Boron, isoelectronic series

Calculations of IIq(O) are very sensitive to the basis set. The venerable Clementi-Roetti wavefunctions [234], often considered to be of Hartree-Fock quality, get the sign of IIq(O) wrong for the sihcon atom. Purely numerical, basis-set-free, calculations [232,235] have been performed to establish Hartree-Fock limits for the MacLaurin expansion coefficients of IIo(p). The effects of electron correlation on IIo(O), and in a few cases IIq(O), have been examined for the helium atom [236], the hydride anion [236], the isoelectronic series of the lithium [237], beryllium [238], and neon [239] atoms, the second-period atoms from boron to fluorine [127], the atoms from helium to neon [240], and the neon and argon atoms [241]. Electron correlation has only moderate effects on IIo(O). 

In a series of papers some thieno fused boron compounds have been subjected to structure determination by X-ray methods. Thus the bis(dithienoborepinyl) ether (26 R = C10H6BS2O) has dimensions close to those of the isoelectronic dithienotropylium ion (74ACS(B)998>. The borepin ring is almost planar, and the B—C bond length was found to be 1.533 A. 

Our group has devoted two articles for the study of systems where the chiral subunits are bonded to a boron or to a metallic atom (Scheme 3.28). In both cases, the weakness of the bonds formed can be easily broken. In the first case, a series of bisdiphenylborates and the effect of the fluoro substitution on the relative stability of the homo vs. heterochiral complexes have been studied by means of DFT calculations (B3LYP/6-31G ) [29]. In addition, the corresponding isoelectronic structures, in which the boron has been substituted by a carbon or a positively charged nitrogen, have been considered. In all the cases, the homo and heterochiral complexes present D2 and S4 symmetries, respectively. 

As later work would show (47, 77), an entire series of polyhedral B Hn2-and isoelectronic C2B 2H carboranes were synthetically accessible for n = 6 through 12. The generally decreased chemical reactivity of these polyhedral species over that of the boron hydrides suggested that they... 

The closo-carboranes of the CrtBn 2H series are isoelectronic with corresponding [B H ]2- ions and have the same closed polyhedral structures with one hydrogen atom bonded to each carbon and boron. 

Carboranes. The accumulation of charge observed in borane anions discussed above may also be afforded by changing some boron atoms by other elements with more valence electrons than boron. The best known examples of this kind of isoelectronic compound are the carboranes. The most important series of carboranes is that with molecular formulas C2B 2Hn, the dicarba-c/oso-boranes, where n is any integer from 5 to 12. These neutral species are isoelectronic with the borane anions BnH . The structures of these compounds are closed deltahedra with terminal hydrogen atoms attached to each carbon or... 

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