Transition‐metal‐free hydrogenation of alkenes can be carried out with simple organocalcium catalysts (20 bar H2, 20 °C). Both steps in the proposed catalytic cycle, hydride addition to the double bond and σ‐bond metathesis with H2, have been confirmed. Alkenes sensitive to polymerization are also hydrogenated in good yields.

Safe and convenient storage of hydrogen is one of the nearfuture challenges. For mobile applications there are strict volume and weight limitations, and these limitations have steered investigations in the direction of compact, solid, lightweight main-group hydrides.[1] Whereas ammonia-borane (NH 3 BH 3 ) is a nontoxic, nonflammable, H 2 -releasing solid with a record hydrogen density of 19.8 wt %, it releases hydrogen in an irreversible process.[2] Metal hydrides such as MgH 2 are less rich in hydrogen (7.7 wt %) but advantageously display reversible hydrogen release and uptake: MgH 2 QMg + H 2 .[3]Although bulk MgH 2 seems an ideal candidate for reversible hydrogen storage, it is plagued by high thermodynamic stability, which translates into relatively high hydrogen desorption temperatures and slow release and uptake kinetics. The kinetics can be improved drastically by doping the magnesium hydride with transition metals [4][5][6] and by ball milling [7,8] or surface modifications.[9] The high hydrogen release temperature (over 300 8C), however, is due to unfavorable thermodynamic parameters (DH = 74.4(3) kJ mol À1 ; DS = 135.1(2) J mol À1 K À1 ), [10] which originate from the enormous lattice energy for [MgÀ1 ) relative to that of bulk Mg (DH = 147 kJ mol À1 ). [11] Although thermodynamic values are intrinsic to the system, recent theoretical calculations demonstrate that for very small (MgH 2 ) n clusters (n < 19), the enthalpy of decomposition sharply reduces with cluster size.[12] Downsizing the particles has a dramatic effect on the stability of saltlike (MgH 2 ) n but much less on that of the metal clusters Mg n . For a Mg 9 H 18 cluster of approximately 0.9 nm diameter a desorption enthalpy of 63 kJ mol À1 was calculated, [12] from which a decomposition temperature of about 200 8C can be estimated. At the extreme limit, molecular MgH 2 is calculated to be unstable even towards decomposition into its elements (DH = À5.5 kJ mol À1

A model solution to hydrogen storage? The recently introduced hydrogen storage material [{Ca(NH2BH3)2}n] has been investigated on a molecular level. A hydrocarbon‐soluble calcium amidoborane complex eliminates H2 spontaneously at the very low temperature of 20–40 °C (see scheme). The decomposition product shows a dimeric species with a bridging [HNBHNHBH3]2− ion that is isolobal to the allylic dianion [HCCHCHCH3]2−.

The reactivity of the hydrocarbon-soluble calcium hydride complex [{CaH(dipp-nacnac)(thf)}(2)] (1; dipp-nacnac=CH{(CMe)(2,6-iPr(2)C(6)H(3)N)}(2)) with a large variety of substrates has been investigated. Addition of 1 to C=O and C=N functionalities gave easy access to calcium alkoxide and amide complexes. Similarly, reduction of the C[triple chemical bond]N bond in a cyanide or an isocyanide resulted in the first calcium aldimide complexes [Ca{N=C(H)R}(dipp-nacnac)] and [Ca{C(H)=NR}(dipp-nacnac)], respectively. Complexation of 1 with borane or alane Lewis acids gave the borates and alanates as contact ion pairs. In reaction with epoxides, nucleophilic ring-opening is observed as the major reaction. The high reactivity of hydrocarbon-soluble 1 with most functional groups contrasts strongly with that of insoluble CaH(2), which is essentially inert and is used as a common drying agent. Crystal structures of the following products are presented: [{Ca{OC(H)Ph(2)}(dipp-nacnac)}(2)], [{Ca{N=C(H)Ph}(dipp-nacnac)}(2)], [{Ca{C(H)=NC(Me)(2)CH(2)C(Me)(3)}(dipp-nacnac)}(2)], [{Ca{C(H)=NCy}(dipp-nacnac)}(2)], [Ca(dipp-nacnac)(thf)](+)[H(2)BC(8)H(14)](-) and [{Ca(OCy)(dipp-nacnac)}(2)]. The generally smooth and clean conversions of 1 with a variety of substrates and the stability of most intermediates against ligand exchange make 1 a valuable key precursor in the syntheses of a wide variety of beta-diketiminate calcium complexes.

A synthetic route to a magnesium-amidoborane complex and its role in the catalytic conversion of a substituted ammonia-borane RNH(2)BH(3) into HB(NHR)(2) is discussed.