Boron isoelectronic complex

In order to estimate and compare the magnitude of the M-B interactions in these isoelectronic complexes, a whole set of structural and spectroscopic parameters determined experimentally and/or computed theoretically were considered. This includes the M - B distance the ratio r between the M -B distance and the sum of covalent radii (to take into account the different sizes of the metals involved), the pyramidaliza-tion of the boron environment XB, the rlB NMR chemical shift <5 11B, the difference AqB between the charge at boron in the metal boratrane and the free ligand TPB, the difference A M between the charge at the metal in the metal boratrane and that in the related borane-free complex [M(i-Pr2PPh)3], and the NBO delocalization energy A NBo associated with the main donor-acceptor M-B interaction found at the second order in the NBO analysis (Table 2). Only the conclusions of this detailed analysis will be recalled here ... 

Although beyond the main focus of this chapter, it is interesting to compare some of the properties of the cationic complexes to isovalent or isoelectronic neutral analogues, such as boron or aluminum pentahydride. Both BHj and AIH3 are electron deficient, and their stable forms are and AUH,. But how much do... 

Replacement of two carbon atoms in nickelocene by boron leads to complex 26 having the ir6/d8 arrangement to which cluster formalism can be applied. The diamagnetic sandwich 26 is isoelectronic with ferrocene... 

While the mercury compound shows localized N—Hg bonds, the TADB-system (which is isoelectronic with cyclopentadiene) forms a low-spin complex with iron. This was the first identified example of a sandwich-compounds of a boron-nitrogen ring. The complex is prepared by adding an etheric solution of TADBLi to a suspension of FeCl2 in tetra-hydrofuran. 

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. 

Fig. 16. Structures of [(tj5-Cp)V]2(t 6-C6H6) (43) (116), [(ij5-Cp )Mo]2(T/6-p6) (44) (117), and [(j 5-Cp)Mo]2(rj4-As5) (45) (119a) from X-ray parameters, showing triple-decker complexes containing isoelectronic units CH, P, and As. Similar complexes containing boron have been prepared by Siebert and colleagues (119b,c).

The use of the boratabenzene heterocycle as a ligand for transition metal complexes dates back to 1970 with the synthesis of (C H5B-Ph)CpCo+ (1) (Cp = cyclopentadienyl).1 Since boratabenzene and Cp are 6 it electron donors, 1 can be considered isoelectronic to cobaltocenium. Many other transition metal compounds have been prepared that take advantage of the relationship between Cp and boratabenzene.2 In 1996, the synthesis of bis(diisopropylaminoboratabenzene)zirconium dichloride (CsHsB-NPr ZrCh (2) was reported Of particular interest is that 2 can be activated with methylaluminoxane (MAO) to produce ethylene polymerization catalysts with activities similar to those characteristic of group 4 metallocenes.4 Subsequent efforts showed that, under similar reaction conditions, (CsHjB-Ph ZrCh/MAO (3/MAO) gave predominantly 2-alkyl-1-alkenes5 while (CsHsB-OEt ZrCh/MAO (4/MAO) produced exclusively 1-alkenes.6 Therefore, as shown in Scheme 1, it is possible to modulate the specificity of the catalytic species by choice of the exocyclic group on boron. 

Boron s strong tendency to 4 covalence is shown in its complexes. The BF - ion, like the isoelectronic BeF42- ion and CF4 molecule, is tetrahedral. The borohydride ion is also tetrahedral, the boron atom again being in the sp valence state. Complex oxo-anions containing 4-covalent boron are formed by the borate ion with some cis-diols ... 

Bis(l,2-azaborolyl)iron sandwich complexes, like the isoelectronic ferrocene compounds, are thermodynamically very stable, but, owing to the presence of the boron and nitrogen atoms, they are in any case sensitive to acids and bases. 

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