Boron 2p orbital

The heightened reactivity of these neutral boraethenes undoubtedly stems from the available boron 2p-orbital and the greater electronegativity... 

Another effect which should be considered is the relativistic shrinkage of the ns valence shell of the metal. This can contribute to the valence repulsion (making it weaker), although, on the other hand, this may also reduce the overlap between the boron 2p orbital and the ns orbital of the... 

The new results in Figure 1.24 show, at least, that it is possible to generate different numerical radial wave functions for the boron 2p orbital [and, of course, any other orbital obtained by approximate solution of the Schrodinger equation]. But, for a light atom like boron, it is unlikely that this is the major reason for any discrepancy. 

Figure 4-4 Overlap of the boron 2p orbital ivith the 2p, orbitals of the fluorine atoms.

Figure i- Two combinatioDS of the fluorine 2pg orbitals that have zero overlap with the boron 2p< orbital. 

Figure 9.3 The bonding in the boron trihalides. The three two-electron bonds in the xy plane use a combination of the boron 2s, and 2p orbitals. This leaves an empty boron 2p orbital above and below the plane, as in (a). Because the molecule is planar, this overlaps with filled p non-bonding orbitals on the halogen atoms, resulting in partial delocalization of halogen non-bonding electrons into the boron 2p orbital. In other words, the B—X bonds are strengthened by n bonding via a n orbital extending over all four atoms, as in (b).

The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

Step 1 A molecule of borane (BH3) attacks the alkene Electrons flow from the 7C orbital of the alkene to the 2p orbital of boron A 7C complex is formed... 

The electrophilic character of boron is again evident when we consider the oxida tion of organoboranes In the oxidation phase of the hydroboration-oxidation sequence as presented m Figure 6 11 the conjugate base of hydrogen peroxide attacks boron Hydroperoxide ion is formed m an acid-base reaction m step 1 and attacks boron m step 2 The empty 2p orbital of boron makes it electrophilic and permits nucleophilic reagents such as HOO to add to it... 

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 boron atom in BFa uses the 2s and two 2p orbitals in bonding. Therefore the bonding is... 

Boron (Z = 5) has five electrons. Two enter the ls-orbital and complete the n = 1 shell. Two enter the 2s-orbital. The fifth electron occupies an orbital of the next available subshell, which Fig. 1.41 shows is a 2p-orbital. This arrangement of electrons is reported as the configuration 1 s22s22p1 or He 2s22/ 1 (5), showing that boron has three valence electrons. 

Because of the necessity of spatially appropriate overlap of the 2p. .-orbital on boron with the adjacent jp3-hybridized carbon orbital (12) or with the tt-MO of the double bond (13), any such ir-bonding should be most sensitive to molecular conformation. Hence, the occurrence of such 7r-bonding might be more evident in certain highly rigid cyclic structures, such as 14 and 15. 

The strength of a Lewis acid is a measure of its ability to attract a pair of electrons on a molecule that is behaving as a Lewis base. Fluorine is more electronegative than chlorine, so it appears that three fluorine atoms should withdraw electron density from the boron atom, leaving it more positive. This would also happen to some extent when the peripheral atoms are chlorine, but chlorine is less electronegative than fluorine. On this basis, we would expect BF3 to be a stronger Lewis acid. However, in the BF3 molecule, the boron atom uses sp2 hybrid orbitals, which leaves one empty 2p orbital that is perpendicular to the plane of the molecule. The fluorine atoms have filled 2p orbitals that can overlap with the empty 2p orbital on the boron atom to give some double bond character to the B-F bonds. 

D) BF3 is a trigonal-planar molecule because electrons can be found in only three places in the valence shell of the boron atom. As a result, the boron atom is sp hybridized, which leaves an empty 2p orbital on the boron atom. BF3 can therefore act as an electron-pair acceptor, or Fewis acid. It can use the empty 2p orbital to pick up a pair of nonbonding electrons from a Fewis base to form a covalent bond. BF3 therefore reacts with Lewis bases such as NH3 to form acid-base complexes in which all of the atoms have a filled shell of valence electrons. 

In the MO treatment of the D3h BF3 molecule the 2p orbital of the boron atom transforms as an a/" representation, as does one of the linear combinations of fluorine 2p, group orbitals (all F-F n bonding). The MO diagram is shown in Figure 6.6. 

The characters in the last line are those corresponding to the e representation. That representation is also that of the two 2p orbitals of the boron atom which lie in the plane. 

A boron atom, B (atomic number 5), in its lowest energy state has four of its five electrons filling the Is and 2s orbitals. Its fifth electron may reside in any one of the 2p orbitals, all of which are at the same energy level ... 

A carbon atom, C (atomic number 6), has six electrons. Five of them occupy the Is, 2s, and 2p orbitals just as the electrons in boron do. Carbon s sixth electron, however, has a choice of either pairing up with the fifth electron in the same 2p orbital or entering a 2p orbital of its own ... 

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