Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Summer 2006
Lecture Notes::Lec 25_6 July	© R. Paselk 2006
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The Chemistry of the Elements

Molecular Orbital Theory for Octahedral Complexes

This is the most sophisticated picture for the interaction of a metal ion and its ligands. It gives the most accurate predictions of the properties of the complexes. The problem is that the calculations are difficult and require lots of computational power to do a good job. As a result, approximations are made to make the computations reasonable, though still not trivial - we still need a good computer and powerful software!

The approximation I want to look at makes the reasonable assumption that most of the contributions of the central metal to the final complex is going to be by the valence (n) and n-1 d-orbitals, and that these atomic orbitals will be the major contributors when the molecular orbitals are created.

• What's wrong with this?
  • Recall that a true MO calculation takes into account ALL of the electrons and potentially occupied orbitals of ALL of the atoms in the molecule.
  • The atomic orbitals will be influenced and modified by the nearby charges and electron densities of the ligands.
• However, as we have seen in the diatomic molecules, not all electrons or orbitals contribute equally, and we may assume to a good approximation that some don't contribute at all.

If we restrict ourselves to the valence and n-1 d-orbitals and look at an octahedral complex, then we again need to look at which orbitals will overlap assuming the x, y, z axis system. Looking at the first set of transition metals, favorable overlaps will then occur between ligand orbitals and:

• the 4s orbital,
• the 4p_x, 4p_y, and 4p_z orbitals,
• the 3d_x²-y² orbital, and
• the 3d_z² orbital.

On the other hand, there will be minimal interaction between ligands along these axis and the 3d_xy, 3d_xz, and 3d_yz orbitals, not only because they are on the diagonals, but also because when we add up the orbitals, the lobes adjacent to any given axis are of opposite algebraic sign, and so add up to zero in the overlap calculation.

So let's look at the results of this approximation. (overhead, Russell 22-10, overlaps, and 22-11, energy levels).

Fourth Period Transition Metals

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

Most t ransition metals form colored compounds. As we saw earlier the color is generally a result of d-electrons in the metal ions.

Periodic Table of the Elements -Fourth Period Transition Metals IIIB IVB VB VI VIIB VIIIB IB IIB 21Sc 4s23d1 22Ti 4s23d2 23V 4s23d3 24Cr 4s13d5 25Mn 4s23d5 26Fe 4s23d6 27Co 4s23d7 28Ni 4s23d8 29Cu 4s13d10 30Zn 4s23d10 E°=-2.08V* E°=-1.63V E°=-1.2V E°=-0.74V E°=-1.18V E°=-0.45V E°=-0.28V E°=-0.26V E°=+0.34V E°=-0.76V   3** (2) 3 4 2 3 4 5 2 3 (4)   6 2 (3) 4   (6) 7 2 3 (4)   (6) 2 3 2 (3) (1), 2 2                     * Reduction potentials from M+2 (or M+3 for Sc & Cr) to the metal. ** Common oxidation states (less common in parenthesis).

Fourth Period Transition Metals:

• Scandium (Sc): rare element, not widely used. Prepared by electrolysis of molton ScCl₃.
  • Scandium forms +3 compounds, thus there are no d-electrons, and consequently, the compounds are mostly colorless and diamagnetic. Chemistry is similar to Aluminum, e.g. Sc(OH)₃ is a gelatinous precipitate and amphoteric, like its Al analog.
• Titanium (Ti): main ores = rutile (TiO₂) and ilmenite (FeTiO₃)
  • Titanium is a widely used metal because of its high strength to weight ratio (as strong as most steels, but 50% lighter, double the strength of Al but only 60% heavier). It is particularly important in aircraft, and more recently for high tech bicycles, and sailing yacht hardware (excellent corrosion resistance as well as weight advantages). Expensive because must be very pure to be machinable (TiC, titanium carbide is used as an abrasive)
  • TiO₂ is widely used as a white pigment in paper, paint etc. especially important since white lead can no longer be used in most applications. Prepared from titanium(IV) chloride: TiCl_4(g) + O₂ TiO_2(s) + 2Cl₂.
  • Titanium(IV) chloride reacts with water to give a dense "smoke" which was used in WW I to from smoke screens via a couple of reactions, one of which is shown here: TiCl_4(l) + 2H₂O_(l) TiO_2(s) + 4HCl.
  • The aqueous ion for Ti(III), Ti(H₂O)₆ ⁺³ is purple (readily oxidizes to +4).

