Van Vleck (1935) and Griffith and Orgel (1957)

Not all the papers we will be looking at are easy to read. Some represent the first stirrings and conceptions about ideas that will later go on to change the field.

The Van Vleck paper perhaps at first does not seem relevant to the discussions we are having. magnetic susceptibilities, what on earth is that?

The Griffith and Orgel paper on the other hand turned into books by each co-author, and those books have been cited thousands of times.

On Wednesday we will be diving deeper into the discussion of the chemistry surrounding this section of inorganic chemistry, before a discussion of these papers on Friday.

Post your questions below before Thursday night.


7 Responses to “Van Vleck (1935) and Griffith and Orgel (1957)”

  1. Hunter Burgin Says:

    I understand that metal complexes determine their electron arrangement based on the delta distance from the low spin state to the high spin state. If the distance between the two states is very large, electrons tend to pair up in the low spin state and when the distance is negligible they tend to remain unpaired and promote themselves to the high spin state. I was wondering if there are ever special circumstances when the d electrons will choose to promote themselves to the unpaired high spin state when the delta distance is not negligible. So in other words, is the spin state determined solely upon the delta distance between the high spin and low spin states?

  2. Josh Ellsworth Says:

    My question may be a bit tangential as it departs a bit from octahedral geometry. I stumbled across hexamethyl Tungsten (VI) while looking at coordination chemistry and saw that it exists as a trigonal prism. Interestingly enough, the compound was incorrectly assigned as having octahedral geometry for close to 20 years until x-ray crystallography proved the trigonal prism.
    After looking a bit more at that geometry, it seems that it occurs only when there is a d0 configuration on the metal, and that the degeneracy of the d orbitals is changed from that of octahedral geometry, with dz2 being lowest in energy, and then dx2-y2 and dxy degenerate, and finally dxz and dyz also degenerate (highest energy).
    My question starts with the synthesis of hexamethyl Tungsten (VI). It is derived from Tungsten (VI) hexachloride, which possesses octahedral geometry. What mechanism accounts for the change in geometry when all the Chlorine ligands are replaced with methyl groups? I’m having a hard time picturing an interchange or an associative (or dissociative) mechanism that would explain the change in degeneracy of the d orbitals. Would there have to be a concerted or simultaneous exchange of all the ligands in order for the orbitals to “shuffle”, or is there a way to picture a stepwise mechanism (replacing one chloro at a time) that could explain an otherwise counter-intuitive change from our beloved doubly and triply degenerate states?
    Seeing as how an empty d shell is requisite for trigonal prisms to occur, and the only electrons available for bonding are coming from the ligand (correct?), do the orbits available to the ligand influence the development of trigonal prisms? Given an understanding of the orbital mixing, can one predict when an empty d shell gives trigonal prism over octahedral geometry?

  3. Adam Settimo Says:

    With respect to the second paper toward the end of the electrostatic theory section, I am getting a little confused with the two complexes of ferrocyanide and ferricyanide. What is the the distinction between the two types of magnetism? It is said that later has five electrons and the former 6, and that they are all in the T2g. This puts 0 unpaired electrons in the ferrocyanide and 1 unpaired in the ferricyanide, making it magnetic. But these are two types of magnetism within the same type of structure? I guess I’m having trouble picturing the structure? There is a Iron metal center in an octahedral conformation, and six cyano ligands, each with a minus one; Yet there is different numbers of d electrons? Why? does this have to do with the solution the molecule is in?

  4. Porter Marsh Says:

    The text explains that because of the orientation of the on and off axis orbitals, electrons in an octahedral complex can be lower in energy if they’re on axis. This explains why there are three t2g orbitals and only 2 eg orbitals. My question is: why do the t2g orbitals become less favorable and the eg more favorable if the complex is a tetrahedral. If the positioning of the 5 d orbitals is what makes on or off axis favorable, why does the number of ligands change the energy of the orbitals and cause eg to be lower?

  5. Kevin Greenwood Says:

    In Griffith and Orgel’s paper I was following along pretty well until the section on molecular orbital theory. On page 387 they discuss the bonding and antibonding eg orbitals, and the proximity of ligands depending on how many unpaired electrons are in those orbitals. They use the example of copper II complexes, stating that the one unpaired electron in the x-squared – y-squared orbital repels the ligands it is nearest to, being the axial ligands. Copper II complexes have an electron configuration of 3d9, with three of the electrons being in the eg orbital, two paired up in the z-squared orbital in their example. Why is it that having an odd number of electrons in the eg orbitals push away the ligand? They use a high spin 3d4 metal cation as an example of the same phenomena, but because the eg orbitals are degenerate, that could put the unpaired electron in either orbital, which would have a different effect on the ligands depending on where it ended up. I do not understand how they could see the same effect unless electrons add first to the x-squared – y-squared orbital for the 3d4 cation example.

  6. Tony Says:

    It was interesting to see what we learned in class in more detail (almost a painful amount of detail). The different theories shown helped me understand it better at times but worse other times (specifically the method of molecular orbitals).

    I have a question concerning the statement made on the second paper just after table 2 that low spin should only occur if delta is more than pi. Why is this the case? In my mind I’d think it’s the opposite. Maybe I need some clarification as to what pi and delta really mean. Delta is the energy gap between on and off axis orbitals. Pi is the energy to pair electrons. Right? So why is this so?

  7. Daniel Begay Says:

    With the Van Vleck paper, everything was making more sense after our crash course in this topic. It’s explained that the authors and others agree that ferro and/or ferrocyanide has a very strong inter atomic force that gives it its diamagnetic characteristic. But the question is which of the three methods do the best job at explaining this anomaly. At the conclusion, they explain that each method does the job of explains the cyanide anomaly but the Method of Molecular Orbitals (and by the definition, is the MO Theory we still use today) was the best due to the fact they it observed the molecule as a whole instead of individual bonds and also isn’t effected by polarity of molecules as well. It also explains that the other two methods can also be explained through MO Theory but are just dealt with as “special cases”. This is where I’m confused, after going over this paper and reading up on these topics, I can’t seem to grasp how the other two methods could be special cases. How would you approach these ferri/ferrocyanide molecules? How would you go about plugging in the zero coefficients for each wave function used for MO Theory?

    Lastly, when the types of coupling (Russel Saunder and Spin Orbit) are destroyed, does that give the electrons free reign to wherever they want to go? Would they be able to fill orbitals in a disorder?

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