Chapter 22: Transition Metal Chemistry and Coordination Compounds

Index:

22.1 Properties of Transition Metals
22.2 Chemistry of Iron and Copper
22.3 Coordination Compounds
22.4 Structure of Coordination Compounds
22.5 Bonding in Coordination Compounds: Crystal Field Theory
22.6 Reactions of Coordination Compounds
22.7 Applications of Coordination Compounds

22.1 Properties of Transition Metals

• incompletely filled d subshells cause:
  • distinctive coloring
  • formation of paramagnetic compounds
  • catalytic activity
  • great tendency to form complex ions
• as you go left to right on periodic table:
  • electrons add to the inner 3d subshell
  • this shields 4s electrons from increasing nuclear charge
  • atomic radii decrease less rapidly
  • electronegativities and ionization increase only slightly
• General Physical Properties:
  • close packed structure

cubic closed packed structure hexagonal closed packed structure

• small atomic size
• very strong metallic bonds
• higher densities, melting and boiling points, and higher heats of fusion and evaporation than the Group 1A, 2A and 2B metals
• Electron Configurations
  • from Scandium across to Copper, electrons are added to the 3d orbital
  • Scandium is 4s^2 3d^1, Titanium is 4s^2 3d2 and so on
  • exceptions: chromium, which is 4s^1 3d^5 and copper, which is 4s^1 3d^10
    • these are a result of the extra stability associated with half filled and completely filled 3d subshells
• Oxidation States

• common oxidation states for each element include +2, +3 or both.
• +3 state is more stable at the beginning of the series and +2 is more stable at the end
• highest oxidation state for transition metal is +7, for manganese
• transition metals exhibit their highest oxidation states in coumpounds with very electronegative compounds
  • for example: V2O5, CrO3, and Mn2O7

22.2 Chemistry of Iron and Copper

• Iron
  • second most abundant metal in Earth's crust
  • pure iron is gray and not very hard
  • the two oxidation states of Iron are +2, and +3
  • in the presence of oxygen, Fe^2+ ions in solution are readily oxidized to Fe^3+ ions
• Copper
  • copper is 6.8 x 10^ -3 percent of the Earth's crust by mass
  • found in the uncombined state as well as ores
  • impure copper can be purified by electrolysis
  • it has the highest electrical reactivity next to silver
  • used in alloys, electrical cables, plumbing pipes and coins
  • only reacts with hot concentrated sulfuric acid and nitric acid and has 1+ and 2+ oxidation states where the 1+ state is less stable
  • all compounds of Cu(I) are diamagnetic and colorless except for Cu2O
  • Cu(II) compounds are paramagnetic and colored

22.3 Coordination Compounds

• transition metals have a tendency to form complex ions
• coordination compounds usually consists of a complex ion and a counter ion
  • Werner's Coordination Theory

Some Coordination Complexes
example	molecular formula	Lewis base/ligand	Lewis acid	donor atom	coordination number
	[Ag(NH3)2]+	NH3	Ag+	N	2
	[Zn(CN)4]2-	CN-	Zn2+	C	4
	[Ni(CN)4]2-	CN-	Ni2+	C	4
	[PtCl6]2-	Cl-	Pt4+	Cl	6
	[Ni(NH3)6]2+	NH3	Ni2+	N	6

A coordination complex is formed when an acid-base reaction occurs in which neutral molecules or anions (called ligands) bond to a central metal atom (or ion) by coordinate covalent bonds.

• Ligands contain at least one pair of electrons to donate to a metal atom/ion.
• Metal atoms/ions are acids in which they can accept pairs of electrons from Lewis bases.
• Within a ligand, the atom that is directly bonded to the metal atom/ion is called the donor atom.
• A coordinate covalent bond is a covalent bond in which the donor atom supplies both electrons. This is different from a normal covalent bond in which each atom supplies one electron.
• If the coordination complex carries a net charge, the complex is called a complex ion.
• Compounds that contain a coordination complex are called coordination compounds.

The coordination sphere of a coordination compound or complex consists of the central metal atom/ion plus its attached ligands. The coordination sphere is usually enclosed in brackets when written in a formula.
The coordination number is the number of donor atoms bonded to the central metal atom/ion.
(above info taken from http://www.chem.purdue.edu/gchelp/cchem/whatis2.html)

• Naming Coordination Compounds

A. To name a coordination compound, no matter whether the complex ion is the cation or the anion, always name the cation before the anion. (This is just like naming an ionic compound.)
B. In naming the complex ion:
1. Name the ligands first, in alphabetical order, then the metal atom or ion. Note: The metal atom or ion is written before the ligands in the chemical formula.
· For anionic ligands end in "-o"; for anions that end in "-ide"(e.g. chloride), "-ate" (e.g. sulfate, nitrate), and "-ite" (e.g. nirite), change the endings as follows: -ide with -o; -ate with -ato; and -ite with -ito
· For neutral ligands, the common name of the molecule is used e.g. H2NCH2CH2NH2 (ethylenediamine). Important exceptions: water is called ‘aqua’, ammonia is called ‘ammine’, carbon monoxide is called ‘carbonyl’, .

2. Greek prefixes are used to designate the number of each type of mono dentate ligand in the complex ion, e.g. di-, tri- and tetra-. If the ligand is a bidentate ligand (ethylenediamine) or if it is polydentate ligands (can attach at more than one binding site) the prefixes bis-, tris-, tetrakis-, pentakis-, are used instead.

