Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Fall 2003
Lecture Notes::Lec 37_5 December	© R. Paselk 2003
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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.

Transition 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 4s23d 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 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 Æ 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) Æ TiO_2(s) + 4HCl.
  • The aqueous ion for Ti(II), Ti(H₂O)₆ ⁺³ is purple.

© R A Paselk

Last modified 5 December 2003