CHEMISTRY THE CENTRAL SCIENCE

23 TRANSITION METALS AND COORDINATION CHEMISTRY

CHAPTER SUMMARY AND KEY TERMS

SECTION 23.1. Metallic elements occur in nature in minerals, which are solid inorganic compounds found in nature. Metallurgy is the science and technology of extracting metals from the earth and processing them for further use. Transition metals are characterized by incomplete filling of the d orbitals. The presence of d electrons in transition elements leads to multiple oxidation states. As we proceed through the transition metals in a given row of the periodic table, the attraction between the nucleus and the valence electrons increases more markedly for d electrons than fors electrons. As a result, the later transition elements in a period tend to have lower oxidation states.

The atomic and ionic radii of period 5 transition metals are larger than those of period 4 metals. The transition metals of periods 5 and 6 have comparable atomic and ionic radii and are also similar in other properties. This similarity is due to the lanthanide contraction.

The presence of unpaired electrons in valence orbitals leads to magnetic behavior in transition metals and their compounds. In ferromagnetic, ferrimagnetic, and antiferromagnetic substances the unpaired electron spins on atoms in a solid are affected by spins on neighboring atoms. In a ferromagnetic substance the spins all point in the same direction. In an antiferromagnetic substance the spins point in opposite directions and cancel one another. In a ferrimagnetic substance the spins point in opposite directions but do not fully cancel. Ferromagnetic and ferrimagnetic substances are used to make permanent magnets.

SECTION 23.2 Coordination compounds are substances that contain metal complexes. Metal complexes contain metal ions bonded to several surrounding anions or molecules known as ligands. The metal ion and its ligands make up the coordination sphere of the complex. The number of atoms attached to the metal ion is the coordination number of the metal ion. The most common coordination numbers are 4 and 6; the most common coordination geometries are tetrahe-dral, square planar, and octahedral.

SECTION 23.3 Ligands that occupy only one site in a coordination sphere are called monodentate ligands. The atom of the ligand that bonds to the metal ion is the donor atom. Ligands that have two donor atoms are bidentate ligandsPolydentate ligands have three or more donor atoms. Bidentate and polydendate ligands are also called chelating agents. In general, chelating agents form more stable complexes than do related monodentate ligands, an observation known as the chelate effect. Many biologically important molecules, such as the porphyrins, are complexes of chelating agents. A related group of plant pigments known as chlorophylls is important in photosynthesis, the process by which plants use solar energy to convert CO2 and H2O into carbohydrates.

SECTION 23.4 In naming coordination compounds, the number and type of ligands attached to the metal ion are specified, as is the oxidation state of the metal ion. Isomers are compounds with the same composition but different arrangements of atoms and therefore different properties.Structural isomers differ in the bonding arrangements of the ligands. Linkage isomerism occurs when a ligand can coordinate to a metal ion through either of two donor atoms. Coordination-sphere isomers contain different ligands in the coordination sphere. Stereoisomers are isomers with the same chemical bonding arrangements but different spatial arrangements of ligands. The most common forms of stereoisomerism are geometric isomerism and optical isomerism. Geometric isomers differ from one another in the relative locations of donor atoms in the coordination sphere; the most common are cis-trans isomers. Optical isomers are nonsuperimposable mirror images of each other. Geometric isomers differ from one another in their chemical and physical properties; optical isomers, or enantiomers, are chiral, however, meaning that they have a specific “handedness” and differ only in the presence of a chiral environment. Optical isomers can be distinguished from one another by their interactions with plane-polarized light; solutions of one isomer rotate the plane of polarization to the right (dextrorotatory), and solutions of its mirror image rotate the plane to the left (levorotatory). Chiral molecules, therefore, are optically active. A 50–50 mixture of two optical isomers does not rotate plane-polarized light and is said to be racemic.

SECTION 23.5 A substance has a particular color because it either reflects or transmits light of that color or absorbs light of the complementary color. The amount of light absorbed by a sample as a function of wavelength is known as its absorption spectrum. The light absorbed provides the energy to excite electrons to higher-energy states.

It is possible to determine the number of unpaired electrons in a complex from its degree of paramagnetism. Compounds with no unpaired electrons are diamagnetic.

SECTION 23.6 Crystal-field theory successfully accounts for many properties of coordination compounds, including their color and magnetism. In crystal-field theory, the interaction between metal ion and ligand is viewed as electrostatic. Because some d orbitals point right at the ligands whereas others point between them, the ligands split the energies of the metal d orbitals. For an octahedral complex, the d or-bitals are split into a lower-energy set of three degenerate orbitals (the t2 set) and a higher-energy set of two degenerate orbitals (the e set). Visible light can cause a d-d transition, in which an electron is excited from a lower-energy d orbital to a higher-energy d orbital. The spectrochemical series lists ligands in order of their ability to increase the split in d-orbital energies in octahedral complexes.

Strong-field ligands create a splitting of d-orbital energies that is large enough to overcome the spin-pairing energy. The d electrons then preferentially pair up in the lower-energy orbitals, producing a low-spin complex. When the ligands exert a weak crystal field, the splitting of the dorbitals is small. The electrons then occupy the higher-energy d orbitals in preference to pairing up in the lower-energy set, producing a high-spin complex.

Crystal-field theory also applies to tetrahedral and square-planar complexes, which leads to different d-orbital splitting patterns. In a tetrahedral crystal field, the splitting of the d orbitals results in a higher-energy t2 set and a lower-energy e set, the opposite of the octahedral case. The splitting by a tetrahedral crystal field is much smaller than that by an octahedral crystal field, so tetrahedral complexes are always high-spin complexes.

KEY SKILLS

• Describe the periodic trends in radii and oxidation states of the transition-metal ions, including the origin and effect of the lanthanide contraction. (Section 23.1)

• Determine the oxidation number and number of d electrons for metal ions in complexes. (Section 23.2)

• Distinguish between chelating and nonchelating ligands. (Section 23.3)

• Name coordination compounds given their formula and write their formula given their name. (Section 23.4)

• Recognize and draw the geometric isomers of a complex. (Section 23.4)

• Recognize and draw the optical isomers of a complex. (Section 23.4)

• Use crystal-field theory to explain the colors and to determine the number of unpaired electrons in a complex. (Sections 23.5 and 23.6)