Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Summer 2006
Lecture Notes::Lec 23_3 July	© R. Paselk 2006
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The Chemistry of the Elements

The Transition Metals

The transition elements have typical metallic properties: high reflectivity, a metallic luster, good electrical conductivity, and good thermal conductivity.

Note the electronic structures of the transition elements going across the periods (see chart below). Note the break in pattern at the 4th and 9th elements in the first and second series due to filling of d subshell (symmetry). This effect breaks down in the third series as the s and d subshells get closer due to the inner f subshells and relativistic effects.

On the other hand they exhibit a wide variation in

• hardness (e.g. Cr is about as hard as corundum [it will readily scratch glass etc.] while Au is very soft)
• melting points (e.g. from Hg = -39°C to W = 3390°C)
  • These variations in lattice strength and stability indicate that some transition metals have extensive covalent bonding interactions via their d-electrons and orbitals supplementing their metallic bonding.
• density (e.g. from Sc = 3g/cm³ to 22.6g/cm³ for Ir and Os)
  • The high densities result from the element's high atomic masses, small atomic volumes (small radii) and close-packed structures of their crystals.

The transition metals tend to be quite similar across Periods as well as within Groups in Chemical properties.

• This is a result of the fact that they all have one or two outermost (s) electrons - the differences are due to the filling of the d-orbitals which are down inside the atoms (see table below).
  • Thus the chemistry of these elements will be close because of the domination of the valence electrons in bonding and the tendency to lose the s-electrons more readily than the d-electrons in ionization. (overhead 206, Figure 21.2)
  • The second and third series of transition metals are even more alike in their group properties because the f-electrons have filled in between, reducing the atomic sizes of these elements so that the third series (5d) elements are nearly identical in size to the second series (4d) elements in the same group. (overhead 207, Figure 21.3)
• The transition elements tend to have multiple oxidation states.
  • First and last elements of series have one oxidation state:
    • First elements (Sc, Y, Lu) = +3 only, corresponding to the removal of the s-electrons and the single d-electron.
    • Last elements (Zn, Cd, Hg) = +2 (but Hg also has a +1 state for the bound Hg-Hg⁺² ion).
  • The oxidation states tend to increase in number and value towards the center of the series (see table below):
    • In the first half of each series the maximum oxidation state corresponds to the loss of the s-electrons and all of the d-electrons (with ruthenium and osmium the maximum goes to +8).
    • In the second half of the series the decreasing maximum oxidation numbers result from the increased stability of the half-filled d subshell.

Periodic Table of the Elements -Transition Metals IIIB IVB VB VI VIIB VIIIB IB IIB 21Sc 4s23d1 E°=-2.08V*   3** 22Ti 4s23d2 E°=-1.63V (2) 3 23V 4s23d3 E°=-1.2V 2 3 4 5 24Cr 4s13d5 E°=-0.74V 2 3 (4)   6 25Mn 4s23d5 E°=-1.18V 2 (3) 4   (6) 7 26Fe 4s23d6 E°=-0.45V 2 3 (4)   (6) 27Co 4s23d7 E°=-0.28V 2 3 28Ni 4s23d8 E°=-0.26V 2 (3) 29Cu 4s13d10 E°=+0.34V (1), 2 30Zn 4s23d10 E°=-0.76V 2  Y Zr Nb Mo 5s14d5 Tc   E°=+0.4V Ru   E°=+0.5V Rh   E°=+0.6V Pd 5s04d10 E°=+1.2V  Ag 5s14d10 E°=+0.80V  Cd Lu  Hf Ta  W 6s24d4 Re Os   E°=+0.9V Ir   E°=+1.0V  Pt 6s14d9 E°=+1.2V Au 6s14d10 E°=+1.7V  Hg   E°=+0.80V * Reduction potentials from M+2 (or M+3 for Sc & Cr) to the metal. ** Common oxidation states (less common in parenthesis).

The reactivities of the transition metals varies significantly for both chemical and physical reasons, e.g.:

• Fe oxidizes readily and continuously because the oxides formed flake off yielding fresh metal to continue reacting.
• Cr and Ni readily form oxides, but they tend to form a continuous and impervious layer which protects the metal from additional corrosion.
• Au, Ag, and Pt do not readily form oxides.

Coordination Compounds

Complex ions are particularly common in the transition metals. When a complex ion is combined with a counter ion the result is a coordination compound. Note that complex ions can be both cations and anions, and a compound could be made up from two different complex ions.

When dealing with complex ions and coordination compounds one of the first things we need to be concerned with is nomenclature - how do we know what someone is talking about, and how do we describe a compound of interest?

