Electron-deficient bonds

We describe here a new structure representation which extends the valence bond concept by new bond types that account for multi-haptic and electron-deficient bonds. This representation is called Representation Architecture for Molecular Structures by Electron Systems (RAMSES) it tries to incorporate ideas from Molecular Orbital (MO) Theory [8T]. 

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

Figure 2-60. Soine examples of electron-deficient bonds a) diborane featuring B-H-B bonds b) diborane in a tentative RAMSES representation c) the orbital in a B-B-B bond (which occurs in boron cage compounds),...

Electrophilic Addition. Electrophilic reagents attack the electron-deficient bond of maleic anhydride (25). Typical addition reagents include halogens, hydrohaHc acids, and water. 

Figure 3.11 Schematic representation of the energy levels in various types of 3-centre bond. The B-H-B ( electron deficient ) bond is non-linear, the ( electron excess ) F-Xe-F bond is linear, and the A-H B hydrogen bond can be either linear or non-linear depending on the compound.

Answer Both epoxidatlons involve eleatrophilia attack on the double bond so attack at the electron-deficient bond (c) is unlikely. Preferential attack at (a) can only be steric as (a) and (b) are identical electronically. In polar solvents, (1) probably colls up so... 

But the discovery of the carboranes in the early 1960s revealed that bonding possibilities other than simple o-- or 7r-bonds between B and C centers were necessary to understand the structure of such compounds as 1,5-dicarba-closo-pentaborane(5) [6 in Eq. (1)], which is obtained in low yield in an electrical discharge.6 Ordinary valence conventions cannot account for the bonding of boron to five other atoms, and hence the concept of electron-deficient bonding must be invoked for boron. Although carbon seems to adhere to normal tetravalence, again it should be remembered... 

Before discussing the structural features of tetraorganomagnesiates in the solid state it should be noted that structures in which the presence of separated anionic and cationic moieties can be distinguished are rare. In most cases such units are linked via electron-deficient bonds, i.e. two-electron three-center bridge-bonded carbon atoms between magnesium and the counter cation. 

A number of papers have appeared recently in which semiempirical quantum mechanical methods, such as the complete neglect of differential overlap (CNDO), incomplete neglect of differential overlap (INDO), or Hiickel methods, have been applied to electron-deficient systems in an attempt to calculate their properties (31, 49, 64, 75, 77, 78, 89, 90, 92). Although the quantitative results of these calculations must be treated with great care, they do provide an indication of some of the parameters that determine formation and stability of electron-deficient bonded systems. 

Radii and Electronegativity Values for Atoms Entering into Electron-Deficient Bonding"... 

The structures of several other symmetrical organoaluminum compounds, which have important implications with regard to electron-deficient bonding, have been determined from X-ray data. The first of these (VI) shows the structure of tricyclopropylaluminum dimer (84, 99). In this compound, the A1—C—A1 bridge is symmetrical and the cyclopropyl rings are oriented so that the p orbital on the bridging carbon atom may interact with the appropriate vacant orbitals on the metal atoms. This interaction appears to increase the stability of the bridge bond as indicated from variable-temperature NMR studies. The... 

A further point of interest is that in both the dimeric and trimeric species shown, the beryllium atom still has a vacant orbital available which may be used in adduct formation without disruption of the electron-deficient bond. This type of behavior leads to the formation of dimers with four-coordinate beryllium atoms, e.g., structure XX (86). This structure has been determined in the solid state and shows that the phenylethynyl-bridging group is tipped to the side, but to a much smaller extent than observed in the aluminum derivative (112). One cannot be certain whether the distortion in this case is associated with a it - metal interaction or is simply a result of steric crowding, crystal packing, or the formation of the coordination complexes. Certainly some differences must have occurred since both the Be—Be distance and Be—C—Be angle are substantially increased in this compound relative to those observed in the polymer chain. 

The only magnesium compounds for which structures have been reported that contain electron-deficient bonds are those of dimethyl (123)- and diethylmagnesium (124). The structure common to these two derivatives is XXI, and the data for these compounds along with those from a variety of other organomagnesium derivatives are collected in Table VI for comparison. Clearly, insufficient data are available to draw any specific conclusions concerning the electron-deficient derivatives, but no unexpected deviations appear in the compounds studied. 

