Coordination Compounds

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A characteristic feature of the transition metals is their ability to form a groupThose elements that comprise a single column of the periodic table. Also called family. of compounds called coordination compounds, complexA central metal and the ligands surrounding it; also called coordination complex. compounds, or sometimes simply complexes. A typical coordination compound is the intensely blue solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. Cu(NH3)4SO4 which can be crystallized from solutions of CuSO4 to which a very large excess of concentratedIncreased the concentration of a mixture or solution (verb). Having a large concentration (adjective). NH3 has been added. These crystals contain two polyatomicContaining two or more atoms. ions, one of which is the sulfate ion, SO42–, and the other of which is the complex ionAn ion formed by the combination of a central metal ion and ligands; a complex that is electrically charged. Cu(NH3)42+ which is responsible for the blue color.

We can regard a complex ion such as Cu(NH3)42+ as being the result of the interaction of :NH3 acting as a Lewis baseA species that donates a pair of electrons to form a covalent bond. with the Cu2+ ion acting as a Lewis acidA species that accepts a pair of electrons to form a covalent bond.. Each NH3 moleculeA set of atoms joined by covalent bonds and having no net charge. can be considered as donating a pair of electrons to a central Cu2+, thus forming four coordinate covalent bonds to it:


Most coordination compounds contain a complex ion similar to Cu(NH3)42+. This ion can be either positively charged like Cr(H3O)63+, or it can be negatively charged like CoCl63–. Neutral complexes like Pt(NH3)2Cl2 are also known. All these species contain a central metalThe metal ion or atom at the center of a coordination complex to which the ligands are attached. ion attached by coordinate covalent bonds to several ligands. These ligands are invariably Lewis bases. Some typical examples of ligands are H2O, NH3, Cl, OH, CN, Br, and SCN. The number of ligands attached to the central metal ion is said to be its coordination numberThe number of atoms to which a particular atom is bonded; in a coordination complex, the number of positions where ligands are attached to the central metal; in a ionic compound, the number of ions of opposite charge surrounding an ion. and is usually 2, 4, or 6. The group of ligands bonded to the metal taken collectively is said to constitute the metal’s coordination sphereIn a coordination complex the region around a central metal where the ligands are attached to the metal..

When writing the formula of a coordination compound containing complex ions, square brackets are usually used to enclose the coordination sphere. Examples are

[Cu(NH3)4]SO4      [Cr(H2O)6]Cl3       [Pt(NH3)2Cl2]
[Cu(NH3)4](NO3)2      K3[Fe(CN)6]      [Pt(NH3)4][PtCl4]

When such compounds are dissolved in H2O, each of the ions present in the solid becomes an independent species with its own chemical and physical properties. Thus, when 1 mol [Cr(H2O)6]Cl3 crystal is dissolved in H2O the solution contains 1 mol Cr(H2O)63+ ion which can be recognized by its characteristic grayish-violet color and 3 mol Cl which can be detected by the precipitate of AgCl which forms when AgNO3 is added to the solution.

Observations on Complex Compounds Containing PtCl2, NH3, and KCl.

Compound Molar Conductivity/

A V–1 dm2 mol–1

Moles AgCl

Precipitated per

Mole Compound

Electrode to which

Pt Migrates During Electrolysis

1. [Pt(NH3)4]Cl2
2 immediately
2. [Pt(NH3)3Cl]Cl
1 immediately; 1 after several hours
3. Pt(NH3)2Cl2
~ 0
2 after several hours
Does not migrate
4. K[Pt(NH3)Cl3]
3 after several hours
5. K2[PtCl4]
4 after several hours

An even better example of how the various ions in a coordination compound can behave independently when dissolved in water is provided by the set of Pt(II) complexes shown in the table. The first of these compounds contains the complex ion [Pt(NH3)4]2+ and in each subsequent compound one of the NH3 ligands in the coordination sphere of the Pt is replaced by a Cl ligand. As a result each compound contains a Pt complex of different composition and also of different charge, and when dissolved in H2O, it shows just the conductivity and other properties we would expect from the given formula. When 1 mol [Pt(NH3)3Cl]Cl is dissolved in H2O, it furnishes 1 mol Pt(NH3)3Cl+ ions and 1 mol Cl ions. The strongest evidence for this is the molar conductivity of the saltAn ionic compound that can be formed by replacing the hydrogen ion of an acid with a different cation. (1.2 A V–1 dm2 mol–1), which is very similar to that of other electrolytes like NaCl (1.3 A V–1 dm2 mol–1) which also yield a +1 ion and a –1 ion in solution, but very different from that of electrolytes like MgCl2 (2.5 A V–1 dm2 mol–1) which yield one + 2 ion and two –1 ions in solution. The conductivity behavior also suggests that the Pt atom is part of a cationA positively charged ion, attracted toward the cathode in an electrolytic cell., since the Pt moves toward the cathode during electrolysis. The addition of AgNO3 to the solution serves to confirm this picture. One mol AgCl is precipitated immediately, showing 1 mol free Cl ions. After a few hours a further mole of AgCl is precipitated, the Cl this time originating from the coordination sphere of the Pt atom due to the slow reaction

[Pt(NH3)3Cl]+(aq) + Ag+(aq) + H2O → [Pt(NH3)3H2O]2+(aq) + AgCl(s)

It is worth noting that in all these compounds, Pt has an oxidation numberA formally defined charge that an atom in a compound or ion would have if the compound or ion consisted entirely of monatomic ions. Based on a Lewis diagram, the charge that an atom would have if all electrons in bonds were assigned to the more electronegative atom or divided equally between atoms of the same electronegativity. of + 2. Thus the combination of Pt with one NH3 ligand and three Cl ligands yields an overall charge of 2(for Pt) – 3(for Cl) + 0(for NH3) = –1. The ion is thus the anionA negatively charged ion. An ion that is attracted toward the anode in an electrolytic cell. [PtNH3Cl3] found in compound 4.

EXAMPLE What is the oxidation stateA formally defined charge that an atom in a compound or ion would have if the compound or ion consisted entirely of monatomic ions. Based on a Lewis diagram, the charge that an atom would have if all electrons in bonds were assigned to the more electronegative atom or divided equally between atoms of the same electronegativity. of Pt in the compound Ca[Pt(NH3)Cl5]2?

Solution Since there are two complex ions for each Ca2+ ion, the charge on each must be –1. Adding the charge on each ligand, we obtain –5(for Cl) + 0(for NH3) = –5. If the oxidationThat part of a chemical reaction in which a reactant loses electrons; simultaneous reduction of a reactant must occur. number of Pt is x, then x – 5 must equal the total charge on the complex ion:

  x – 5 = –1

or     x = +4

The compound in question is thus a Pt(IV) complex.