Nomenclature of Complex Ions: Note that we still write, in formulae and names, the cation first and the anion second. The question then is how do we name the components of the complex ions themselves:

• Ligand Naming: Since complex ions are made up of a central metal atom and the ligands surrounding it, the first thing we need is a way of designating the ligands.
  • Anionic Ligands have names which are modified from the standard names:
    • If the anion name ends in -ide the ligand name ends in -o instead:

Anion Name Ligand Name Cl- chloride -Cl chloro I- iodide -I iodo CN- cyanide -CN cyano OH- hydroxide -OH hydroxo

• If the anion name ends in -ate then the ligand name ends in -ato instead:

Anion Name Ligand Name SO42- sulfate -OSO3 sulfato C2O42- oxalate -O(CO)2O- oxalato

• Some anions may bind to a central atom in more than one way, so different names are used for the different arrangements. Two important examples are given below:

Anion Name Ligand Name SCN- thiocyanate -SCN thiocyanato     -NCS isothiocyanato NO2- nitrite -NO2 nitro     -ONO nitrito

• Molecular (neutral) and cationic ligands are generally not renamed when used as ligands. The three most important exceptions to this rule are listed below:

Molecule Name Ligand Name H2O water -OH2 aqua (old = aquo) NH3 ammonia -NH3 amine CO carbon monoxide -CO carbonyl

• Complex Ion Naming:
  1. The ligands in a complex ion are named first, even though they are listed after the cation in the formula.
  2. The number of each ligand is indicated using Greek prefixes (di- = 2, tri- = 3, tetra- = 4, penta- = 5, and hexa- = 6).
  3. When there is more than one kind of ligand, they are listed alphabetically by ligand name (prefixes like tetra- and bis- are ignored in the ordering).
  4. The oxidation number of the central atom is indicated with a Roman numeral as in the Stock System for naming cations, e.g. iron(III).
  5. Complex anions have the suffix -ate added to the name of the central atom. Note that some atoms use their Latin names, e.g. cuprate vs. nickelate.

Examples

Formula Name [Cr(H2O)6]3+ Hexaaquachromium(III) ion [CoCl4(NH3)2]- Diamminetetrachlorocobaltate(III) ion [Cr(OH)2(H2O)4][FeCl2(CN)2(NH3)2] Tetraaquadihydroxychromium(III) diamminedichlorodicyanoferrate(III)* * I don't know if this compound actually exists, but for nomenclature examples I refuse to be constrained by reality!

Stereoisomerism

When a number of different ligands bind to a single central atom their arrangement may result in sterioisomerism - the situation where substances with the same bonds (e.g. same Lewis Structures) are different species because of differing spatial arrangements of bonds around the central atom. Isomerism is extraordinarily important in biology and organic chemistry. Here we'll look at complex ion examples.

The stereoisomerism of complex ions can be organized by coordination number, with the most important coordination numbers for stereoisomers being 2, 4, and 6.

• Coordination number 2: Complex ions with coordination number 2 (e.g. [Ag(NH₃)₂]^- or [CuCl₂]^-) are linear. There is thus no possibility of stereochemistry.
• Coordination number 4: Complex ions with coordination number 4 (e.g. [Cu(NH₃)₄]²⁺, [Ni(CN)₄]^2-) may take two different forms exhibiting quite different stereoisomerism:
  • Square planar: In square planar isomerism we see geometrical or cis-trans isomerism. Though most chemical and physical properties of these isomers are identical, they can have quite different biological properties. Thus, for example the cis-isomers of platinum compounds, such as [PtCl₂(NH₃)₂], are frequently potent antitumor agents useful in cancer chemotherapy, while the trans-isomers have no therapeutic value:

• Tetrahedral: In these complex ions the four ligands are arranged at the corners of a regular tetrahedron (e.g. [ZnCl₄]^2-, [MnO₄]^-). Note that for these complex ions cis-trans isomerism is not possible - no way to get a trans isomer on a tetrahedron! (models). However, another kind of isomerism appears when different ligands are added. Look at two different situations and their results:
  • MA₂BC: No stereoisomerism (models).
  • MABCD: With four different ligands a new type of stereoisomerism occurs - enantiomerism or optical isomerism (so-called because the mirror-image (chiral) isomers have different optical (polarization) properties).
    • With four different ligands can be arranged to give two different isomers which are mirror images of each other.(models). These isomers will also rotate light in opposite directions, so if a polarized beam of light is passed through a solution of one isomer it will rotate it clockwise, while the other will rotate counter-clockwise (the direction and angle of rotation is not determined trivially, generally found experimentally).
      • Polarization and polarized light discussion (see text as well).
• Coordination number 6: Complex ions with coordination number 6 are the most common of the complex ions. They are almost always based on octahedral geometry. Again, isomerism depends on the number of different ligands:
  • For MA₆ and MA₅B no stereoisomerism occurs.
  • cis-trans geometrical isomerism occurs for MA₄B₂, where if the two "B" ligands are on adjacent corners we have cis- and where they are at opposite corners we have trans-. (models)
  • The MA₃B₃ structure gives a new kind of geometrical isomerism: fac:mer
    • In this type of geometric isomer the fac- (facial) isomer has each triad of identical ligands on the corners of a single face of the octahedron. (model)
    • On the other hand the mer- (meridional) isomer has its triads of ligands arranged along a meridian of the octahedron (the great circle arc passing from pole to pole on a sphere, in this case the sphere just enclosing the octahedron). (model)

  Polydentate (multidentate) ligands or Chelating Agents: Ligands occur which can bind to the central atom of a complex in more than one place. These ligands can give rise to additional kinds of isomerism.

© R A Paselk

Last modified 3 July 2006