80353

COMPLEX OR COORDINATION COMPOUNDS

Лекция

Химия и фармакология

The formation of one common type of complex ion can be demonstrated also by adding colorless, anhydrous copper (II) sulfate to water. The resulting blue color of the solution is caused by the complex ion formed between water and copper (II) ions.

Английский

2017-02-21

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COMPLEX  OR  COORDINATION COMPOUNDS

One of the distinguishing characteristic of most transition elements is their ability to formcomplex ionsand coordination compounds. Coordination compounds are prevalent both in nature and in chemical laboratories. Dyes containing coordination compounds were used thousands of years ago. The red color of blood is caused by the presence of hemoglobin, a coordination compound containing Fe(II). Chlorophyll, which is found in plants, is a coordination compound similar in structure to hemoglobin but containing Mg(II) instead of Fe(II). A number of substances incorporated into fertilizers and foods are coordination compounds.

Complex ions. Central ion and ligands

Coordination compounds contain complex ions.

An ion which is surrounded by and bonded to a discrete group of coordinating particles is calleda complex ion

Complex ions may becationic neutralor anionic. An example of complex ion formation is

Because of its charge, a complex ion can combine with oppositely charged ions and form ionic crystals. For example, Cu(H2O)42+ is a complex ion which crystallises from an aqueous sulphate solution as an ionic coordination compound, CuSO4•5H2O.

The formation of one common type of complex ion can be demonstrated also by adding colorless, anhydrous copper(II) sulfate to water. The resulting blue color of the solution is caused by the complex ion formed between water and copper(II) ions.

The central ion of a complexis sometimes calledthe nuclear ion, while the surrounding particles (H2O, NH3, CO, F-, Cl-, CN-, SCN-) are calledligands. Most ligands have a lone pair of electrons that becomes coordinated to a central cation, and the charge on the resulting complex ion depends on the relative charges of the cations and the ligands.

.Theunfilleddorbitals,the relatively high positive charges,andthesmall radiiof the ions of the transition elements lead to the formation of a large variety of complex ions. Ions with a large positive charge density (intense electric fields) have a strong tendency to interact withligands all of which have highly electronegative atoms and unshared electron pairs.

In general, the smaller the positive ion and the larger the charge, the greater will be the tendency to form stable complexes.

Thus, the relatively large ions of the Group IA andIIA elements do not have as great a tendency to form stable complex ions as do the smaller, more highly charged ions of the transition elements. The general relationship between the charge/radius ratio of an ion and its tendency to form complex ions is illustrated in Table.

Table 1. Relationship between charge to radius ratio of ions and their complexing tendency

Ion

Radius (A)

Charge/Radius Ratio

Tendency to Form Complexes and Behaviour in Aqueous Solution

K+

1.33

0.71

No complexes; weak electrostatic forces between the ion and mantle

of water molecules surrounding it

Hg2+

1.10

1.8

Few weakly bonded complexes

Cd2+

0.97

2.0

Several complexes, ions weakly hydrated

Co2+

0.72

2.8

Generally complexed, ions hydrated

Fe3+

0.64

4.7

Always complexed, ions hydrated

Pt4+

0.65

6.2

Always complexed, ions hydrated

The green colour of aqueous solutions of Ni(II) ions, the blue of Cr(III), and the red of Co(II) ions result from the presence of aqua complexes of these ions.

Coordination number and coordination sphere

The number of ligands attached directly to the central species is known as thecoordination number of the complex.Thus, the coordination number for [Cu(H2O)4]2+ is 4. Most of the transition element complexes have coordination numbers of 2, 4, or 6, with 6 being the most common.

The central ion and the ligands coordinated about itare alwaysenclosed in square bracketsand constitutewhat is calledtheinner coordination sphere. Outside this sphere other less strongly held groups may be attached. Conductivity measurements, freezing-point measurements, chemical tests, and other instrumental measurements can be used to determine which groups are within and which are outside with the inner coordination sphere.

For example, all of the chloride in CoCl3 6NH3 can be precipitated as AgCl by addition of AgN03. In addition, conductance measurements indicate the presence of four moles of ions per mole of compound. Other experiments show that one mole of this compound furnishes three moles of C1- ions and one mole of [Co(NH3)6]3+ ions. This suggests that the chlorine is in the form of readily available ions which are not a part of the inner coordination sphere. Therefore the correct formula is [Co(NH3)6]Cl3.

When CoCl3 • 5NH3 is subjected to the same tests, only two thirds of the chlorine is precipitated by AgNO3. Furthermore, conductance tests indicate that three moles of ions are formed per mole of compound. On this basis the correct formula is[Co(NH3)5Cl]Cl2.

Nomenclature

Because of the large number of complicated coordination compounds, it has been necessary to develop a systematic method for naming them. Complex species may be acation such as [Cu(H2O)4]2+, an anion such as [Fe(CN)6]4-, or a neutral molecule such as [Cr(NH3)3Cl3]. Many different electronically satisfied entities may act as electron-pair donor ligands. The names of some common ligands are given in the margin. It may be seen that the names of negatively-charged ligands end ino. The name of the molecule is generally used for neutral ligands. Water and ammonia are the two important exceptions. The rules listed below enable you to name a large number of common complex substances. The rules for naming chemical compounds are established by nomenclature committees of the International Union of Pure and Applied Chemistry (IUPAC).

Rules for naming coordination complexes

The name of the positive ion is written before the name of the negative ion.

The name of the ligand is written before the name of the metal to which it is coordinated.

The Greek prefixesmono-,di-,tri-,tetra-,penta-,hexa-, and so on are used to indicate the number of ligands when these ligands are relatively simple. The Greek prefixesbis-,tris-, andtetrakis- are used with more complicated ligands.

The names of negative ligands always end ino, as influoro (F-),chloro (Cl-),bromo (Br-),iodo (I-),oxo (O2-),hydroxo (OH-), andcyano (CN-).

A handful of neutral ligands are given common names, such asaquo (H2O),ammine (NH3), andcarbonyl (CO).

Ligands are listed in the following order: negative ions, neutral molecules, and positive ions. Ligands with the same charge are listed in alphabetical order.

The oxidation number of the metal atom is indicated by a Roman numeral in parentheses after the name of the metal atom.

The names of complexes with a net negative charge end in-ate. Co(SCN)42-, for example, is the tetrathiocyanatocobaltate(II) ion. When the symbol for the metal is derived from its Latin name,-ate is added to the Latin name of the metal. Thus, negatively charged iron complexes are ferrates and negatively charged copper complexes are cuprates.

Table 2.Name of ligands

Ligand

Name of ligand

H2O

Aquo

NH3

Ammine

CN-

cyano

SCN-

thiocyano

Cl-

chloro

OH-

hydroxo

NO3-

nitro

SO42-

sulphato


 

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