Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Fall 2003
Lecture Notes::Lec 35_1 December	© R. Paselk 2003
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The Chemistry of the Elements

Stereoisomerism, cont.

Last time we looked at coordination numbers 2 and 4, today we will look at coordination number 6:

• 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.

• Polydentate ligands can have different numbers of bonding sites.
  • Polydentate ligands are often referred to as chelating agents and their complexes are known as chelates (from the Greek for claw).
    • Chelating agents can have 2, 3, 4, 5, or 6 points of attachment with hexacoordinate central atoms.
  • One of the most common and simplest of polydentate ligands is ethylenediamine (often abbreviated en in complex ion formulae): H₂NCH₂CH₂NH₂. Each of the nitrogens has a free electron pair which can act as a Lewis base to interact with the Lewis acid sites of the central metal ion (e.g. tetrachloro(ethylenediamine)cobaltate(III) ion, [CoCl₂(en)]^-.
    • Note that, due to its small size ethylenediamine can only bond cis- corners, its too short to reach around for trans- bonding.
  • Polydentate ligands like ethylenediamine give geometrical isomers, cis- and trans-
  • Polydentate ligands like ethylenediamine can also give rise to enantiomerism (mirror image or chiral isomers) in hexacoordinate complex ions. (models)

Transition Metal Ion Colors & Complex Ion Bonding

Introductory discussion of color and electronic energy levels in atoms and ions.

• Why are transition metal ions often colored?
  • Must have small electronic energy differences in ions or complexes to have transitions in the visible region (recall hydrogen atoms spectra - ground level spectral lines are all in UV).
  • Apparently the outermost (valence) electrons in transition metal ions have access to orbitals at only slightly higher energies.
    • Note that metal ions of the Representative element Groups I-IV are not colored.
    • Note also lack of color in Sc, Zn, Cd, and Hg - all explained by a lack of valance electrons in ions.
• How do we account for the small differences in energy levels required to explain these colors?

To approach these questions in greater depth we need to discuss the bonding in complexes from a quantum point of view. We'll approach this much as we did when we introduced molecular bonding theory, beginning with a hybrid orbital (localized electron orbital) model and then going to molecular approximations to give us energy levels etc.

Hybrid Orbital Theory for Transition Metal Ions

As we've mentioned before the interactions between ligands and transition metal ions are best considered to be covalent in nature. So how do we come up with a set of bonds giving octahedral symmetry with a transition metal ion? Let's look at one of the more intensely colored complex ions we've seen in lab, ferricyanide ion (more properly, hexacyanoferrate(III) ion): [Fe(CN)₆]^3-.

First, let's look at the electronic configuration of the electrons in the ground-state free atom:

Fe (Z = 26): [Ar] 3d 4s 4p

To get the ferric ion we remove three electrons:

Fe3+ (Z = 26): [Ar] 3d 4s 4p

(Note - You can see why iron prefers to lose two electrons [the s-electrons] to give ferrous ion, or three electrons [the s-electrons, plus one d-electron to give a stable half-filled d-shell.]

Now the question is, how do we hybridize the remaining orbitals? Remember that in hybridization the orbitals must overlap to give good addition. Also, we need to come up with hybrid orbitals along the x-, y-, and z-axis to give octahedral symmetry, so we expect to hybridize with the p-orbitals which lie along thee axis, and the s-orbital which is symmetrical over all axis. If we look at the d-orbitals we see that the 3d_xy, 3d_xz, and 3d_yz orbitals lie on the diagonals of these axis, and thus do not overlap with the p-orbitals as required to give good hybridization: (computer animations below)

On the other hand the 3d_x²-y² and 3d_z² orbitals do have their lobes aligned along the axis:

But how do we get the electrons arranged so that these orbitals are available for hybridization?

First, recall that though the hybridization model is for the localized electron picture, that doesn't mean it applies to an isolated atom or ion, Thus we will assume that the ligands will influence the energy levels of our ion in some complex way, and for our purposes we will assume that it results in all of the d-electrons "bunching up" into the 3d_xy, 3d_xz, and 3d_yz orbitals giving:

Fe3+ (Z = 26): [Ar] 3d 4s 4p

If we now "mix" the two empty d-orbitals and the s- and p-orbitals we will get the d²sp³ hybrid orbital set:

Fe3+ (Z = 26): [Ar] 3d d2sp3

Our hybridized ferric ion can now act as a Lewis acid to accept six electron pairs from cyanide ion acting as a Lewis base. Note that the cyanide ion has been shown experimentally to donate the electron pair on the carbon atom, not the nitrogen pair: [:C:::N:]^-

The electronic configuration for iron in the final complex is then:

Fe3+ (Z = 26): [Ar] 3d d2sp3

where the electrons in the d²sp³ hybrid orbitals are all contributed by the ligands.

© R A Paselk

Last modified 1 December 2003