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 (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
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