DOES THE TETRAHYDROBORATE SPECIES AUBH4 EXIST - AB-INITIO MO STUDY OFTHE STRUCTURE AND STABILITY OF CUBH4, AGBH4, AND AUBH4

The MP2, MP4(SDTQ), QCISD(T), and CCSD(T) methods in conjunction with the 19-valence electron Hay-Wadt relativistic effective core potential have been used to study geometries, energies, and possible rearrangem ents of tetrahydroborate compounds of Cu, Ag, and Au. For the CuBH4 an d AgBH4 molecules only the tetrahydroborate structure MBH(4) can exist ; the hydride borane structure, HMBH(3), is thermodynamically (about 2 5-26 kcal/mol) and kinetically unstable and rearranges into the MBH(4) structure. The bidentate (b) and tridentate (t) MBH(4) structures are energetically very close and structurally nonrigid, and at ambient te mperature their bridge and terminal hydrogen atoms will easily interch ange by the pathway (b) reversible arrow (t) reversible arrow (b') rev ersible arrow (t') reversible arrow.... However, the Au analog cannot exist as a tetrahydroborate species, AuBH4, which rearranges with a sm all (similar to 1 kcal/mol) barrier to the hydrido(borane)gold, HAuBH3 , structure lying 11.7 kcal/mol lower. The thermal stability of MBH(4) (M = Cu, Ag, and Au) molecules for the lowest decomposition path, MH +BH3, decreases in the order Cu > Ag > Au.