Coordination compounds octet rule

In Section 8-5 we found that gaseous BeCb is linear. The Be atoms in BeC molecules, however, act as Lewis acids. In the solid state, the Cl atoms form coordinate covalent bonds to Be, resulting in a polymeric structure. In such compounds. Be follows the octet rule. 

Similarly, while diagram 15.3 is a resonance form for H2SO4 which describes the S atom as obeying the octet rule, structures 15.4 and 15.5 are useful for a rapid appreciation of the oxidation state of the S atom and coordination environment of the S atom. For these reasons, throughout the chapter we shall use diagrams analogous to 15.2, 15.4 and 15.5 for h5rpervalent compounds of S, Se and Te. 

Suboctets and coordinate covalent bonds Another exception to the octet rule is due to a few compounds that form suboctets—stable configurations with fewer than eight electrons present around an atom. This group is relatively rare, and BH3 is an example. Boron, a group 3 nonmetal, forms three covalent bonds with other nonmetallic atoms. 

A third characteristic of the uniqueness principle is the lack of availability of low-lying d orbitals for participation in bonding. Hence, the second series elements cannot violate the octet rule in the formation of compounds. Consider, for example, the mixed halogens with fluorine as the central atom Fj and CIF, and BrF compared with those of chlorine, bromine, or iodine, which take higher coordination numbers CIF3, CIF5, BrF3, BrFj, IF3, IF5, and IF7. Similarly, the carbon atom in CF4 is sp hybridized, while the Si atom in SiF is d sp hybridized. The extent to which the... 

For the transition metal complexes possessing the coordination number w, the number of bonding and nonbonding orbitals equals nine and, in the case of the main group compounds, the number of these orbitals is four. This leads to the effective atomic number rule (Sidgwick s rule) and the octet rule, respectively. In and EL ... 

Each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions. The coordination numbers of both ions are six. In the formation of NaQ, the sodium atom loses its valency electron (3s ) to the chlorine atom (3s 3p ) so that Na and Cl" have the electronic configurations of neon and argon, respectively, in accordance with the octet rule (discussed in Section 5.3). Because the electron attachment enthalpy of chlorine (-355 kJ mol" ) does not provide compensation for the enthalpy used to ionize the sodium atom (500 kJ mol" ), the stability of the compound is produced by the attractive forces operating between the oppositely charged ions in a crystal lattice. 

As depicted in Figure 2.1 and outlined in the Summary of Chapter 2, viable bonding theories started to emerge from the quantum-mechanical model of the atom in the 1920s. G. N. Lewis proposed his now familiar electron-dot diagrams and octet rule for simple compounds in the early 1920s, and by the end of the decade, Nevil Sidgwick applied these ideas to coordination compounds. It was he who first proposed the idea of the coordinate-covalent bond referred to in earlier chapters. 

These elements (E) have four valence electrons and are tetravalent as carbon below which they are located. The compounds ER4 thus follow the octet rule which confers them a great stability. The tetraalkyl-element complexes are almost apolar and particularly robust and inert. However, the energy of the E-C bond decreases upon going down in the column of the periodic table. Thus, the tetraalkyllead complexes are less robust towards thermolysis than the lighter analogs. They decompose between 100 and 200°C, which was eventually applied to provide their antiknock properties. The chemistry of these elements is largely dominated by the oxidation state and coordination number 4, but the oxidation state and coordination number 2 is also known for all, and its stability increases upon going down in the column of the periodic table. 

In Sip5 and SiFg we can see again the idea that not all compounds obey the stable octet rule. It is common for elements of the third row to increase their coordination number to five or six (and higher in later rows), and we shall meet more examples in the chemistry of phosphorus and sulfur. 

---