Hydrogen storage and the 18-electron rule.

We show that the 18-electron rule can be used to design new organometallic systems that can store hydrogen with large gravimetric density. In particular, Ti containing organic molecules such as C(4)H(4), C(5)H(5), and C(8)H(8) can store up to 9 wt % hydrogen, which meets the Department of Energy target for the year 2015. More importantly, hydrogen in these materials is stored in molecular form with an average binding energy of about 0.55 eV /H(2) molecule, which is ideal for fast kinetics. Using molecular orbitals we have analyzed the maximum number of H(2) molecules that can be adsorbed as well as the nature of their bonding and orientation. The charge transfer from the H(2) bonding orbital to the empty d(xy) and d(x(2)-y(2) ) orbitals of Ti has been found to be singularly responsible for the observed binding of the hydrogen molecule. It is argued that early transition metals are better suited for optimal adsorption/desorption of hydrogen.