Synergic Effect
Synergic effect is the bonding interactions between metal atoms and σ-donor ligands. σ-donor ligands (such as Ammonia. Water, Ethylenediamine, Carbon monoxide, Chlorine etc.) donate electron pairs to the metal(M), simultaneously, metal also donate electron density into the vacant 𝜋* orbitals of σ-donor ligands. This dual interaction is called synergic effect which strengthens the metal-ligand bond and increasing the stability of the complex by removing the excess negative charge which gets accumulated on the metal as a result of the coordination of σ-donor ligands to it(M).
During the formation of above type of 𝜋 bonds, electronic charge is transferred from the filled nonbonding orbitals of the metal (M) to the 𝜋* orbitals of the σ-donor ligand (CO). This reduces the bond order of CO. Thus, while, the 𝜋 back bonding strengthens the M-C bond bond order, it weakens the CO bond order. This means that M-C and CO bond orders in carbonyls are roughly complementary to each other and any factor that increase metal 𝜋 electron transfer to CO would increase the M-C and decrease the CO bond order simultaneously.
Since, the bond order between M and C is generally higher than that due to 𝜋 back bonding the single line in M-C bond simply represents a metal-carbon bond without any reference to the actual bond order in this bond.
Let us consider the following isoelectronic and isostructural species:
Ni(CO)4, [Co(CO)4]−,and [Fe(CO)4]2−
The overall negative charge in case of the last two species is assumed to reside on the metal. Greater the negative charge on the metal, greater would be the ease of the electron transfer from metal to the 𝜋* orbitals of CO. Accordingly, the M-C bond order increases in the direction
Ni-C < Co-C < Fe-C
Simultaneously, the C-O bond order decreases in the direction
Ni(CO)4 > [Co(CO)4]− > [Fe(CO)4]2−
The actual bond orders determined by spectroscopy are given below in the table.
| Metal Carbonyls | M—C Bond Order |
C—O Bond Order |
M—C + C—O Bond Order |
|---|---|---|---|
| Ni(CO)4 | 1.33 | 2.46 | 3.97 |
| [Co(CO)4]− | 1.89 | 2.14 | 4.03 |
| [Fe(CO)4]2− | 2.16 | 1.85 | 4.01 |
