Isoelectronic species bonding

There are several important chemical species that consist of four atoms and have a total of 24 valence-shell electrons. Some of the most common isoelectronic species of this type are C032-, N03 , S03, and P() j (known as the metaphosphate ion). Because four atoms would require a total of 32 electrons for each to have an octet, we conclude that eight electrons must be shared in four bonds. With four bonds to the central atom, there can be no unshared pairs on that atom if the octet rule is to be obeyed. Therefore, we can draw the structure for CO, 2 showing one double C=0 bond and two single C-O bonds as... 

The polyhedra in Fig. 1 thus represent suitable shapes for cluster species with n skeletal atoms (each of which can furnish three AO s for use in skeletal bonding) and with (n + 1) skeletal bond pairs. Since it is the cluster symmetry that determines the number of bonding MO s, the same polyhedra can serve as the basis for the structures of a whole range of isoelectronic species, including neutral carboranes of formula C2B 2Hn, bismuth clusters, such as the trigonal-bipyramidal Bis " ",... 

The standard state of fluorine is the difluorine molecule, F2, which has an electronic configuration identical with that of the peroxide ion. The two species are isoelectronic. The bond order is 1, and the bond dissociation energy of 155 kj mol-1 and bond length of 144 pm are very similar to the values for 022-. 

Some isoelectronic species of C2B10H12, such as CBnH12-, NB11H12, and their derivatives, are known. Since they have the same number of skeletal atoms and the same bond valence, these compounds are isostructural. 

The above structure is in contrast to that of the isoelectronic species [B6HU]-whose structure was predicted by Shore and coworkers from NMR data125). This structure is essentially that of B5H8 with a BH3 inserted into the basal B—B bond, i.e.,[n-BH3B5H8]-. 

The diagrams, with minor modiheation. also describe the bonding in isoelectronic species such as c/oso-CBsHfi, ... 

The structure of N2O4 in the gas phase is planar (D2h) with a remarkably long N -N bond, and these features persist in both the monoclinic crystalline form near the mp and the more stable low-temperature cubic form. Data for the monoclinic form are in the lower diagram together with those for the isoelectronic species B2P4 and... 

What about the bonding situation of heavier diatomic molecules The EDA results for the isoelectronic species N2, CO and BF are given for comparison in Table 13.1. The zero-point corrected interaction energy AEjn, of N2 is — 232.2 kcal/mol which yields a BDE of Dq = 228.8 kcal/mol. This is in excellent agreement with the experimental value of Dq = 225.0 kcal/mol [16]. 

The generalizations of the VSEPR model are useful, but there are hmitations to its use. In this section, we give examples that illustrate some problems. The isoelectronic species IF7 and [TeFy] are predicted by VSEPR theory to be pentagonal bipyramidal and this is observed. However, electron diffraction data for IF7 and X-ray diffraction data for [Me4N][TeF7] reveal that the equatorial F atoms are not coplanar, a result that cannot be predicted by the VSEPR model. Moreover, in IF7, the I-F x and I-F q distances are 179 and 186 pm respectively, and in [TeF7], the Te-Fju bond distance is 179 pm and the Te-Fgq distances lie in the range 183 to 190 pm. 

NHJ) leaves only two electrons for bond formation. In this case, the nitrogen atom (as N ) is isoelectronic with the neutral oxygen atom, and angular bonds are formed. We can thus compare, sterically, the following isoelectronic species ... 

Thus, for the atoms whose valence shell consists of the s, Px, Py and p orbitals, the geometry of compounds that involve only single bonds is based on a tetrahedral orbital geometry. The arrangement of the nuclei in the molecule, the molecular geometry, depends on how many of the tetrahedral orbitals are occupied by unshared pairs. The following group of isoelectronic species illustrates the point. 

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