Boron atomic configuration

This compound, which contains atoms arranged tetrahedrally around the boron atom, can readily be isolated from a mixture of dimethyl ether and boron trichloride. On occasions a chlorine atom, in spite of its high election affinity, will donate an electron pair, an example being found in the dimerisation of gaseous monomeric aluminium chloride to give the more stable Al2Clg in which each aluminium has a tetrahedral configuration ... 

The boron atom in boron trifluoride is hybridized to the sp planar configuration and consequently is coordinatively unsaturated, ie, a Lewis acid. Its chemistry centers around satisfying this unsaturation by the formation with Lewis bases of adducts that are nearly tetrahedral sp [ The electrophilic properties (acid strengths) of the trihaloboranes have been found to increase in the order BF < BCl < BBr < BI (3,4). 

To show how orbital diagrams are obtained from electron configurations, consider the boron atom (Z = 5). Its electron configuration is ls22s22p1. The pair of electrons in the Is orbital must have opposed spins (+j, or f j). The same is true of the two electrons in the 2s orbital. There are three orbitals in the 2p sublevel. The single 2p electron in boron could be in any one of these orbitals. Its spin could be either up or down. The orbital diagram is ordinarily written... 

There is no more room in the 2s orbital for a fifth electron, which appears when we move on to the boron atom. However, another orbital with principal quantum number 2 is available. A 2p orbital accepts the fifth electron, giving the configuration Is ls-lfi. Continuing this process, we obtain the following configurations ... 

The electron configuration (41) is somewhat higher in energy than (40). It is necessary to promote a 2s electron to the 2p state to obtain (41). In return, however, the boron atom gains bonding capacity. Whereas a boron atom can form only one covalent bond in configuration... 

Glembotskii, I. I., Kibartas, V. V., and Iutsis, A. P., Soviet Phys. 2, 476, Fock self-consistent field for the neutral boron atom in the two configuration approximation."... 

Similar to the four- and five-coordinate complexes 120-126, for RCo (dioxime-BR2)2L 127 and Fe(dioxime-BR2)LL 128 different conformations are possible in solution and in the solid state, in which the substituents of the boron atoms may adopt cis- or trans-configurations and in which the alkyl group R may have a parallel or an antiparallel orientation with respect to the BR2 substituents [173-180]. 

Figure 11.1 The hydroboration-oxidation of 1-methylcyclopentene. The first reaction is a syn addition of borane. (In this illustration we have shown the boron and hydrogen both entering from the bottom side of 1-methylcyclopentene. The reaction also takes place from the top side at an equal rate to produce the enantiomer.) In the second reaction the boron atom is replaced by a hydroxyl group with retention of configuration. The product is a trans compound (trans-2-methyl-cyclopentanol), and the overall result is the syn addition of -H and -OH.

Although the boron atom (with electron configuration Is2 2s2 2p ) has three valence electrons, only one of them is unpaired in the ground state. 

Electron configurations, as they are used in this book, provide information about the first two quantum numbers, n and 1. (Electron configurations may also reflect the third quantum number, mi, but this notation goes beyond the scope of this chemistry course.) The electron configuration below represents a boron atom in its ground state. 

The case of boron as a network former cation is somewhat specific in that this element has no available d orbitals. However, a p orbital is available when the boron has a coordination number of 3, which allows stabilisation of an electronic doublet of the oxygen or sulphur introduced by the modifier. This oxygen or sulphur giving up a doublet to another boron atom increases the cross-linking by the formation of two BO4 tetrahedra. In hybridisation terms, the boron is altered from the sp configuration to the sp configuration. The coordination change of boron has been especially well observed by NMR (Bray and O Keefe, 1963 Muller-Warmuth and Eckert, 1982). 

Reaction of borabenzene with Li2 gives structure (116) in which the tetracoordinate boron atom has a planar bond configuration [MNDO (90ZOR210)]. A structure with the tetrahedral bond configuration has an energy higher by 9.6 kcal/mol compared to structure (116) and represents a transition state in the topomerization of the latter. 

If the molecule is rotated around the z axis by 120° (360°/3), an equivalent configuration of the molecule is produced. The boron atom does not change its position, and the fluorine atoms exchange places depending upon the direction of the rotation. The rotation described is the symmetry operation associated with the C3 axis of symmetry, and the demonstration of its production of an equivalent configuration of the BF3 molecule is what is required to indicate that the C3 proper axis of symmetry is possessed by that molecule. 

There are other proper axes of symmetry possessed by the BF3 molecule. The three lines joining the boron and fluorine nuclei are all contained by C2 axes (from hereon the term proper is dropped, unless it is absolutely necessary to remove possible confusion) as may be seen from Figure 2.2. The associated symmetry operation of rotating the molecule around one of the C2 axes by 360°/2 = 180° produces an equivalent configuration of the molecule. The boron atom and one of the fluorine atoms do not move whilst the other two fluorine atoms exchange places. There are, then, three C2 axes of symmetry possessed by the BF3 molecule. 

It was then discovered that the configurations of the molecules correspond to an increase in ligancy of the boron atoms to 5 or 6 and of some of the hydrogen atoms to 2. These configurations provide strong support of the suggestion made by Lewis77 that the electron pairs reso-... 

Trimethylbortme Dih).2 We may assume that the methyl groups will have their usual configuration found in organic compounds. The Lewis structure of (CHj)jB will place six electrons in the valence shell of the boron atom, and in order that the-electron pairs be as fair apart as possible, the methyl groups should be located at the comers of an equilateral triangle. This results in sp2, or trigonal Ur), hybridization for the bcron atom (Fig. 6.1b). 

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