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.

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:

On the other hand, there will be minimal interaction between ligands along these axis and the 3dxy, 3dxz, and 3dyz 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:

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

Other transition metals:


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Last modified 6 July 2006