Let us now focus our attention on the interaction between lithium alkyls and Group III derivatives. These species are often considered to be metalates with discrete MR4 ions present, but a variety of studies show that substantial metal-anion interactions occur both in solution and in the solid state (45, 96, 131). More thorough examination of both of the structures and spectroscopic properties of these derivatives shows that they must be included in any treatment involving electron-deficient bonding. 

In each of these cases, we might expect some type of electron-deficient bonding, but only limited data are available relating to this problem. 

Examination of the 1 1 mixtures has shown the formation of crystalline LiMgPha (132), LiHg(SiMe3)3 (101), and of Na2Be2H2Et4 (1) and NajjZnaEte (52) in which electron-deficient bond structures have been proposed (28). Unfortunately, none of these have been determined and are, therefore, only speculative. 

XXXVII we also see a bridging group with Al—C distances very close to that observed in other bridged aluminum compounds. The distance between the metal centers in this compound is similar to that observed in the simpler aluminum derivatives but greater than the sum of the covalent radii of the two metals (2.54 A), which may be an indication that Ti—Al interactions do not increase the stability of the bridged system. Structures on Cp2MMe2AlMe2 (M = Y, Er, and Yb) have been recently completed and clearly show stable electron-deficient bonds between the aluminum and the transition metal moiety (XXXVIII) (12). 

Dimethylberyllium, which at room temperature exists as a solid, high polymer having three-center electron-deficient bonds, may enter reversible equilibria with low polymers. 

Alkyllithium compounds have solubility and stability because of their ability to associate to form aggregates of definite structure. Such aggregation confers stability but is not extensive enough to cause insolubility. Methyllithium and n-butyllithium, for instance, exist in a highly associated form, typical of electron-deficient bonding, e.g., (MeLi) and (BuLi)4. 

If the system contains three electrons, the two occupying 4 will be stabilized, and the other one, localized in XV2, destabilized. Here, the stability of the molecule depends upon the relative energies of 4, Tf, and the AOs thus, HHe dissociates spontaneously, but the three-electron bond in He2+ is moderately robust. Note that, in contradiction with Lewis theory, a covalent bond may be formed with one or three electrons. Electron-deficient bonds (where there are fewer than two electrons per bond) are particularly prevalent amongst boron compounds. 

Fig. 37. The structure of HW2(CO)9(NO) as determined by neutron diffraction (Ref. 23a). Note that the axial ligand-metal vectors do not point at the bridging H atom, but at the center of the W-H-W triangle. This observation was taken as evidence that there is significant metal-metal overlap in a M-H-M 3-center-2-electron bond. The structure shown here illustrates one advantage of using metal complexes to study electron-deficient bonding the ML moiety on each metal atom serves as a convenient coordinate system to pinpoint the direction of the orbital used by tungsten to participate in W-H-W overlap...

The electronegativity of the carbon atom is 2.5, which means that the carbon atom cannot easily gain or lose electrons to form an anion or cation. As the number of valence orbitals is exactly equal to the number of valence electrons, the carbon atom cannot easily form a lone pair or electron-deficient bonds. Carbon has a small atomic radius, so its orbitals can overlap effectively with the orbitals of neighbor atoms in a molecule. 

Electronic Imperfection Electrons in solids are considered as imperfections. The electron deficient bond produce by the removal of electron by heating solids above OK, is referred as a hole. Holes give rise to electrical conductivity. The concentration of holes and electrons will be equal in pure crystals. Electrons and holes can be preferentially produced in such covalent crystals by adding impurities. 

The Lewis acidity of organoaluminium compounds is the reason of their association. The association of alkylaluminium molecules and aluminium hydrides proceeds by way of electron-deficient bonds [143a], The associates of dimethylberyllium, dimethyl- and diethylmagnesium, methyl- and ethyl-lithium are of the same type each alkyl group is simultaneously bound to two or three metal atoms [143b],... 

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