Transargononic structures

The use of the electroneutrality principle in assigning electronic structures to molecules and crystals is discussed in the following examples and in the following section and later chapters. 

Example 6-13. Should the hydrogen cyanide molecule be assigned the structure HCN or the structure HNC  

Solution. The electronic structure H—C=N makes the atoms nearly neutral. The partial ionic character of the bonds (4% for H—C and 7% for each C—N) leads to the charge +0.04 on H, +0.17 on C, and —0.21 on N. These charges are small, and are compatible with the electroneutrality principle. For HNC the structure H—N=C which completes the octet about N and C, assigns four valence electrons to N and five to C, and hence corresponds to N+ and C. The partial ionic character of the bonds then leads to the charges +0.04 on H, +0.75 on N, and —0.79 on C. These charges on N and C are much larger than for the structure H—C=N , and correspond to instability. Hence HCN is the preferable structure. 

Example 6-14. What is the electronic structure of the anesthetic gas nitrous oxide, N2O  

Solution. A ring structure is not likely, because of the strain of the bent bonds. The linear structure N—N=Q completes the octet for each atom, but we reject it because of the double negative charge on the end nitrogen atom. The two other structures that complete the octet for each atom are N=N—O and N=N=Ot, each of which has formal charges on two atoms as shown. These two structures look equally good, and we conclude that the molecule can best be described as the resonance hybrid with the two structures contributing about equally. 

---

Here the phosphorus atom has four shared electron pairs and one unshared pair, using five orbitals. (In PC15, eg, the transargononic phosphorus atom has five shared pairs in its outer shell.) However, because of the electroneutrality principle such a structure is allowed only for structure 1. Transargononic structures do not occur for first-row atoms, so this phenomenon is not found in NF3. These ideas concerning the bonding in NF3 and PF3 are implicit in the discussion by Marynick, Rosen and Liebman61 of the inversion barriers of these molecules. 

A transargononic structure for sulfur, with six bonds formed by sp3d2 hybrid orbitals, was suggested for sulfur in the octahedral molecule SF6 long ago, and also for one of the sulfur atoms, with ligancy 6, in binnite (Pauling and Neuman, 1934). Some transargononic structures of metal sulfides have been proposed recently by Franzen (1966). 

Structures for many sulfides that do not occur as minerals have been reported in which sulfur forms four bonds directed to one side, leaving room for the unshared pair of the sulfur atom. There is little doubt that these sulfides, too, have transargononic structures, as suggested by Franzen (1966). 

Sometimes an atom forms so many covalent bonds as to surround itself with more than four electron pairs it assumes a transargononic structure. An example is phosphorus pentachloride, PCI5 in the molecule of this substance the phosphorus atom is surrounded by five chlorine atoms, with each of which it forms a covalent bond (with some ionic character) ... 

The stability of transargononic structures will be discussed in Sections 7-8 and 8-1. 

In the PCI5 molecule the phosphorus atom has a transargononic structure, with five shared electron pairs in the outer shell. It forms five covalent bonds, with the bond orbitals formed by hybridization of a 3>d orbital with the 35 orbital and the three 3p orbitals. The valence-bond structure for the molecule is... 

A transargononic P—Cl bond is 152 kJ mole" less stable than an ar-gononic P—Cl bond. This smaller stability results from the smaller stability of a 2>d electron in phosphorus than a 3p electron, partially neutralized by the greater bond-forming power of spd hybrid bond orbitals (greater overlap) than of p bond orbitals. In addition, a significant contribution to the normal state of PCI5 is made by ionic structures such as... 

The importance of ionic structures in stabilizing transargononic compounds is shown by the great stability of transargononic fluorides. The bond energy for each of the two added fluorine atoms in PF5 is 425 kJ mole" ... 

---