Sigma-bonding order

The experiments with the varioug —M-Re(CO)3L species establishes that population of the a level is not required to achieve sufficient M-M bond lability to yield homolytic cleavage within the lifetime of the excited state. However, when the sigma bond order is reduced from one to approximately one-half by the depopulation of the level, the rate constant for M-M bond cleavage appears to be only 105 s-1. By way of contrast, M-M bond cleavage seems to occur with a rate of >1010 s 1 for species... 

Note that the two electrons (one from each hydrogen atom) have both gone into the sigma bonding MO. We can determine the bonding situation in MO theory by calculating the MO bond order. The MO bond order is the number... 

As a general rule, pi bond orders, pi overlap populations and total pi overlap populations allow a diagnosis of the type of pi interactions which obtain between two AO s. The situation is much more uncertain in the case of sigma AO interactions, and one has to rely upon bond orders and/or overlap populations. 

A final useful index of sigma nonbonded interactions between lone pairs is the partial bond order p (Xm, Yn) which is evaluated over the MO s which result from the interaction of the lone pair group MO s with the sigma HOMO and vacant MO s of the coupling unit. This index is intimately connected with the type of analysis employed in this work. In our survey of a variety of problems of molecular structure we shall provide computational results pertinent to the analysis outlined, i e. all or some of the following indices will be provided ... 

A fascinating situation arises when one inquires as to what happens when a sigma lone pair is replaced by a sigma bond, i.e. when the sp2 lone pair of H20 is protonated giving rise to H30+. In order to answer this question, one has to compare the bending tendencies in linear H20 and in a T shaped H30+. 

This example is said to be a shift of order (1,5), ora 1,5-shift, which means that one end of the sigma bond is involved with just one atom (i.e. it remains attached to this atom, the hydrogen) and the other end of the sigma bond migrates over five atoms inclusive. 

In summary, it appears that poly silanes and polygermanes have large third order nonlinear susceptibilities, particularly for materials containing only a saturated sigma bonded polymer backbone. This is consistent with their large polarizabilities and the extensive sigma delocalization along the backbone. In some instances, the... 

There are several types of hybrid orbitals sp sp2 sp dsp- d spl etc. In order to figure out the type of hybrid orbital formed by an atom on the MCAT, simply count the number of sigma bonds and lone pairs of electrons on that atom. Match this number to the sum of the superscripts in a hybrid name (letters without superscripts are as- H sumed to have the superscript l ). Remember,... 

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