Transition Structure, definition bonding

We also conclude from our ab initio DF SCF calculations that the 5d, 6d and 5f DFAOs (and their associated electrons) are definitely involved (due to relativistic effects in the electronic structure and bonding of the diatomics of the heavy third-row transition elements and actinides, and they present the formidable dual challenge to quantum chemists of the accurate calculation of the relativistic and electron correlation effects for such systems. 

Figure 7.11. Terminology definitions for hydroboration transition structures [141] (a) The auxiliary may be either syn or anti to the alkene substituents, but anti to the substituent (R) on the nearest carbon, (b) A stereocenter attached to boron, in a staggered conformation with respect to the forming C-B bond, has substituents in anti, inside, and outside positions, (c) Definition of the Large, Medium, and Small substituents of IpcBH2.

Now we would like to use a transition state ring bond order uniformity (n-molecular orbital delocalization) as a measure of its stability, and therefore the selectivity between two or more isometric transition state structures. A view that transition state structures can be classified as aromatic and antiaromatic is widely accepted in organic chemistry [54], A stabilized aromatic transition state will lead to a lower activation barrier. Also, it can be said that a more uniform bond order transition state will have lower activation barriers and will be allowed. An ideal uniform bond order transition state structure for a six-membered transition state structure is presented in Scheme 4. According to this definition, a six-electron transition state can be defined through a bond order distribution with an average bond order X. Less deviation from these ideally distributed bond orders is present in a transition state which is more stable. Therefore, it is energetically preferred over the other transition state structures. 

The efforts to rationalize the formulas and structures of Zintl ions and related species predated extensive definitive structural information on anionic post-transition metal clusters obtained by Corbett and his group in the 1970s [8, 9]. After enough such structural information on the bare post-transition metal clusters became available, the resemblance of their polyhedra to the known polyhedral boranes became apparent. For this reason, the simple Zintl-Klemm concept has been largely superseded by newer, more advanced descriptions of chemical bonding in such clusters, initially those applied to the polyhedral boranes. 

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