Borane Anions and their Derivatives

Borane Anions and their Derivatives.—number of rules, analogous to the 18-electron rule of organo-transition-metal chemistry, have been proposed for metalloboranes. Thus, (i) a closo-, n-vertex polyhedral heteroborane with one transition-metal vertex requires 2n 4- 14 electrons, (ii) niWu-polyhedra of this type require 2 + 16 electrons, and (iii) c/ojo-polyhedra with two transition-metal vertices require 2n + 26 electrons. Further rules can also be derived for cowwo-compounds in which two c/ojo-polyhedra are fused at the metal atom.  

Hydrogen-deuterium exchange accompanying protolysis and deuterolysis of BH4 and AIH4 ions is consistent with the intermediate formation of BH5 and AlHg. BHg is isoelectronic and isosteric with the methonium ion CH, and theoretical calculations (CNDO/2 method) favour a C, structure (4). According to these data, electrophilic attack on BH (or AIH4) is at the 

M—cr-bond, via the formation of a two-electron, three-centre bond, not through direct linear hydride-ion transfer.  

The reduction of molybdate ion by NaBH4 in acid, neutral, and borate buffer solutions takes place via the action of BH(OH 3, to Mo + in strongly acid solution, and to MoO(OH)3 at pH 5.6.  

Data on the enthalpies of solution of KBH4 and LiBH4 in O.OllM-NaOH have been used to calculate the enthalpy of formation of solid KBH4 and LiBH4  

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