21.1 - The Transition Metals: A Survey

• One striking characteristic of the representative elements is that their chemistry changes markedly across a given period as the number of valence electrons changes

• The chemical similarities occur mainly within the vertical groups

• The transition metals behave as typical metals, possessing metallic luster and relatively high electrical and thermal conductivities

• Silver is the best conductor of heat and electric current

• The chemical reactivity of the transition metals also varies significantly

• Some react readily with oxygen to form oxides

• More than one oxidation state is often found. For example, iron combines with chlorine to form FeCl2 and FeCl3.

  • The cations are often complexions, species where the transition metal ion is surrounded by a certain number of ligands

• Many compounds are paramagnetic

• The chromium configuration occurs because the energies of the 3d and 4d orbitals are very similar for the first-row transition elements

  • In transition metal ions, the 3d orbitals are lower in energy than the 4s orbitals

  • The differences between the 4d and 5d elements in a group increase gradually going from left to right

• Niobium and molybdenum are important alloying materials for certain types of steel

• Tantalum, which has a high resistance to attack by body fluids, is often used for the replacement of bones.

21.2 - The First-Row Transition Metals

• They all have one or more electrons in the 4s orbital and various numbers of 3d electrons

• All exhibit metallic properties:

  • A particular element often shows more than one oxidation state in its compounds

• Most commonly form coordination compounds containing a complexion involving ligands (Lewis bases) attached to a central transition metal ion

  • The number of attached ligands (called the coordination number) can vary from 2, 8, with 4 and 6 being the most common

• Many transition metal ions have major biologic importance in molecules such as enzymes and those that transport and store oxygen

  • Chelating ligands form more than one bond to the transition metal ion

• About 90% of the zinc produced is used for galvanizing steel

21.3 - Coordination Compounds

• Transition metal ions characteristically form coordination compounds, which are usually colored and often paramagnetic

• Coordination compound: A transition metal ion with its attached ligands

  • Coordination compounds have been known since about 1700, but their true nature was not understood until the 1890s when a young Swiss chemist named Alfred Werner proposed that transition metal ions have two types of valence

• One type of valence, which Werner called secondary valence, refers to the ability of a metal ion to bind to Lewis bases to form complex ions.

  • The other type, the primary valence, refers to the ability of the metal ion to form ionic bonds with oppositely charged ions

• The number of bonds formed by metal ions to ligands in complexions varies from two to eight depending on the size, charge, and electron configuration of the transition metal ion

• Ligand: a neutral molecule or ion having a lone electron pair that can be used to form a bond to a metal ion

  • A ligand that can form one bond to a metal ion is called a monodentate ligand

  • Some ligands have more than one atom with a lone electron pair that can be used to bond to a metal ion

21.4 - Isomerism

• Isomers: Two or more compounds with the same formula but different properties

• Coordination isomerism: The composition of the coordination sphere varies

• Linkage isomerism: The point of attachment of one or more ligands varies

• Stereoisomerism: Isomers have identical bonds but different spatial arrangements

• Geometric isomerism: Ligands assume different relative positions in the coordination sphere; examples are cis and trans isomers

• If this light is passed through a polarizer, only the photons with electric fields oscillating in a single plane remain, constituting plane-polarized light

• Optical isomerism: Molecules with non-superimposable mirror images rotate plane-polarized light in opposite directions

• The most important biomolecules are chiral, and their reactions are highly structured dependent

21.5 - Binding in Complex Ions: The Localized Electron Model

• The VSEPR model for predicting structure generally does not work for complexions

  • However, we can safely assume that a complexion with a coordination number of 6 will have an octahedral arrangement of ligands, and complexes with two ligands will be linear

  • Complexions with a coordination number of 4 can be either tetrahedral or square planar

• The interaction between a metal ion and a ligand can be viewed as a Lewis acid-base reaction with the ligand donating a lone pair of electrons to an empty orbital of the metal ion to form a coordinate covalent bond

• A linear complex requires two hybrid orbitals 180 degrees from each other

• Although the localized electron model can account in a general way for metal-ligand bonds, it is rarely used today because it cannot readily account for important properties of complexions

21.6 - The Crystal Field Model

• The main reason the localized electron model cannot fully account for the properties of complex ions is that it gives no information about how the energies of the d orbitals are affected by complex ion formation

• The crystal field model focuses on the energies of the d orbitals

• It is an attempt to account for the colors and magnetic properties of complex ions

• It also has been observed that the magnitude  ▵ for a given ligand increases as the charge on the metal ion increases

• The crystal field model also applies to square planar and linear complexes.

• We can obtain the square planar complex by removing the two-point charges along the z-axis

• We can obtain the linear complex from the octahedral arrangement by leaving the two ligands along the z-axis and removing the four in the XY plane

• A given photon of light can be absorbed by a molecule only if the wavelength of the light provides exactly the energy needed by the molecule

21.7 - The Biological Importance of Coordination Complexes

• The ability of metal ions to coordinate with and release ligands and to easily undergo oxidation and reduction makes them ideal for use in a biological system

  • For example, metal ion complexes are used in humans for the transport and storage of oxygen, as electron-transfer agents, as catalysts, and as drugs.

• A protein is a large molecule assembled from amino acids, which have the general structure in which R represents various groups

• Iron plays a central role in almost all living cells

  • In mammals, the principal source of energy comes from the oxidation of carbohydrates, proteins, and fats

• The principal electron-transfer molecules in the respiratory chain are iron-containing species called cytochromes,

• Hemoglobin dramatically demonstrates how sensitive the function of a biomolecule is to its structure

  • Our knowledge of the workings of hemoglobin allows us to understand the effects of high altitudes on humans

• Our understanding of the biological role of iron also allows us to explain the toxicities of substances such as carbon monoxide and the cyanide ion

  • Cyanide coordinates strongly to cytochrome oxidase, an aniron-containing cytochrome enzyme that catalyzes the oxidation-reduction reactions of certain cytochrome

• The coordinated cyanide thus prevents the electron-transfer process and rapid death results

21.8 - Metallurgy and Iron and Steel Production

• The steps in metallurgy:

  • Mining

  • Pretreatment of the ore

  • Reduction to the free metal

  • Purification of the metal

  • Alloying

• The processes connected with separating a metal from its ore

  • The minerals in ores are often converted to oxides (roasting) before being reduced to metal (smelting)

• The metallurgy of iron: most common method for reduction uses a blast furnace; process involves iron ore, coke, and limestone

• Impure product (90% iron) is called pig iron

• Steel is manufactured by oxidizing the impurities in pig iron

• The production of iron from its ore is fundamentally a reduction process, but the conversion of iron to steel is basically an oxidation process in which unwanted impurities are eliminated

• Oxidation is carried out in various ways, but the two most common are the open-hearth process and the basic oxygen process

• The rate of heating and cooling determines not only the amounts of cementite present but also the size of its crystals and the form of crystalline iron present