Experiment 5 - Investigating Complex Ions of Copper (II)

Experiment 5 - Investigating Complex Ions of Copper (II) -...

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Investigating Complex Ions of Copper(II) Background: Transition metal ions in aqueous solutions generally exist as complex ions in which water molecules, acting as Lewis bases, "coordinate" or bond with the small cation (which acts as a Lewis acid). The water molecules in these structures are known as ligands. Historically this kind of attachment has been called either a coordinate covalent bond or a dative bond . The distinguishing characteristic of such bonds is that the shared electron pairs which constitute the bonds come from only one of the bonded species. In normal covalent bonding the assumption is that each atom donates one electron to the shared pair that is the bond. The number of ligand attachments to the metal ion is called the coordination number . Ligands which can make only one bond with an ion are called monodentate ligands ("one tooth"). Bidentate ligands are generally larger structures which can attach twice to an ion (e.g. ethylene diamine). A few ligands are polydentate (such as EDTA). The existence of metal ion-water complexes is mainly due to the attraction of the lone pairs of the water molecules for the high, concentrated, positive charge on the metal cations. Silver ion, for example, is typically coordinated with two water molecules. Although it is usual to write aqueous silver ions as Ag + , a more accurate representation would be Ag(H 2 O) 2 + [diaquasilver ion]. Similarly, aqueous copper(II) ions are generally coordinated with four water molecules resulting in the species Cu(H 2 O) 4 2+ [tetraaquacopper(II) ion]. Although these examples include only water molecules as ligands, other neutral molecules, anions, and even some cations are also possible. The structure of such ions and their compounds (in copper(II) sulfate pentahydrate, only one of the waters is actually a water of hydration; the others are coordinated with the copper ion) was a subject of much interest to Alfred Werner who began a systematic study of them in the 1890's. Although many of the substances had been known for nearly 200 years before Werner began his work, very little was known about the structure or bonding of the compounds. Werner's work led to a better understanding of synthetic methods for producing different compounds and eventually various models for describing how the bonding might take place and how some of the more interesting properties of the compounds might come about. One of the early approaches to understanding the bonding in complex ions was to adapt Valence Bond Theory. Werner's work showed that complexes had definite geometries (some of which gave rise to isomers similar to the geometric and optical isomers of hydrocarbon compounds) which were the same as geometries of ordinary molecular compounds. Accordingly, bonding in complexes can be treated as occurring within hybrid orbitals. In the example of Ag(H 2 O) 2 + cited earlier, the orbital diagram for Ag + would be represented as: 4 d (recall that silver has an irregular electron configuration of [Kr]5s
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This note was uploaded on 11/07/2010 for the course CHEM 257 taught by Professor Asdf during the Spring '10 term at Allan Hancock College.

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Experiment 5 - Investigating Complex Ions of Copper (II) -...

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