without destruction of inner sphere.

1.(NH4)2[Hg(SCN)4] + ZnSO4 → (NH4)SO4 + + Zn[Hg(SCN)4]↓

2. 2K4[Fe(CN)6] + H2O2 → 2K3[Fe(CN)6] + 2KOH

2│[Fe(CN)6]4- - ℮ → [Fe(CN)6]3-

1│H2O2 + 2℮ → 2OH-

2[Fe(CN)6]4- +H2O2 → 2[Fe(CN)6]3- + 2OH-

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II. With destruction of inner sphere.

1.[Ag(NH3)2]Cl + 2HNO3 → AgCl↓ + 2NH4NO3

2.H2[SnCl6] + Zn → ZnCl2 + SnCl2 + 2HCl

1 │ [SnCl6]2- + 2℮ → Sn2+ + 6Cl-

1 │ Zn0 - 2℮ → Zn2+

[SnCl6]2- + Zn0 → Sn2+ + 6Cl- + Zn2+

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Thank You!