Effectively using the EAN rule

In addition to the 18- and 16-electron rules, there is also the Effective Atomic Number (EAN) rule, which looks to the periodic table for verification of what can be calculated using the 18-electron rule. To put this rule simply, it says that an organometallic complex (as a system) binds as many ligands as possible to reach the number of electrons of the next noble gas configuration. 

For example, for carbon the noble gas configuration is that of neon and it s achieved with a total of ten electrons [Ne]. For iron, the next noble gas configuration is krypton, and it has 36 atoms [Kr]. 

Consider the example of iron pentacarbonyl (Fe(CO)5) that s used in the catalytic growth of nanotubes. Table 15-2 illustrates how both the 18-electron and the EAN rules can predict a stable configuration. In this example, the covalent convention is used. 

Effective Atomic Electmn Count Number Rate Covatent 18-Etectron Rufe Electron Count 

When using the 18-electron rule, you should count the number of metal electrons according to the group number. For example, iron is in Group 8 of the periodic table, so you give it eight electrons. But for the EAN, the number 26 is used instead because this is the atomic number of iron. 

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