In most main group element compounds, the valence electrons of the isolated atoms combine to form chemical bonds that satisfy the octet rule. For instance, the four valence electrons of carbon overlap with electrons from four hydrogen atoms to form CH4. The one valence electron leaves sodium and adds to the seven valence electrons of chlorine to form the ionic formula unit NaCl (Figure 1a). Transition metals do not normally bond in this fashion. They primarily form coordinate covalent bonds, a form of the Lewis acid-base interaction in which both of the electrons in the bond are contributed by a donor (Lewis base, e.g. ammonia molecule ) to an electron acceptor (Lewis acid, e.g. cobalt ion) (Figure 1b). The Lewis acid in coordination complexes, called a central metal ion (or atom), is often a transition metal or inner transition metal. The Lewis base donors, called ligands, can be a wide variety of chemicals - atoms, molecules, or ions. The only requirement is that they have one or more electron pairs, which can be donated to the central metal. Most often, this involves a donor atom (nitrogen atom, Figure 1b) with a lone pair of electrons that can form a coordinate bond to the metal.
Figure 1 (a) Covalent bonds involve the sharing of electrons, and ionic bonds involve the transferring of electrons associated with each bonding atom, as indicated by the colored electrons. (b) However, coordinate covalent bonds involve electrons from a Lewis base being donated to a metal center. The lone pairs from six ammonia molecules form bonds to the cobalt ion to form an octahedral complex.
The coordination sphere consists of the central metal ion or atom plus its attached ligands. Brackets in a formula enclose the coordination sphere; species outside the brackets are not part of the coordination sphere. The coordination number of the central metal ion or atom is the number of donor atoms bonded to it. The coordination number for the silver ion in [Ag(NH3)2]+ is two; for the copper(II) ion in [CuCl4]2−, the coordination number is four; whereas for the cobalt(II) ion in [Co(H2O)6]2+ the coordination number is six.
The Naming of Complexes
The nomenclature of the complexes is patterned after a system suggested by Alfred Werner, a Swiss chemist and Nobel laureate, whose outstanding work more than 100 years ago laid the foundation for a clearer understanding of these compounds. The following five rules are used for naming complexes:
When the complex is either a cation or a neutral molecule, the name of the central metal atom is spelled exactly like the name of the element and is followed by a Roman numeral in parentheses to indicate its oxidation state.
When the complex is an anion, the suffix -ate is added to the stem of the name of the metal, followed by the Roman numeral designation of its oxidation state. Sometimes, the Latin name of the metal is used when the English name is awkward. For example, ferrate is used instead of ironate, plumbate instead leadate, and stannate instead of tinate.
The oxidation state of the metal is determined based on the charges of each ligand and the overall charge of the coordination compound. For example, in [Cr(H2O)4Cl2]Br, the coordination sphere (in brackets) has a charge of 1+ to balance the bromide ion. The water ligands are neutral, and the chloride ligands are anionic with a charge of 1− each. To determine the oxidation state of the metal, the overall charge is set equal to the sum of the ligands and the metal: +1= −2 + x, so the oxidation state (x) is equal to +3.
This text is adapted from Openstax, Chemistry 2e, Chapter 19.2 Coordination Chemistry of Transition Metals.
Copyright © 2024 MyJoVE Corporation. All rights reserved