Bonding in Transition Structures

Considerable effort has been expended in discussion of polar 

Gandour (eds.), Transition States of Biochemical Processes, Plenum Press, New York, 1978. 

In the application of free energy relationships to the study of bond order in a reaction the following considerations need to be addressed  

Albery, Transition-state Theory Revisited, Adv. Phys. Org. Chem., 1993,28, 139. 

V Anslyn and D.A. Dougherty, Modern Physical Organic Chemistry, 

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Fig. 12.9. Structure and relative energies of four modes of hydrogen bonding in transition structures for epoxidation of 2-propen-l-ol by peroxyformic acid. Relative energies are from B3I.YP/6-311G -level computations with a solvation model for CH2C12, e = 8.9. Reproduced from / Org. Chem., 64, 3853 (1999), by permission of the American Chemical Society.

The general understanding of the electronic structure and the bonding properties of transition-metal silicides is in terms of low-lying Si(3.s) and metal-d silicon-p hybridization. There are two dominant contributions to the bonding in transition-metal compounds, the decrease of the d band width and the covalent hybridization of atomic states. The former is caused by the increase in the distance between the transition-metal atoms due to the insertion of the silicon atoms, which decreases the d band broadening contribution to the stability of the lattice. 

A new look at structure and bonding in transition metal complexes. J. K. Burdett, Adv. Inorg. Chem. Radiochem., 1978, 21,113-146 (73). 

A New Look at Structure and Bonding in Transition Metal Complexes Jeremy K. Burdett... 

The interest in the structures of simple R2Si(OH)2 compounds lies in the fact that one of them, Bu 2Si(OH)2, forms a discotic liquid crystalline phase (308,309). Despite many attempts, it has not proved possible to obtain crystals of Bu 2Si(OH)2 suitable for a crystallographic study, the material obtained from various solvents usually being of a fine fibrous nature. The discotic phase of Bu 2Si(OH)2 has been proposed (309) to be due to the formation of dimeric disks of molecules which remain on breaking the interdimer hydrogen bonds in a structure of type 65 at the transition between crystal and mesophase. As has been described, structure type 65 is found for several diols similar to Bu 2Si(OH)2, and it is thus quite likely that Bu 2Si(OH)2 does indeed have the proposed structure. 

The major carbon centered reaction intermediates in multistep reactions are carboca-tions (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [ ]+, [ ]-, [ ], and [ CI I21-... 

The stereochemistry of the reaction between alkenes and isocyanates has been studied experimentally [117] and computationally [109]. It was found that the concerted nature of the reaction should result in retention of configuration of the starting olefin. However, in one case in which a strong Ji-donor was present it was possible to characterize a stepwise mechanism (Scheme 37). Reaction between vinyl alcohol (147) and chlorosulfonyl isocyanate (148) was calculated to proceed at the MP2(SCRF)/6-31G //RHF(SCRF)/6-31G level via zwitterionic intermediate (150), whose rotation about the C4-OH bond through transition structure (151) opens the possibility of a loss of stereoselectivity in this kind of reactions, a phenomenon observed in some cases in the reaction between (148) and vinyl ethers [118]. 

Certainly most useful at the present time for understanding the bonding in transition metal pentadienyl compounds is the series of structural results for the M(2,4-C7HU)2 (M = V, Cr, Fe) and Ru(2,3,4-C8H13)2 compounds241 243. While Ti(2,4-C7Hn)2 is known as a liquid244, its structure has not yet been determined, but it appears that it should be readily inferrable from the other structural data (vide infra). 

A NEW LOOK AT STRUCTURE AND BONDING IN TRANSITION METAL COMPLEXES... 

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