Complex compounds

 

Coordination compound. You have come across compounds like Na[Ag(CN)2] and Na2[Zn(CN)4]. Such compounds are referred to as coordination compounds or complex compounds. Coordination compounds play an important role in the chemical industry and in life itself. For example, the Ziegler- Natta catalyst which is used for polymerization of ethylene, is a complex containing the metals aluminum and titanium. Metal complexes play important role in biological systems. For example, chlorophyll, which is vital for photosynthesis in plants, is a magnesium complex and hemoglobin, which carries oxygen to animal cells, is an iron complex. These are the compounds that contain a central atom or ion, usually a metal, surrounded by a number of ions or molecules. The complexes tend to retain their identity even in solution, although partial dissociation may occur. Complex ion may be cationic, anionic or nonionic, depending on the sum of the charges of the central atom and the surrounding ions and molecules [29, p. 1].

 

Werner’s Coordination Theory. Coordination compounds were known in eighteenth century. It was a mystery for the chemist, of those days to understand as to why a stable salt like CoCl3 reacts with varying number of stable molecules or compounds such as ammonia to give several new compounds: CoCl3 × 6NH3, CoCl3 × 5NH3 and CoCl3 × 4NH3; and what are their structures? These compounds differed from each other in their chloride ion reactivity. Conductivity measurements on solutions of these compounds showed that the number of ions present in solution for each compound are different. Several theories were proposed, but none could satisfactorily explain all the observable properties of these compounds and similar other series of compounds which had been prepared by then. It was only in 1893 that Werner put forward a set of ideas which are known as Werner’s coordination theory, to explain the nature of bonding in complexes. His theory has been a guiding principle in inorganic chemistry and in the concept of valence. The important postulates of Werner’s theory are:

1.      Metals exhibit two types of valence:

a)      Primary valence (ionizable)

b)      Secondary valence (non-ionizable).

  

Primary or ionizable valence is satisfied by negative ions and corresponds to oxidation state of the metal. The secondary or non-ionizable valence, which is satisfied by negative, positive or neutral groups, is equal to the coordination number of metal ion.

Every metal tends to satisfy both its primary and secondary valence.

 

2.      The secondary valence is directed toward fixed positions in space i.e. this has spatial arrangement corresponding to different coordination number.

 

For the complexes CoCl3 × 6NH3, CoCl3 × 5NH3 and CoCl3 × 4NH3, the number of ionizable ions in these complexes are three, two and one, respectively. It has been proved by precipitation reactions and conductivity measurements. On the basis of Werner’s postulate these compounds are formulated as:

[Co(NH3)6]Cl3, [Co(NH3)5Cl]Cl2 and [Co(NH3)4Cl2]Cl, respectively, the species inside the square brackets being the complex ion and outside the square brackets the ionisable ions.

One of the three chloride ions satisfy both primary and secondary valence. He also postulated that octahedral, tetrahedral and square planar shapes are more common for coordination compounds of transition elements. Six coordinated complexes such as [Ni(NH3)6]2+ and [Co(NH3)6]3+ are octahedral whereas four coordinated such as [NiCl4]2- and [Ni(CN)4]2- are tetrahedral and square planar, respectively [29, p. 2].

 

Classification of complex compounds. Currently, there are several classifications of complex compounds.

Classification on the nature of the ligands. The basis of this classification is the nature of the ligands of complexing agent:

·         Acidocomplexes (from latin acidum - acid) ligands are the residues of acids: CN-, Cl-, Br-, I-, for example H[AuCl4], K2[HgI4], the residues of many organic acids: oxalate ion C2O42-, aminopolycarbonic acid residues, etc.;

·         Amminecomplexes. The ligands are molecules of ammonia NH3, for example, [Cu (NH3)4](NO3)2, [Ag (NH3)2]Cl;

·         Aquacomplexes. The ligands are water molecules: [Cr(H2O)6]Cl3, [Cu(H2O)4](NO3)2;

·         Hydroxocomplexes. The ligands are hydroxide ions: K3[Co(OH)6];

·         Carbonyl complexes. The ligands are CO molecules: [Ni(CO)4], [Cr(CO)6].

 

The inner sphere may be acid residues and neutral groups, for example: [Cr(NH3)4Cl2]Cl. Such complexes are mixed.

 

Chelation-complexes are complexes in which the metal atom-complexing agent is associated with organic ligands in several relationships (see table 10.1). The complexing agent and ligands form loops, the strength of which is change due to Chugaev’s rule (most stable of these compounds contain in the inner sphere of five- or six-membered rings).

