Bond, electron pair directional characters

Since the most direct evidence for specihc solvation of a carbene would be a spectroscopic signature distinct from that of the free carbene and also from that of a fully formed ylide, TRIR spectroscopy has been used to search for such car-bene-solvent interactions. Chlorophenylcarbene (32) and fluorophenylcarbene (33) were recently examined by TRIR spectroscopy in the absence and presence of tetrahydrofuran (THF) or benzene. These carbenes possess IR bands near 1225 cm that largely involve stretching of the partial double bond between the carbene carbon and the aromatic ring. It was anticipated that electron pair donation from a coordinating solvent such as THF or benzene into the empty carbene p-orbital might reduce the partial double bond character to the carbene center, shifting this vibrational frequency to a lower value. However, such shifts were not observed, perhaps because these halophenylcarbenes are so well stabilized that interactions with solvent are too weak to be observed. The bimolecular rate constant for the reaction of carbenes 32 and 33 with tetramethylethylene (TME) was also unaffected by THF or benzene, consistent with the lack of solvent coordination in these cases. °... 

In the foregoing treatment of the water molecule, which we shall use as an example, each of the two bond orbitals of the oxygen atom has been calculated to have 6 percent s character and 94 percent p character. Each of the two unshared-pair orbitals then has 44 percent s character and 56 percent p character. The maxima for the unshared-pair orbitals lie in directions making an angle of 142° with one another and such that their resultant is opposed to that for the two bond orbitals, which have their maxima at 93.5° with one another. The component for the four unshared-pair electrons is determined by the direction cosine —0.34, and that of the two bonding electrons of the oxygen atom by the direction cosine 0.68 hence the contribution of the four unshared-pair electrons to the dipole moment is just balanced by that of the two bonding electrons.18... 

All ionic bonds have some covalent character. To see how covalent character can arise, consider a monatomic anion (such as Cl ) next to a cation (such as Na ). As the cation s positive charge pulls on the anion s electrons, the spherical electron cloud of the anion becomes distorted in the direction of the cation. We can think of this distortion as the tendency of an electron pair to move into the region between the two nuclei and to form a covalent bond (Fig. 2.11). Ionic bonds acquire more covalent character as the distortion of the electron cloud on the anion increases. 

The chloride ion is not attached directly to the nitrogen but is held simply through electric forces by the ammonium ion/ The two valence spheres of Werner s complexes thus had a simple explanation. The inner valences were electron pair bonds often formed by the donor-acceptor process. The outer valences were electrovalent in character. 

Figure 2 id) The structural unit of [XeaFu[+[AuF6] b) A representation of the influence of Xe non-bonding valence-electron pairs upon the polarizing character of the XeF ion arrows indicate directions of maximum polarization of anions)... 

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