Percentage ‘s’ character

The hybridization parameter X can be estimated at each R by expressions such as (3.1 lb), which in turn are related to visual plots of the s-pz overlap. A procedure for numerically estimating X will be given at the end of this section. For present purposes, we need only recognize from the percentage s characters that the relative energies of the starting atomic hybrids satisfy... 

Comparisons with Other Metal Systems. Although as complete a series as that reported here is not available for other metal systems, the limited data reviewed in the literature (15, 16, 50) suggest that both steric bulk and electronic effects will influence the M-P bond length. As the bulk of L is changed, the percentage s character in the M-P bond also changes (51, 52). The spectroscopic trends whenX is varied... 

C-charge and Cl-charge NBO charges (in au). Pol(%C) is the C-Cl bond polarization (% at C-atom), s(%C) is the percentage s-character in C orbital of the C-Cl bond. The last line of the table ( ) presents characteristics of the CF3CI monomer not involved in any interaction... 

The structure adopted by a molecule will always try and maximise the number of electron pairs in bonding molecular orbitals. A structure that is able to accommodate electron pairs in non-bonding orbitals will always be preferred to a more symmetrical structure which results in the population of antibonding molecular orbitals. Where more than one structure is possible with the same number of non-bonding orbitals, the one that maximises the character of the most stable atomic orbital is preferred. For example, both planar and pyramidal NH3 have a non-bonding orbital but the latter has a higher percentage s character and, therefore, is preferred. 

On extending the study to cyclopropane, the relationship was found no longer to hold, probably because of the unsaturated character associated with the three-membered ring. A semi-empirical linear relationship between the percentage s-character of a C—H bond and the carbon-hydrogen coupling constant (Ji3c h)... 

All of these trends can be accommodated by the orbital hybridization model The bond angles are characteristic for the sp sp and sp hybridization states of carbon and don t require additional comment The bond distances bond strengths and acidities are related to the s character m the orbitals used for bonding s Character is a simple concept being nothing more than the percentage of the hybrid orbital contributed by an s orbital Thus an sp orbital has one quarter s character and three quarters p an sp orbital has one third s and two thirds p and an sp orbital one half s and one half p We then use this information to analyze how various qualities of the hybrid orbital reflect those of its s and p contributors... 

Bond distances for some important bond types are given in Table 1.5. As can be seen in this table, carbon bonds are shortened by increasing s character. This is most often explained by the fact that, as the percentage of s character in a hybrid orbital increases, the orbital becomes more like an s orbital, and hence is held more tightly by the nucleus than an orbital with less s character. However, other explanations have also been offered (see p. 37), and the matter is not completely settled. 

The articles by J. R. Anderson, J. H. Sinfelt, and R. B. Moyes and P. B. Wells, on the other hand, deal with a classical field, namely hydrocarbons on metals. The pattern of modem wTork here still very much reflects the important role in the academic studies of deuterium exchange reactions and the mechanisms advanced by pioneers like Horiuti and Polanyi, the Farkas brothers, Rideal, Tw igg, H. S. Taylor, and Turkevich. Using this method, Anderson takes ultrathin metal films with their separated crystallites as idealized models for supported metal catalysts. Sinfelt is concerned with hydrogcnolysis on supported metals and relates the activity to the percentage d character of the metallic bond. Moyes and Wells deal with the modes of chemisorption of benzene, drawing on the results of physical techniques and the ideas of the organometallic chemists in their discussions. 

In this expression, the hybridization parameter A, varies between 0 (pure s) and oo (pure p), with any integer or non-integer value in this range being physically allowed. The general relationships between k and percentage s and p characters are given by... 

Figure 3.18 Comparison of accurate (%p) and estimated (%p, Eq. (3.71)) percentage p character for hydride bond (circles) and lone-pair (triangle) hybrids of group 15-17 AH hydrides (r.m.s. deviation = 2.82%).

Fig. 8.3 The bond integral, , between directed hybrids as a function of the percentage of s character of the hybrid, 100/(1 + X2), for two choices of the ratios pp

Jp-c values decrease regularly on going from methyl to tert-butyl this has been connected with the decreasing percentage of s character in the hybrid orbital comprising the P—C bond. Both electronic and steric factors were considered to have some influence in that respect (substitution of a more electronegative element than carbon for R increased lJf-clps<. further). 

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