Synergic bonding involves transference of electrons from ligands to metal. The transference of electrons takes place from filled metal orbitals to anti-bonding orbitals of ligands. It is a bond between a carbonyl group acting as a ligand and a metal. Synergic bonding means self strengthening bond. More the number of d electrons on the central metal ion more the synergic bonding. In metal carbonyls' oxidation state of the metal is zero.

In the complex $\left[\mathrm{Mn}{\left(\mathrm{CO}\right)}_5\right]$, the electronic configuration of $\mathrm{Mn}$ is $3{\mathrm{d}}^{5}4{\mathrm{s}}^2$. As carbon monoxide is a strong field, pairing of electrons occurs. Hence, the new electronic configuration of $\mathrm{Mn}$ is $3{\mathrm{d}}^7$.

In the case of $\left[{\mathrm{Mn}}_2{\left(\mathrm{CO}\right)}_{10}\right]$ also electronic configuration after pairing of electrons is $3{\mathrm{d}}^7$.

In the complex $\left[\mathrm{Cr}{\left(\mathrm{CO}\right)}_6\right]$ , the electronic configuration of $\mathrm{Cr}$ is $3{\mathrm{d}}^{5}4{\mathrm{s}}^1$ . As carbon monoxide is a strong field, pairing of electrons occurs. Hence, the new electronic configuration of $\mathrm{Cr}$ is $3{\mathrm{d}}^6$

All of these compounds can show synergic bonding.