What is a complex connection

In complexes, negatively charged anions or molecules are arranged symmetrically around positively charged metal ions (cations) or metal atoms. A complex consists of one Central particles(or Central atom) and the Ligands. Complexes contain complexes as building blocks. These are indicated by square brackets. The underlying coordination theory goes back to the Swiss chemist Alfred Werner (1866–1919). Werner received the Nobel Prize in Chemistry in 1913 for his investigations and interpretations of the complexes.
If concentrated ammonia solution is added to a concentrated copper (II) sulphate solution, a deep blue precipitate is formed. After filtering with the aid of a frit and washing with ethanol at the same time, the complex tetraammine copper (II) sulfate can be obtained after drying:
CuSO4 + 4 NH3 + H2O [Cu (NH3)4]SO4•H2O

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When ammonia solution is added to copper (II) sulfate solution, an ultramarine blue precipitate is formed.

After filtering and drying, a cobalt blue powder is obtained. It is not stable in air, it weathers to a green powder. It is also visually different from the turquoise-blue copper (II) sulfate pentahydrate.

In the case of tetraammine copper (II) sulfate, the copper ions go Cu2+ a complex with the ammonia molecules NH3 a. To the Cu2+-Ion, four ammonia molecules are arranged as ligands. But there is also a complex with copper (II) sulfate pentahydrate. Here are around a Cu2+-Ion four water molecules arranged as ligands, the fifth water molecule is connected to the sulfate ion. In fact, the common name is not exact. The well-known turquoise-blue compound is correctly called tetra-aquacopper (II) sulfate monohydrate according to the complex nomenclature.
Naturalized nameCopper (II) sulfate pentahydrateTetrammine copper (II) sulfate
Exact nameTetraaqua copper (II) sulfate
Tetraammine copper (II) sulfate
formula[Cu (H2O)4]SO4 •H2O[Cu (NH3)4]SO4 •H2O
When both copper compounds dissolve in water, a new complex is formed, in which two additional water molecules appear as ligands. So when you dissolve white copper (II) sulfate in water you get hexaaquakupfer (II) sulfate:

CuSO4 + 6 H.2O [Cu (H2O)6]2+ + SO42-  
During drying, this disintegrates with the release of water to form the well-known tetra-aqua-copper (II) sulfate monohydrate (copper (II) sulfate pentahydrate):
[Cu (H2O)6]2+ + SO42-  [Cu (H2O)4]SO4 •H2O + H2O

The number of ligands that are bound directly to the central particle determines the Coordination number of the central atom. In the case of the tetra aqua copper complex, this coordination number is 4, in the hexa aqua copper complex it is 6. Complexes with coordination numbers from 2 to 12 are known. One also imagines the complexes spatially. Complexes with the coordination number 4 form a tetrahedron (or a square), complexes with the coordination number an octahedron. The hypothetical, spatial figures are called Coordination polyhedra designated.
Most complex compounds are colored, a typical example being the pigment phthalocyanine blue. Red blood liquor salt and yellow blood liquor salt also contain complexes. Potassium hexacyanoferrate (II) is called potassium hexacyanidoferrate (II) according to the complex nomenclature, the formula with the representation of the complex is K4[Fe (CN)6]. In this complex there are around one Fe2+-Ion six cyanide ions arranged as ligands.
Attempts at explaining the ties

The cohesion between central particles and ligands can be described both by ionic bonds and by electron pair bonds. According to the crystal field theory, electrostatic forces act between the ions, so that the positively charged central particle interacts with the negatively charged ligands. According to the electron pair bond theory, the ligands provide free electron pairs for bonding with the central particle. Molecular orbital theories describe the complex bonds much more precisely.
Complexes in metabolic processes in nature 
Complex compounds often occur in nature when metabolic processes are necessary. The red blood pigment hemoglobin is located in the red blood cells, it is responsible for the transport of oxygen in the blood. The heme molecule contains a chelate complex of porphyrin, which has four ligands with the central Fe2+-Ion is connected. At the Fe2+-Ion two further coordination positions are available. One is occupied by the protein globulin, an oxygen molecule can bind to the other. The Fe2+-Ion releases an electron to the oxygen molecule, creating oxyhemoglobin. An Fe is formed in the process3+-Ion and an O2--Ion. Instead of oxygen, carbon monoxide can also be bound. Since the resulting complex is much more stable than oxyhemoglobin, prolonged inhalation of carbon monoxide leads to suffocation.


The green leaf pigment chlorophyll enables photosynthesis in plants. It is found, for example, in the chloroplasts of the waterweed. A chlorophyll molecule contains a similar building block as hemoglobin, but instead a magnesium ion occurs as the central particle.


In many cases, the complex formation serves as chemical evidence. White copper (II) sulfate forms the blue hexaaqua copper (II) complex with water. Copper (II) ions are identified based on the reaction with ammonia described above. Iron (II) ions react with potassium hexacyanidoferrate (III) to form Prussian blue, as do iron (III) ions with potassium hexacyanidoferrate (II). The reaction of iron (III) ions with potassium thiocyanate is also based on the formation of a blood-red complex in aqueous solution.


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When adding a potassium hexacyanidoferrate (III) solution to an iron (II) sulfate solution, Berlin blue is produced.

After adding a 1% dimethylgyloxime solution in alcohol to a (dilute) nickel (II) sulfate solution, a red complex is formed. This is how nickel (II) ions can be detected:

In complexometric titration, the cations to be determined form a complex with an indicator. A color change takes place at the equivalence point, and all cations are converted into complexes. The binding of molecular groups or ions at the coordination points enables a variety of other technical applications:
  • When gold is extracted, the finely ground rock is mixed with a cyanide solution with the addition of atmospheric oxygen. The gold enters into a cyano complex, from which it can be obtained in pure form by reduction with zinc shavings.
  • Certain phosphates form chelate complexes in the water, which calcium ions can bind to themselves. They are therefore suitable as softeners in detergents. Today they are no longer used because they lead to eutrophication of the waters. Certain zeolites, which can hold calcium or magnesium ions in their lattice structure, are suitable as substitutes.
  • Ethylenediamine tetraacetate (EDTA) forms a complex with certain metal ions such as calcium ions. It is suitable as an additive in blood samples and prevents blood clotting, as the calcium ions necessary for clotting are chemically bound.
  • When starch is detected with an iodine-potassium iodide solution, a polyiodide starch complex is formed, which has a typical blue or violet color.