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EDTA

Some ligands can bond to a metal atom using more than two pairs of electrons. An example is ethylenediamminetetraacetate ion (EDTA^4-), the Lewis structure of which is shown below. EDTA^4- forms very stable complexes with most of the transition metals.

Other Structures

By far, the most common coordination numbers for coordination compounds are 2, 4 and 6. Shown below are several coordination compounds which have coordination numbers of 3, 5 and 7.


[HgCl₃]^- In this complex ion, the coordination number of 3 results in a trigonal planar structure. Note that the chloride ligands are bonded to the Hg atom at the corners of an equilateral triangle.


Fe(CO)₅ In this compound, the coordination number of 5 results in a trigonal bipyramidal structure. Note that three CO molecules are bonded to the Fe atom at the corners of an equilateral triangle while the other two CO molecules are bonded to the metal above and below the plane of the triangle.	[VOCl₄]^2- In this compound, the coordination number of 5 results in a square pyramidal structure. Note that the four Cl ligands are bonded to the V atom at the corners of a square while the O ligand is bonded to the metal above the plane of the square.


IF₇ This compound is an example of a compound containing only main-group elements. The coordination number of 7 results in a pentagonal bipyramidal structure. Note that five F ions are bonded to the I atom at the corners of an pentagon while the other two F ions are bonded to the I atom above and below the plane of the pentagon.	[NbOF₆]^3- In this compound, the coordination number of 7 results in a capped octahedron - triangular face structure. Note that the six F ions are bonded to the Nb atom in an octahedral arrangement. The O atom is bonded to the Nb atom at one of the triangular faces of the octahedron.


[NbF₇]^2- In this compound, the coordination number of 7 results in a capped trigonal prism structure.

Aqueous Metal Ions

Because water can function as a Lewis base and transition metal ions can function as Lewis acids, most salts containing transition metals form coordination complexes when they are dissolved in water.

All salts which contain a first-row transition metal (i.e., Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) form octahedral complex ions when they are dissolved in water.

In aqueous solution, transition metal cations are usually symbolized as Mⁿ⁺(aq), where M is the atomic symbol of the metal ion and n is the charge on the ion. For example, Fe³⁺ in aqueous solution is written as Fe³⁺(aq). The (aq) symbol indicates that the metal ion is aquated (i.e., the metal ion is bonded to several water molecules).


[Fe(OH₂)₆]³⁺ The octahedral species produced when solid Fe(NO₃)₃ is dissolved in water (NO₃^- ions are not shown). Note that this species is also written as: Fe³⁺(aq).	[Co(OH₂)₆]²⁺ The octahedral species produced when solid Co(NO₃)₂ is dissolved in water (NO₃^- ions are not shown). Note that this species is also written as: Co²⁺(aq).

Complex Formation In Aqueous Solution

What happens when SCN^- is added to an aqueous solution containing Fe(NO₃)₃?

In aqueous solution, [Fe(OH₂)₆]³⁺ ions react with SCN^- (not free Fe³⁺ ions). In the reaction shown above, the SCN^- ion replaces a water molecule in the coordination sphere to produce the complex ion, [Fe(OH₂)₅(SCN)]²⁺.

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Complex Formation In Aqueous Solution