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Between 1912 and 1936, A. Stock and others in Germany first prepared many borohydrides such as diborane (B2H6) as well as their derivatives, and identified them. Borane (BH3) has never been synthesized, and the physicochemical properties of diborane differ from those of the hydrocarbon ethane (C2H6); therefore, it is not possible to represent the structure of diborane using classical valence concepts or conventional bond theory. Initially, it was written as two BH3 molecules, or H2BBH2 molecule and H2 molecule, but there were no electrons connecting these two molecules. Thus, it prompts people to consider the reasons behind it. Because in diborane, each boron atom has 4 atomic orbitals (valence orbitals) that can be used to form valence bonds, while each hydrogen atom has 1 such orbital; in total, there are 4×2 + 1×6 = 14 valence orbitals. Each boron atom possesses 3 valence electrons, and each hydrogen atom has 1 valence electron, giving a total of 3×2 + 1×6 = 12 valence electrons. Since the number of valence electrons is less than the number of valence orbitals, a special molecular structure must exist. Since 1954, W.N. Lipscomb and others have carried out a series of studies on boranes and their derivatives. Regarding the structure of diborane, they proposed the concept of a \"three-center bond.\" They suggested that each of the 4 valence orbitals of a boron atom in diborane overlaps with two other orbitals, and these combined with hydrogen atoms to form bridge bonds with two hydrogen atoms situated between the two boron atoms. Such bridge bonds are those that connect three atoms using a pair of electrons; they are thus known as \"three-center bonds,\" or two-electron three-center bridge bonds. The structural formula of diborane B2H6 is as follows: http://www.cnmhg.com/d/cnmhgimg/f1btlrfem0t.png In this way, the positions of hydrogen atoms in the structure of molecular borohydrides are almost entirely determined. The valence bond structure suitable for them is also perfectly described by three-center bonds. Of course, borohydride molecules also contain the usual B—B and B—H bonds, which are typical two-center bonds. Atoms with covalent properties such as boron, whose number of valence electrons (3) is less than the number of valence shell orbitals (4), are known as electron-deficient atoms. Compounds formed by covalent bonds between electron-deficient atoms that do not possess an octet structure are called electron-deficient compounds. Molecular borohydrides, as well as many known borohydride ions other than [BH4]-, are all electron-deficient compounds. There are not many other electron-deficient compounds; representative ones include carboranes and their derivatives, carbocations, [Al2Cl6], [Al2(CH3)6], {Be(CH3)2}∞, etc. Compounds of boron such as BF3 and B3N3H6 are not electron-deficient compounds.