I was going through some crystal structures in 5 coordinated Iron porphyrins and got curious to see azide binding to iron center in bend mode whereas Isothiocyanate binds in linear mode. What could be possible reason to this?
Hi Firoz,
in a simple way, both SCN and NNN should be drawn in two resonance forms (see attached Scheme).
Form A shows a triple bond between CN or NN and the SC and second NN bond is then a single bond. Form B) has each two double bonds.
Hybridisation as a simple concept then says that in the Form A the N atom of SCN and one of the N atoms of NNN are sp hybridisised, thus allowing a linear coordination (180°). The S and N coordinating atoms are sp3 hybridisised, thus forcing a binding angle of 109°. In the Form B all atoms are sp2 hybridisised, thus binding angles should be ideally 120°.
From this simple view the two ligands might bind 180° or might be bend (120 or 109°), depending on the electron distribution and the preference of the metal atom to bind S (soft) or N (hard) according to the HSAB principle.
Also in this case, crystal packing or steric bulk in the complex might force linear or bend binding since the energies of the two states (resonance forms) are not far away from each other.
In a more sophisticated picture using MO therorie you will find that the coordinative bond from these ligands to metals is "floppy" thus allowing a huge range of angles. Any covalent or coordinative bond can be described as a Morse potential (has roughly the form of a parable) and the walls of these potentials can be steep or shallow. The latter case is called floppy and means that the energy for a bond deformation (elongation, shortening, bond bend) is small and might be overcome by other forces (steric bulk in a molecule, crystal packing in crystals).
Another such floppy molecule is NO+. Although it should clearly show a linear binding to metals, angles between 120 and 180° were found in crystal structures.
Hope this helps
Dear Prof. Klein,
Many thanks for clarifying my doubt. As in both the cases (azide as well as isothiocyanate) only the N atom binds to the Iron center, so the small intermolecular interactions arising through crystal packing might be responsible for such variable bond angles. NO+ is indeed a great example.
Thank you,
Firoz
I'm curious, is the azide/isothiocyanate molecule itself bent? Or is the bond between the molecule and the iron center that is not linear?
Good question.
We are talking about a bent in the angle Fe-N-X (X = C or N) not about the angle N-C-S or N-N-N.
The latter would be interesting, but I never saw the two ligands with a bend structure on themselves. It would be contradictory to the resonance form stabilising the linear form as resonance stabilises the planar structure of aromats. However, while for aromats this resonance stabilisation is huge it might be smaller for N3- and NCS- and the energy might be overcome by packing effects in the crystal.
Has anyone examples for this?
Yes, indeed we are interested in the Fe-N-X (X=C/N) bond angles only. The bond was bend in case of N3 while it was almost linear (178° or so) for NCS.
Regarding N-C-S or N-N-N, it is linear owing to the conjugation between atoms. I haven't come across any such case where this bond is particularly bent. Crystal packing might affect it but it would be very very minor rather negligible.
this link is useful
www.sciencedirect.com/science/article/pii/S0065279208600643/pdf?md5...pid..
regards
https://www.researchgate.net/deref/http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0065279208600643%2Fpdf%3Fmd5...pid