• Vanadium (V): The +3 and +4 oxidation state are used by some invertebrates (tunicates) as an oxygen transporter instead of iron. These organisms concentrate V from sea water by more than a million-fold, and their fossil remains have created important vanadium deposits.
  • very hard, strong, and corrosion resistant, but much denser than Ti. Not used much as the pure metal, but an important alloying metal for steels - makes them stronger and more ductile.
  • VO, V₂O₃, VO₂, and V₂O₅ are known.
• Chromium (Cr):
  • silvery white, very hard, brittle and shiny metal.
  • High corrosion resistance due to the formation of a thin, impervious oxide coat.
  • Used for alloying steels (e.g. 'chrome-vanadium' steel for tools), and in particular, its a main component of many stainless steels.
  • Used as a protective and decorative plating for metal objects.
  • Four important oxidation states
    • Cr(II), chromous ion: aqueous solutions (blue hexaaquachromium(II) ion) of Cr(II) are often used as reducing agents.
    • Cr(III), chromic ion: Forms many octahedral complexes. The hexaaquachromium(III) is violet. Chromium(III) hydroxide is a gray-green gelatinous precipitate. Chromic oxide is used as a green pigment.
    • Cr(IV): CrO₂ is the most important compound of Cr(IV). It is a metallic conductor and can be magnetized. It is an important coating in magnetic tapes etc.
    • Cr(VI), chromate: This oxidation state is seen in the yellow, tetrahedral, chromate ion, CrO₄^2-. In acidic aqueous solution chromate is converted to the orange dichromate ion: 2CrO₄^2- + 2H⁺ Cr₂O₇^2- + H₂O.
      • Chromium(VI) is highly toxic and carcinogenic.
• Manganese (Mn):
  • A white, brittle metal.
  • Used in steel alloys.
  • We see three oxidation state of manganese in the lab:
    • Mn(II), manganous ion: this is the very pale pink ion of aqueous solutions (the d-d transition is forbidden).
    • Mn(IV) is seen in manganese dioxide (manganese(IV) oxide) a solid brown to black precipitate.
    • Mn(VII), permanganate: the most important Mn(VII) species is the permanganate ion. Permanganate is intensely purple (easily seen in 10^-4M solutions), and a powerful oxidizing agent.
• Iron (Fe): Iron is very abundant (most abundant transition element, 4.7 mass% of Earth's crust) with readily accessible ores (hematite, Fe₂O₃, magnetite, Fe₃O₄, siderite, FeCO₃, and iron pyrite, FeS₂). Much of the iron ore (various iron oxides) used is the result of biological deposition or bio-induced deposition by a variety of bacteria.
  • Has been the most important industrial metal since the beginning of the industrial revolution, and though being displaced for some uses (plastic bumpers and siding in cars etc., aluminum in beverage cans etc.) it is still the predominant metal of our civilization.
  • Iron is ferromagnetic - it has a very strong attraction to magnets. Results from many atoms in a crystal aligning so that the paramagnetic spins line up. Thus its cause is paramagnetism, but enhanced because normally atoms oriented randomly.
  • Pure iron is a silvery-white, soft metal that corrodes readily in moist air. It does not form a protective coat, and so can readily be converted to oxides.
    • Iron is prepared in blast furnaces - see text.
      • Product is "pig iron" which has up to 5% carbon and another 5% of silicon, phosphorus, manganese and sulfur.
    • Iron is further purified by burning off excess carbon etc. to give steels - see text.
  • Iron is most important in its various alloys known as steels:
    • Carbon steel is an alloy which contains essentially no metal but iron. It is generally classified in three grades:
      • Mild steel - contains <0.2% carbon. It is malleable and ductile, and is used where strength is not paramount. It cannot be tempered.
      • Medium steels - contain 0.2-0.6% carbon. Used for structural steel like beams and girders in construction etc.
      • High-carbon steels - contain 0.8-1.5% carbon. Used to make knives, tool bits, drills etc. where hardness is important. High-carbon steels' hardness can be adjusted from relatively soft to very hard and brittle by tempering. This characteristic is one of steels most endearing properties since one can work it while soft with files etc., then temper it to get a hard (but often brittle) object. (see Heat Treatment of Steel in text)
    • Alloy steels add other transition metals (and often carbon) to give steels with special properties.
      • Some of the metals added and properties they convey are:
        • V - improves springiness, strength and ductility
        • Cr - resistance to corrosion, increased hardness
        • Mn - increased wear-resistance
        • Ni - improves toughness, resistance to corrosion
        • Co, W, and Mo are added to increase heat resistance for engine parts and tool steels (e.g. cobalt steels can be used at red-heat without losing their temper).
      • Stainless steels generally have high concentrations of chromium and nickel to give corrosion resistance. 18:8 stainless (18%Cr, 8%Ni) is one of the most common.
  • Iron is also essential for life as we know it. It is used by all oxygen using organisms in the various proteins of the electron transport system, and in many animals as part of their oxygen transport systems.
  • Note Fe chemistry in lab book.

Cobalt, Nickel, Copper, and Zinc all have +2 as the most common and important oxidation state.

• Cobalt (Co): is fairly rare.
  • Hard, bluish-metal.
  • Important for alloying.
  • Note Co chemistry in lab book.
• Nickel (Ni): 22nd most abundant element in the Earth's crust. Occurs in various sulfide ores.
  • Silvery metal that polishes very well, which, along with its corrosion resistance accounts for its wide use as a decorative metal or as a plating.
  • In fine-particulate form Ni is an important catalyst.
  • As seen above it is an important alloying element, particularly for stainless steels.
  • Note Ni chemistry in lab book.
• Copper (Cu): copper is the only one of these elements with an important oxidation state other than +2, +1. It is a little less abundant than nickel, generally found in sulfide ores, though other ores are also common, and it occurs rarely in the native (metal) form.
  • Soft, ductile, reddish metal. Resistant to corrosion.
  • Most electrically conductive metal other than Ag.
  • Important alloys as brasses and bronzes.
    • Classically brass is an alloy of copper and zinc
    • Classically bronze is an alloy of copper and tin. Other alloying metals are used however and often referred to as bronzes.
  • Note Cu chemistry in lab book.
• Zinc (Zn): Widely distributed and about as abundant as copper. Most common ore is the sulfide, ZnS called sphalerite (zinc blende).
  • Note Zn chemistry in lab book.
  • Most widely used in galvanizing steel to protect it from corrosion. Also the base for a variety of die-casting alloys.

Other transition metals:

• Ag & Au: (Note Ag chemistry in lab book.)
• Hg: (Note Hg chemistry in lab book.) Only metal which is liquid at room temperature.
• Pt: The platinum metals (Ru, Rh, Pd, Os, Ir, Pt) are all distinguished by exceptional resistance to corrosion.
• W: Highest melting temperature for any metal (second only to carbon).
• Cd: Note Cd chemistry in lab book.

© R A Paselk

Last modified 6 July 2006