 

Table 10.1

The most common ligands:

Types of ligand

 

Chemical formula

The name of the ligand

Negative ions

 

CH3COO-

acetato-

 

F-, Cl-,Br-, I-

the fluoro -, chloro- bromo-, iodo-

 

OH-

hydroxo-

 

CN-

cyano-

 

SCN-

thiocyano, tiocianato-

 

NO2-

nitro-

 

NO3-

nitrato-

 

SO42-

sulphato-

 

SO32-

sulphito-

 

S2-

thio-, sulphido-

 

S22-

disulphido-

 

O22-

peroxo-

 

Positive ions

 

NH4+

ammonium-

 

 

OH3+

hydroxoniy-

 

Neutral molecules

 

NH3

ammine-

 

CO

carbonyl-

 

H2O

aqua-

 

NO

nitrosyl-

 

N(CH3)3

trimethylamine-

 

Chelateformation particles

 

C2O42-

oxalate-

 

NH2-CH2-COO-

glicinato-

 

NH2-CH2-CH2-NH2

ethylenediamine-

 

 

  

The ligands are characterized by denticity - ability to form multiple coordination bonds with ions-complexing agent. There are monodentate and polydentate ligands. The coordination number is equal to the number of monodentate ligands coordinating by the central atom. For example, [Cu(NH3)4]2+, the coordination number is 4. Due to NH3 (ammonia) is monodentate ligand, and 4 molecules of ammonia are filled 4 places around central atom (Cu2+) [30, p. 15].

 

The nomenclature of complex compounds. The name of the complex compounds are composed as follows: 

Name the ligands first, in alphabetical order, then the central atom or ion. Name of the complex is written in one word. Neutral ligands called without changes; in the names of negatively charged ligands add «o» to the end. Greek prefixes are used to designate the number of each type of ligand in the complex ion, e.g. di-, tri- and tetra-

Name of complexing agent depends on the charge of the complex. For neutral and cationic – English name of the cation. For the anionic complex - the Latin suffix «ate». Indicate the degree of oxidation of complexing agent using Roman numerals in parentheses.

Examples of names of the ligand: Aqua - H2O, NH3 - ammine,CO – carbon monoxide, NO - nitrosyl, OH- - hydroxo, CN- - cyano, NO2-, SO32-nitro-, carbonato-, sulphito-, SO42- - sulphato- , Cl- -chloro-.

For Example:

[Ag (NH3)2]Cl Diamminesilver(I) chloride;

[Cu(NH3)4](OH)2 Tetraamminecopper(II) hydroxide;

[Al(H2O)5OH]Cl2 Hydroxopentaaqaaluminium(III) chloride;

[Pt(H2O)3OH]NO3 Hydroxotriaquaplatinum(II) nitrate;

[Co(NH3)4CO3]Cl Carbonatotetramminecobalt(III) chloride;

[Al(H2O)6]Cl3 Hexaaquaaluminium(III) chloride.

K3[Fe (CN)6] potassium hexacyanoferrate(III);

[Cr(H2O)3F3]-trifluorotriaquachromium(III) [30, p. 17]

 

Isomerism of the complex compounds. Isomerism is a phenomenon, when substances have the same qualitative and quantitative composition, but have a different structure, and hence different properties.

There are geometric, optical, hydrated, ionization, coordination, etc. types of isomerism of complex compounds.

Geometric (spatial) isomerism is common for complex compounds with different (heterogeneous) ligands. The geometric isomers have different placement of inhomogeneous ligands in a complex which has square-planar or octahedral structure.

For example, for the complex [Pt(NH3)2Cl2] with square-planar structure, there are two geometric isomers (CIS-and TRANS-isomers) (see figure 10. 1), which explains the difference of their properties (different color, dipole moment, reactivity):

 

10.1.png

Figure 10.1 CIS-andTRANS-isomers

 

CIS (cis) refers to one side, close; and TRANS - (trans) - on different sides. For complex [Co(NH3)4Cl2] is octahedral structure (see figure 10.2) of geometrical isomers can be shown schematically                                                                

 

10.2.png

Figure 10.2 Complex [Co(NH3)4Cl2] is octahedral structure

 

Hydrate isomers are substances which have the same composition, but is different allocation of molecules of the solvent between the internal and external spheres of complex compounds. For example, crystalline hydrate corresponds to four CrCl3 × 6H2O isomer:

 

[Cr(H2O)6]Cl3;

[Cr(H2O)5Cl]Cl2 × H2O;

[Cr(H2O)4Cl2]Cl × 2H2O;

[Cr(H2O)3Cl3] × 3H2O.

 

All isomers have different colour.

So:

 

[Cr(H2O)6]Cl3 - purple;

[Cr(H2O)5Cl]Cl2 × H2O - green;

[Cr(H2O)4Cl2]Cl × 2H2O - dark green.

 

Ionization isomers are substances which have the same composition, but isdetermined by the different distribution of charged ligands between internal and external spheres of the complex. These isomers are also distinguished by colour:

 

[CoBr(NH3)5]SO4 - red-purple

[CoBr(NH3)5]SO4 → [CoBr(NH3)5]2+ + SO42-.

[CoSO4(NH3)6]Br - red

[CoSO4(NH3)6]Br → [CoSO4(NH3)6]+ + Br-

 

Coordination isomerism. When both positive and negative ions of a salt are complex ions and the two isomers differ in the distribution of ligands between the cation and the anion occurs coordination isomerism. For example [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6], [Pt(NH3)4][PtCl4] and [Pt(NH3)3Cl][Pt(NH3)Cl3]

That is, this type of isomerism is possible for compounds composed of two or more systems, and complexing agnt exchange their ligands [30, p. 18].

