Vitamin $\mathrm{B}_{12}$ is a complex of cobalt. It is cyanocobalamin $\left(\mathrm{C}_{63} \mathrm{H}_{88} \mathrm{CoN}_{14} \mathrm{O}_{14} \mathrm{P}ight)$. Its main sources are meat, fish, egg, milk etc. Its deficincy is responsible for pernicious anaemia.