Chapter 21 - Transition Metals and Coordination Chemistry

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 (ôŹ°µ90% 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