Sidgwick, in his theory, used the Lewis concept of the covalent bond between two atoms in a molecule. He introduced a new concept of the coordination bond, also called the dative or semipolar bond. According to him, the ligands donate electron pairs to the central metal ion to form coordination compounds. These ligands are donors, and the central metal ions are acceptors. The bonds between the donors and an acceptor are called coordinate, dative, or semipolar bonds; such bonds are not very different from covalent bonds. The coordinate bond formed is generally represented by an arrow starting from the donor (L) pointing towards the acceptor (M) as L→ M. For example, the complex ion, [Co(NH3)6]³+ is represented as shown below.
Sidgwick's Effective Number Rule (EAN Rule)
The Effective Atomic Number rule is also known as the 18-electron rule or noble gas rule. According to Sidgwick, the total number of electrons around the central metal ion, including those gained through coordination by the ligands, is known as the Effective Atomic Number (EAN) of the central metal ion. In many cases, this number is equal to the atomic number of the following higher inert compounds that are said to obey the EAN rule. EAN for a central metal ion can be obtained as follows.
EAN = Atomic number of the central metal, the number of electrons gained or lost in ion formation + the number of electrons gained through coordination. (In rare cases, the metal may form a complex in its negative oxidation state; that is why the sign is used here.)
Some examples of complexes that obey the EAN rule are given in the following table:
| Complex | Central metal M | Atomic no.ofM | Oxidation number of M | Electron gained through Coordination | EAN | Atomic number of the next higher inert gas |
|---|---|---|---|---|---|---|
| Ni(CO)4 | Ni | 28 | 0 | 8 | 28-0+8 = 36 | 36 |
| Fe(CO)5 | Fe | 26 | 0 | 10 | 26-0+10 = 36 | 36 |
| [Fe(CN)6]4- | Fe | 26 | 2+ | 12 | 26-2+12 = 36 | 36 |
| [Co(NH3)6]3+ | Co | 27 | 3+ | 12 | 27-3+12 = 36 | 36 |
| [Zn(NH3)4]2+ | Zn | 30 | 2+ | 8 | 30-2+8 = 36 | 36 |
| [V(CO)6]- | V | 23 | 1- | 12 | 23+1+12 = 36 | 36 |
There are exceptions to this rule. Some of these exceptions are given in Table
| Complex | Central metal M | Atomi no. of M | Oxidation numer of M | Electron gained through Coordination | EAN | Atomic no. of the next higher inert gas |
|---|---|---|---|---|---|---|
| V(CO)6 | V | 23 | 0 | 12 | 23-0+12 = 35 | 36 |
| [Mn(CN)4]2- | Mn | 25 | 2+ | 8 | 25-2+8 = 31 | 36 |
| [Fe(CN)6]3- | Fe | 26 | 3+ | 12 | 26-3+12 = 35 | 36 |
| [Co(NH3)6]2+ | Co | 27 | 2+ | 12 | 27-2+12 = 37 | 36 |
| [Ni(NH3)6]2+ | Ni | 28 | 2+ | 12 | 28-2+12 = 38 | 36 |
Defects of Sidgwick's Theory
Sidgwick’s electronic theory is also not free from defects. Some of the defects of this theory are listed below.
The donation of electrons by the ligands to the central metal ion would cause an accumulation of unfavourable negative charge over the electropositive central metal. This will reduce the stability of the complex.
The electron pair donated by ligands like H₂O and NH3 to the central metal ion is the 2s2 pair of electrons. This 2s2 pair has no bonding characteristics. In order to make them useful for bonding purposes, these electrons should be promoted to higher energy levels. This requires more energy than what is usually available during bond formation.
According to Sidgwick, coordination compounds must be covalent. But there are complexes which are predominantly ionic in nature. Hence, the forces acting between the central metal ion and the ligands may be essentially electrostatic.
