Number of complexes which will exhibit synergic bonding amongst, $\left[\mathrm{Cr}{\left(\mathrm{CO}\right)}_6\right],\left[\mathrm{Mn}{\left(\mathrm{CO}\right)}_5\right]$ and $\left[{\mathrm{Mn}}_2{\left(\mathrm{CO}\right)}_{10}\right]$ is