to SOs (Figure 3-7). These structures, however, result in formal charges (2- on Be and 1 + on F in BeF2, and 1 - on B and I + on the double-bonded F in BF3), which are unlikely by the usual rules. It has not been experimentally determined whether the bond lengths in BeF2 and BeC12 are those of double bonds, because molecules with clear-cut double bonds are not available for comparison. In the solid, a complex network is formed with coordination number 4 for the Be atom (see Figure 3-7). BeC12 tends to dimerize to a 3-coordinatestructure in the vapor phase, but the linear monomer is also known at high temperatures. The monomeric structure is unstable; in the dimer and polymer, the halogen atoms share lone pairs with the Be atom and bring it closer to the octet structure. The monomer is still frequently drawn as a singly bonded structure with only four electrons around the beiyllium and the ability to accept more from lone pairs of other molecules (Lewis acid behavior, discussed in Chapter 6). Bond lengths in all the boron trihalides are shorter than expected for single bonds, so the partial double bond character predicted seems reasonable in spite of the formal charges. Molecular orbital calculations for these molecules support significant double bond character. On the other hand, they combine readily with other molecules that can