3. After naming the ligands, name the central metal. If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is call cobalt and Pt is called platinum. If the complex ion is an anion, the name of the metal ends with the suffix –ate. For example, Co in a complex anion is called cobaltate and Pt is called platinate. For some metals, the Latin names are used in the complex anions e.g. Fe is called ferrate (not ironate).

22.4 Construction of Coordination Compounds

• there is more than one way to arrange ligands around the central atom
• compounds have distinctly different physical and chemical properties

(A) Linear
(B) Square Planar
(C) Tetrahedral
(D) Octahedral

Coordination Number	Structure
2	Linear
4	Tetrahedral or Square Planar
6	Octahedral

Stereoisomers: compounds that are made up of the same types and numbers of atoms bonded together in the same sequence but with different special arrangements. There are two different types of stereoisomers: geometric isomers and optical isomers, but most compounds do not have stereoisomers.

• Geometric Isomers
  • isomers that cannot be interconverted without breaking a chemical bond
  • come in pairs and are named by "cis" and "trans"
  • Cis- two particular atoms are adjacent to each other
  • Trans- the atoms are on opposite sides of the structural formula
  • "cis" and "trans" have varying boiling points, colors, dipole moments, and chemical reactivities

• Optical Isomers
  • non superimposable mirror images of each other
  • optical isomers have identical physical and chemical properties
  • differ in their reactions with plane-polarized light
  • two optical isomers have the same relationship as your left and right hand
    • if you put your left hand in front of a mirror, the image will look like your right hand
  • described as chiral (greek word for "hand")
    • chiral molecules are non superimposable
    • isomers that are superimposable with their mirror images are called achiral
  • chiral molecules are optically active because of their ability to rotate the plane of polarization of polarized light as it passes through them
    • plane polarized light vibrates only in a single plane unlike ordinary light which is vibrated in all directions
  • a polarimeter is used to measure the rotation of polarized light by optical isomers

image provided by google images

22.5 Bonding in Coordination Coordination Compounds: Crystal Field Theory

• Octahedral Crystal Fields
  • Each Mn2+ ion in manganese(II) oxide is surrounded by six O2- ions arranged toward the corners of an octahedron, as shown in the figure below. MnO is therefore a model for an octahedral complex in which a transition-metal ion is coordinated to six ligands.

structure

• repulsion energy between the 4s and 4p orbitals greatly increases from the surrounding six electrons, but the three 4p orbits are still degenerate.
• These orbitals still have the same energy because each 4p orbital points toward two O2- ions at the corners of the octahedron.

• Tetrahedral Crystal Fields
  • Each Cu+ ion in copper(I) chloride is surrounded by four Cl- ions arranged toward the corners of a tetrahedron, as shown in the figure below. CuCl is therefore a model for a tetrahedral complex in which a transition-metal ion is coordinated to four ligands.

structure

• Once again, the negative ions in the crystal split the energy of the d atomic orbital on the transition-metal ion. The tetrahedral crystal field splits these orbitals into the same t2g and eg sets of orbitals as does the octahedral crystal field.
• Because a tetrahedral complex has fewer ligands, the magnitude of the splitting is smaller. The difference between the energies of the t2g and eg orbitals in a tetrahedral complex (delta t) is slightly less than half as large as the splitting in analogous octahedral complexes (delta o)
• Square Planar Complexes
  • The crystal field theory can be extended to square-planar complexes, such as Pt(NH3)2Cl2. The splitting of the d orbital in these compounds is shown below.

diagram

22.6 Reactions of Coordination Compounds

• complex ions undergo ligand exchange (or substitution) reactions in solution. Rates vary depending on nature of metal ion.
• kinetic lability- thermodynamic property, which is measured in terms of the species' formation constant Kf
• labile complexes- complexes that undergo rapid ligand exchange reactions.
• inert complexes- complex ion that undergoes very slow exchange reactions.
  • shows that a thermodynamically unstable species is not necessarily chemically reactive.
  • rate of reaction is determined by the energy of activation
• most complex ions that contain Co^3+, Cr^3+, and Pt^2+

22.7 Applications of Coordination Compounds

Coordination compounds are found in living systems and have many uses in the home, in industry, and in medicine:

• Mettalurgy
  • extraction of silver and gold by the formation of cyanide complexes
  • purification of nickel is done by converting the metal to the gaseous compound Ni(CO)4
• Therapeutic Chelating Agents
  • chelting agent EDTA is used in the treatment of lead poisoning.
  • certain platinum-containing compounds can effectively inhibit the growth of cancerous cells
• Chemical Analysis
  • other chelates are more selective in binding
  • dimethylglyoxime forms an insoluble brick-red solid with Ni^2+ and an insoluble bright-yellow solid with Pd^2+.
  • these characteristic colors are used in qualitative analysis to identify nickel and palladium
  • knowing the formula of a complex, we can readily calculate the amount of nickel present in the original solution
• Detergents
  • the cleansing actino of soap in hard water is hampered by the reaction of the Ca^2+ ions in the water with the soap molecules to form insoluble salts or curds
  • a "builder" was introduced (usually sodium tripolyphosphate) to help counteract this problem
  • because phosphates are plant nutrients, waste waters containing phosphates discharge into rivers and lakes cause algae to grow, resulting in oxygen depletion
  • this process is called eutrophication
  • many states have banned phosphate detergents because of its harm to aquatic life

Main Source: "Chemistry" eighth edition by Raymond Chang









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