The CaBH4-and SrBH, free radicals were synthesized by the reaction of Ca or Sr vapor with diborane, The A2Al-X2AI and B2E-X2AI electronic transitions were detected by laser-induced fluorescence. The spectra are consistent with tridentate molecular structures with three bridging hydrogens and C3 symmetry. The tetrahydroborate anion (BH,-) forms a large number of interesting ionic and covalent complexes with metals.'S2 The coordination between the metal and the BH, ion invariably occurs through bridging hydrogens, and rapid interconversion between the various coordination modes is known in the liquid phase.'q2 BH4- is a commonly used reducing agent. Nevertheless, very little is known about the spectroscopic properties of the metal-BH, complexes, particularly in the gas phase. Of the alkaline earth borohydrides, only Be(BH4)2 has been analyzed in the gas phase, but the molecular structure remains uncertain. Gundersen, Hedberg, and Hedberg3 studied the Be(BH4)2 structure using electron diffraction methods, while Nible8 examined the infrared and Raman spectra. The gas-phase NMR spectrum shows only a single set of equivalent protons, consistent with fluxional be- ha~ior.~ A recent theoretical calculation by Stanton, Lipscomb, and BartletP has provided new insight into this long-standing problem. In contrast to the covalent Be(BH4)2, the alkali borohydrides and the heavier alkaline earth borohydrides are all ionic solids with low vapor pressures.'s2 We report here on the discovery of the gas-phase calcium and strontium monoborohydride free radicals, CaBH, and SrBH,. These molecules are made by the reaction of Ca and Sr vapors with diborane, B2H6 The observation of CaBH, completes the isoelectronic pblock family of molecules, CaBH,, CaCH3,' CaNH2,8 CaOH? and CaF.IO We have pre- viously studied many monovalent derivatives of Ca and Sr in our laboratory, including carboxylates,' I azides,12 i~ocyanates,'~ ~lopentadienides ,'~ acetylide~,'~ formamidates,16 pyrrolates, alkanamides,'* and alko~ides.'~ CaBH, and SrBH, are the first examples of gas-phase metal monoborohydrides to be experimentally characterized. However, there has been considerable theoretical activity in this area. Ab initio predictions for the properties of LiBH,,225 NaBH4,23-26 KBH424 and CUBH,~, are available in the literature. The structures are all predicted to be fluxional with either tridentate