Coordination bonds are formed when ligands donate an electron pair to transition element cations, forming complex ions.
Syllabus reference R3.4.8Reactivity 3.4.8 - Coordination bonds are formed when ligands donate an electron pair to transition element cations, forming complex ions. (HL)
- Deduce the charge on a complex ion, given the formula of the ion and ligands present.
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Complex formation in transition metals
Transition metals of the first row of the 'd' block, scandium to zinc, form complex ions by accepting electron pairs into specialised molecular orbitals involving the fourth energy level 4s, 4p and 4d sub-shells.
Owing to the relatively large size of these molecular orbitals it is possible to accomomodate many coordinated electron pairs into these orbitals. Commonly, four-coordinate and six-coordinate complexes are found.
The hexaaquacopper(II) ion
[Cu(H2O)6]2+
In the above complex ion, there are six water ligands, each coordinating a lone pair from an oxygen atom into the transition metal 4th level orbitals.
Each water ligand is behaving as a Lewis base and the central copper ion is behaving as a Lewis acid.
Ligands
The bonding strength of ligands, i.e. the ease with which they can coordinate the lone pair, depends on several factors. For example, the hydroxide ion, OH- is a better ligand than the water molecule, H2O, as there are less protons in the structure attracting the lone pair on the oxygen.
The relative strength with which ligands form coordinate bonds is shown in the spectrochemical series (chemistry databooklet).
Cyanide ions, CN-, are very strong ligands, while chloride ions are not. Water and hydroxide ions lie in the middle of the series.
Variable oxidation state
The transition metals of the first row can form many oxidation states. The actual oxidation state of the central transition metal ion can be determined from the charge on the complex ion and the number and type of ligands involved in the complex.
For example, the complex ion hexaaqucopper(II) has the formula: [Cu(H2O)6]2+.
We know that water molecules carry no charge, they are neutral, therefore the only species responsible for the overall +2 charge is the transition metal ion, which must have a +2 oxidation number, which is, of course, shown in the name.
However, what if we are given a formula such as: [Fe(CN)6]3-?
The same logic applies. We know that the formula of the cyanide ligand is CN- and there are six cyanide ions. Hence, the cyanide ions contribute 6- charges to the complex. But the complex as a whole has only 3- charges. Hence, the iron ion has cancelled out three negative charges - it must have a 3+ charge. The complex can now be named as hexacyanoferrate(III)
Notice that the iron ion has changed its name to ferrate, as it forms part of a negative ion.
The tetrachlorocobaltate(II) ion
[CoCl4]2-