Efficient catalytic systems based on arene-Ru(II) complexes bearing bis-imidazole methane-based ligands were developed to achieve additive-free hydrogen generation from formaldehyde and paraformaldehyde in water. Our findings inferred the influential role of bis-imidazole methane ligands in the observed catalytic performance of the studied catalysts. Among the screened complexes, [(η6-p-cymene)RuCl(L)]+Cl- ([Ru]-2) (L = 4,4'-((2-methoxyphenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole) outperformed others to generate hydrogen gas from paraformaldehyde in water with an exceptionally high turnover number (TON) of >20,000. A detailed mechanistic pathway for hydrogen gas generation from formaldehyde has been proposed on the basis of identified several crucial catalytic intermediate species involved in the hydrogen production process.