 

Properties of coordination compounds. Complex compounds are involved in various chemical reactions-substitution, exchange, isomerization, redox processes.

1.      Stability of the complex compounds in solutions.

Neutral complexes (coordination compounds with no external coordination sphere) are non-electrolytes, i.e. does not dissociate in the aqueous solutions on ions.

Complex compounds with complex ion (cation or anion) dissociate in aqueous solutions on Internal and external coordination spheres. This dissociation of complex compounds is called the primary. It is almost completely. There are complex compounds that have external coordination sphere, dissociate in aqueous solutions as strong electrolytes. For example, [Cu(NH3)4]SO4, K[AuCl4] such as simple salt dissociation:

 

[Cu(NH3)4]SO4 → [Cu(NH3)4]2+ + SO42-

K[AuCl4] → K+ + [AuCl4]-

 

In turn complex ion is also capable to dissociate as electrolyte, but weak or middle force, that is, reversible (secondary dissociation):

 

[Cu(NH3)4]2+ ⇄ Cu2+ + 4NH3

[AuCl4]- ⇄ Au3+ + 4Cl-

 

The stability constant characterizes the equilibrium process of formation of a complex that also happens speed. For example, the formation of the complex of [Cu(NH3)4]2+, you can show the following equations:

 

I-st step Cu2+ + NH3 ⇄ [Cu(NH3)]2+;

II-nd step [Cu(NH3)]2+ + NH3 ⇄ [Cu(NH3)2]2+;

III-d step [Cu(NH3)2]2+ + NH3 ⇄ [Cu(NH3)3]2+;

IV-th step [Cu(NH3)3]2+ + NH3 ⇄ [Cu(NH3)4]2+.

 

Each stage corresponds to a certain value of the stability constant (or β).The General equation of complex formation is

 

[Cu(NH3)4]2+ ⇄Cu2+ + 4NH3 ⇄ [Cu(NH3)4]2+

 

In the reaction of complex compound [Ag(NH3)2]Cl, ions of hydrogen of nitric acid react with ammonia molecules to produce a strong ammonium ion, which can show the equation:

 

[Ag (NH3)2]Cl + 2HNO3 → AgCl ↓ + 2 [NH4] NO3

 

(NH4+ is usuall simplistic designation a complex ion of ammonium).

As noted above, complex compounds participate in various chemical reactions, such as substitution, exchange, isomerization, ox/red processes. Here are some examples:

2.      Reactions of substitution of ligands in a complex ion internal coordination sphere:

 

[Cu(H2O)4]SO4 + 4NH3 → [Cu(NH3)4] SO4 + 4H2O;

Zn + 2Na[Au(CN)2] → 2Au + Na2 [Zn(CN)4]

 

3.      Exchange reactions. Exchange reactions of complex compounds often find application in analytical chemistry for the qualitative detection of certain ions.

For example, detection of the Zn2+ cation with hexacyanoferrate (II) potassium (yellow blood salt). As a result of the reaction a white sediment hexacyanoferrate (II) potassium zinc formation can be shown by equation:

 

3ZnCl2 + 2K4[Fe(CN)6] → К2Zn3[Fe(CN)6]2↓ + 6KCl

3Zn2+ + 2К+ + 2[Fe(CN)6]4- → К2Zn3[Fe(CN)6]2↓

 

Reactions of Turnbule blue and the Prussian blue formation is used for detection of the the cations Fe2+ and Fe3+, respectively. It is proved that Prussian blue and Turnbule blue are identical in composition, as a result of the reaction they appear as dark blue precipitation:

 

FeCl2 + К3[Fe(CN)6]→ КFe[Fe(CN)6]↓ + 2KCl;

Potassium hexacyanoferrate(ІІІ)Turnbule blue

 

КFe[Fe(CN)6] ⇄К[FeFe(CN)6];

potassiumhexacyanoferrate(ІІ, ІІІ)

Fe2++ [Fe(CN)6]3- → КFe[Fe(CN)6]↓ ⇄ К[FeFe(CN)6]↓.

 

FeCl3 + К4[Fe(CN)6] → КFe[Fe(CN)6]↓ + 3KCl;

Potassium hexacyanoferrate (ІІ) Prussian blue

 

КFe[Fe(CN)6] ⇄ К[FeFe(CN)6];

potassiumhexacyanoferrate(ІІ, ІІІ)

Fe3++ [Fe(CN)6]4- → КFe[Fe(CN)6]↓ ⇄ К[FeFe(CN)6]↓.

 

4.      Redox reactions:

 

2K4[Fe(CN)6] + Cl2 → 2K3[Fe(CN)6] + 2KCl;

4[Co(NH3)6]2+ + O2 + 2H2O → 4[Co(NH3)6]3+ + 4OH- [30, p.20]