Prof. Eluvathingal D. Jemmis
Department of Inorganic and Physical Chemistry
Indian Institute of Science, Bangalore
Chemistry Building, Room 400
Physical Seminar

We show that the continuum behavior in the variation of the X-H stretch from blue- to red- through zero- shifting observed in H-Bonds also holds good for X-Z stretch in X-Z---Y complexes where Z = a main group fragment. This has been studied by ab-initio electronic structure calculations using Z= Hydrogen, Halogens, Chalcogens and Pnicogens as prototypical examples. Our analysis revealed that, the competition between negative hyperconjugation within the donor (X-Z) molecule and Charge Transfer (CT) from the acceptor (Y) molecule is the primary reason for the X-Z bond length change. Proper tuning of X- and Y-group for a particular Z- can change the blue-shifting nature of X-Z bond to zero-shifting and further to red-shifting. In contrast to the weak intermolecular interactions in the main group compounds, electron-saturated (18-electron) transition metal complexes show reluctance towards weak metal-bond formation. We propose that the nature of valence electron density distribution in transition metal complexes is the primary reason for this reluctance. We propose that weak interactions form a part of the larger continuum of the nature of the chemical bond. The strong among the so-called weak interactions are stronger than the weak among regular X-Z bonds. We end with an unconventional use of halogen bond to strengthen the sextuple bond in Cr2.

References:

1. J. Joseph, EDJ; Red- Blue- or No-Shift in Hydrogen Bonds: A Unified Explanation, J. Am. Chem. Soc., 129, 4620-4632 (2007).

2. J. Joy, J. Anex, EDJ; Continuum in the X-Z---Y Weak Bonds: Z= Main Group Element, Journal of Computational Chemistry (P.Schleyer Issue), 37, 270-279 (2016).

3. J. Joy, EDJ; Contrasting Behavior of the Z Bonds in X-Z..Y Weak Interactions: Z = Main Group Elements Vs Transition Metals, Inorg. Chem., 56, 1132−1143 (2017).

4. J. Joy, EDJ; A Halogen Bond Route to Shorten the Ultrashort Sextuple Bond in Cr2 and Mo2, Chem. Commun. 53, 8168-8171(2017).