CHEM1902 (C10K) Coordination Chemistry.

Some definitions

The transition metals are generally defined as the elements existing between the Alkali metals and earths (Groups 1 and 2) and the non-metal elements (Groups 13 - 18). But note that IUPAC uses the definition that a transition element is one for which an atom has an incomplete d-subshell, or which gives rise to a cation with an incomplete d-subshell, so that the elements in group 12 (which are within the d-block) are not considered transition elements.

For this reason, in the case of the first-row transition metals, Scandium and Zinc are usually ignored in simple discussions, since Sc(III) and Zn(II) are their major oxidation numbers and these are d0 and d10 respectively.

The oxidation number of a central atom in a coordination entity is the charge it would bear if all the ligands were removed along with the electron pairs that were shared with the central atom. It is represented by a Roman numeral e.g in FeCl3 the oxidation number of iron is Fe(III). This is often called the oxidation state and for this course they can be treated as being the same, however this is not always the case.

A coordination compound, sometimes called a coordination complex, contains a central metal atom or ion surrounded by a number of oppositely charged ions or neutral molecules (possessing lone pairs of electrons) which are known as ligands.

If a ligand is capable of forming more than one bond with the central metal atom or ion, then ring structures are produced which are known as Metal Chelates, the ring forming groups are described as chelating agents or polydentate ligands.

The coordination number of the central metal atom or ion is the total number of sites occupied by ligands. Note: a bidentate ligand uses two sites, a tridentate three sites etc.

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