[blaisem]

Hello everyone.  I am running b3lyp/def2-TZVP (slightly higher quality than 6-311G**) geometry optimization calculations on a Nickel compound complexed with a bridged four-ring pentadentate ligand that has 5 coordinating Nitrogens.  The SCF Energy is compared between this complex bound with either CO₂, CO or MeCN.  I also have a set of calculations with Nickel complexed to a bridged 3-ring tetradentate ligand (4 coordinating Nitrogens) and MeCN, along with either CO₂or CO to complete the octahedral complex.

For each structure, I set the charge to either +1 or +2 and include the two likely Spin States for each charge.  I am noticing that the calculated energies for a given complex are almost exactly the same, sometimes even within 1 Hartree of another.  It's as if my designation of the charge and multiplicity were practically arbitrary.  This has made me question:

What kind of role does the oxidation state of a metal play in coordination chemistry?

What kind of role does the spin state (doublet vs. quartet, singlet vs. triplet) play in coordination chemistry?

It also has me worried that my basis set is not accounting for the differences appropriately, but I can't really explore that possibility until I know what these differences might be.  Links to further reading are always welcome.  I had trouble finding anything beyond counting electrons and summaries of high spin vs low spin, but I might be using the wrong terms to search. I did read that high oxidation state is not conducive to coordination with strong pi acidic ligands, as pi-backbonding is not as favorable.  I was hoping someone could enlighten me to other, possibly more general, implications.

Thank you very much for reading and any advice!

[Corribus]

It's been a while (about 10 years) since I did any serious Gaussian calculations, and at the time I was using basis sets likt 6-311G**, but the common wisdom THEN was that density functionals were not at a state yet to model transition metals (particularly open shell metals) very well.  This may have changed in the time since, but it's something to keep in mind.

As to your question, oxidation state and spin state can have a profound affect on coordination geometry.  So profound, in fact, that without it we wouldn't be here.  After all, the coopertive binding ability of hemoglobin is related to structural alterations in the protein that occur as a result of spin state changes at heme centers in response to oxygen bonding by iron.

You might check out the textbook Bertini Gray Lippard Valentine, Bioinorganic Chemistry, which has a good section on this subject.

(EDIT: I removed the paper citation; I think the textbook is a better source for information on the topic.)

[blaisem]

Thank you as always, Corribus I have ordered the book at my library; I'll be looking forward to reading it!

Edit: Incidentally, I have discovered I made a mistake.  Since the units are given in Hartrees, the 1 Hartree difference I noted actually converts to a 2625 kJ/mol difference.  So even energy differences to the second decimal place in Hartrees are larger than I realized.