B-X bonds

The broken bonds (boldface = dissociated fragment) BDEs (boldface = recommended data Methods  

The broken bonds (boldface=dissociated fragment) BDEs (boldface= recommended data reference in parentheses) Methods (reference in References 

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Reductive methods form B—B bonds from B—X bonds. For B2X4 (X = Cl, Br, I) from BXj, an electric discharge is supplemented by the presence of a metal, or metal atoms, as halide scavenger. The passage of BX3 at low pressure through a rf discharge in the presence of Hg produces the diboron tetrahalides B2X4 at 300 mg h ... 

In 6.5.2.1 it is shown that BF3 and BCI3 add to phosphine Pt complexes without cleaving the B—X bond. In contrast, aryl and alkyl boron halides are added oxidatively with phosphine Pt complexes - ... 

The addition proceeds through (a) oxidative addition of the B-X bond to a low-va-lent metal (M=Pd, Pt) giving a ds-B-M-X complex (92), (b) migratory insertion of alkene or alkyne into the B-M bond (93 94), and finally (c) reductive elimination... 

Let us suppose that molecular ions of the ABXY molecule may fragment, with the rupture of the B-X bond, or rearrange, with formation of a new bond between A and Y, accompanied by the cleavage of A-B and X-Y bonds. 

Ion abundances are consistent with increasing B-N and decreasing B-X bond strength as X varies from F to I. 

There are some molecules in which the fourth orbital of boron is used for formation of another bond and is not available to permit double-bond character to be assumed by the B—X bonds. In these molecides the bond lengths should approximate the values calculated for B—X single bonds (Table 9-1 column 2). An example is ammonia-boron trifluoride, with the structure... 

For the formation of BX +1, the converse will hold the acceptance of X will be favoured by a strong B-X bond, a large atom B and small X, with little or no increase in promotion energy required to achieve the valence state for the higher coordination number. It will be apparent that for the reaction ... 

Simultaneous complexation of anions and cations, in a similar way to that reported for 15-crown-5 uranyl salene complexes,80 has been observed in the 21-membered ring boronate crown ether 40. Ligand 40 is capable of dissolving stoichiometric amounts of KF in dichloromethane at room temperature to give the KF adduct shown at Figure 13.91 The fluoride ion is bound to the boron atom out of the plane of the crown ether, which contains the K+ ion. Ligand 40 fails to dissolve either KC1 or KBr as a consequence of the weaker nature of the B-X bonds. KI and KSCN are dissolved by 40 but without complexation of the boron atom. The stabilization of the K+ ion by the crown ether moiety is apparently sufficient in these... 

In the reaction of N- organyldipropargylamines with halogenoboranes both C=C triple bonds insert into B—X bonds to form the f 1,2]azaborolo[ 1,2- ][ 1,2]azaborole system by intramolecular coordination (equation 77) (81CB2519). 

The requirement for an initial dissociation step implies that halogen redistribution rates should parallel the ease of dissociation of one of the halogens, and this expectation is confirmed in fluorine - heavier halogen redistributions where the rates change from slow to very fast over the series Cl, Br, I as the B—X bond becomes weaker. Rates should also be related to stabilization of the residual boron trihalide formed on dissociation. The boron trihalide with the greatest number... 

Generally, cation-anion bonding is soft for large cations and anions. In other words, the coefficients of thermal expansion and compressibility are both larger for the A X bond than for the B-X bond. Thus, not only temperature but also pressure should be considered to affect the bond length matching to be estimated in terms of tolerance factor. 

This is opposite to the order suggested by the relative electronegativities of the halogens the inductive eflect is evidently outweighed by another—a tendency for electrons from the smaller halogen atoms to be partly back-donated to the boron, to give some double-bond character to the B-X bonds and to reduce the electron deficiency on the boron. 

Ketones and aldehydes insert into B-X bonds, and the products vary with the substituents on the substrateU Thus, with BX3 either mono-, bis-, or tn s-inserted products may be obtained in which B-0 bonds are formed. Electron-withdrawing substituents favor insertion. Reactions with ketenes lead to insertion into both the C=C and C=0 bonds . 1,3-Addition of PhBCU occurs with a-diazoethylacetates to form PhClBOC(OEt)=CRCl under mild conditions. Cyclic perfluoroketones react with... 

Boron halides react with isocyanates and isothiocyanates forming insertion products with -N(R)-CEC1, (E = O, S), unitsE Insertions into the B-X bond by nitriles and isonitriies occur and for o-aminophenylnitrile, two products are formed, the expected 1,2-insertion product and a ring-closed insertion product ... 

Insertions and (3-eliminations are also the microscopic reverse of each other. In an insertion, an A=B 77 bond inserts into an M-X bond (M-X + A=B —> M-A-B-X). The M-X and A=B bonds are broken, and M-A and B-X bonds are formed. Insertion is usually preceded by coordination of the A=B 77 bond to the metal, so it is sometimes called migratory insertion. In an insertion, an M-X bond is replaced with an M-A bond, so there is no change in oxidation state, d electron count, or total electron count. However, a new a bond is formed at the expense of a 77 bond. The nature of the reaction requires that the new C-M and C-H bonds form to the same face of the A=B 77 bond, resulting in syn addition. The reaction of a borane (R2BH) with an alkene to give an alkylborane is a typical insertion reaction that you have probably seen before. 

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