Metal-boron bonds

Some group-IB and -IIB boranes or carboranes, especially of Cu, possess M — H—B structure units and are not discussed here because no direct metal-boron bond exists. 

A series of Rh and Ir complexes is also basic enough to add boron halides, although the metal-boron bonds are weak, e.g., dppe and PPh3-Rh complexes serve as electron-pair bases when they react with BF3 and BCI3 . Table 1 summarizes the results. 

Dihydro-l,2,5-thiadiboroles (Et2C2B2R2S) have remarkable acceptor properties toward metal-ligand moieties. Dihydrothiadiborole-derived complexes can be achieved thermally or photochemically, e.g., Co and Fe sandwich, triple-decked and even tetradecked complexes are known, all having direct metal-boron bonds. 

Boron resonances (Table III) appear in the range expected for sandwich-type complexes with metal-boron bonding (for reference data see Ref. 50). In cases where pairs of B-methyl and B-phenyl derivatives (as, e.g., 9 and 17) are known, the nB resonance of the methyl compound is at lower field by 2.0-3.7 ppm (average, 2.6 ppm). 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

Compounds containing transition metal-boron bonds are produced by the reaction of 2-CIB5H8 with NaM(CO)s when M = Re and Mn 190>. These compounds are formulated as 2-[(CO)5M]BsH8 on the basis of infra-red and nmr studies 190>. 

Protic reagents such as primary amines, alcohols, and water led to the corresponding substituted borylene complexes [n-BX (C5H4Me) Mn(CO)2 2] (13, X = NHtBu 14, X = NHPh 15, X = OMe 16, X = OEt 17, X = Oz Pr 18, X = OH) in high yields of up to 94% (Fig. 3).100 102 It should be noted that due to the kinetic lability of the metal-boron bond such reactions at the boron center with retention of the M-B linkage are very rare for both boryl- and borylene complexes and were observed subsequently by Roper only in the case of one particular boryl complex.103... 

The nature of the transition metal-boron bond in [LXM = B-R] is obviously strongly influenced by the boron bonded substituent R. Descriptions based on calculations range from an almost purely... 

RB —> TM o-bonded model when R is a good 71-donor up to a strong TM —> BR 71-back donation when R is a weaker 71-donor.132 134 However, the most recent studies made the point that the sum of the covalent interactions gives a bond order less than unity, and argued the case for a mainly ionic transition metal-boron bond.133 135... 

All of these compounds are characterized in solution by their UB-NMR signals ranging from S= 50 to 5= 70 which are distinctly lowfield shifted with respect to the signals of the starting boranes. The iron compound 4a shall serve as a typical example for all structurally characterized compounds 4 - 8 to discuss the nature of the metal boron bond (Figure 5). The molecule adopts Ci symmetry in the crystal which is due to an almost... 

The chemistry of metalloborane compounds is vast, although most are not prepared by oxidative addition of the B—H bond to a metal complex neither do they contain metal-hydrogen bonds. The synthesis of metalloborane hydrides involves many routes, the mechanisms of which are obscure. Reactions between borohydrides and transition-metal complexes involve hydride transfer without the formation of a metal-boron bond (see 1.10.7). 

Borane Anions and Metallo-derivatives.—Greenwood and Ward have reviewed the synthesis and structures of metalloboranes, with a discussion of metal-boron bonding.29... 

Organoboron-Co compounds are precursors to metal-boron bonds as established for Sn (see 5.3.17.3.1) and provide a RT route to otherwise inaccessible organoboron-Pb compounds ... 

Due to the formation of two metal-boron bonds, the compounds 23a,b-26a,b show characteristically deshielded nB-NMR signals in solution. In the case of aminoborylene complexes, resonances in the range of S = 100-120 are observed. For the tert-butylborylene complex 26a an even more deshielded signal is found at 8= 170.0. The observed CO stretching frequencies of 23a,b-26a,b are comparable to those of the related /x-methylene complexes [48-50]... 

Finally, we note that the chapter ordering in the current Volume places borylene complexes before boryl complexes. We have ordered the chapters according to the degree of metal-boron bonding, from highest to lowest, rather than following a historical perspective. 

Noth and co-workershave considered the B shifts of several metal derivatives of bis(dimethlamino)boranes these shifts are, for convenience of comparison, shown in Table XIV. Noth and Schmid have discussed co-ordination compounds containing metal-boron bonds and the B shifts are also reproduced in Table XIV. 

---