Tetrahydroborate complexes/ligands

A large number of covalent tetrahydroborate complexes with first-row transition metals has been described. The most common coordination mode of the tetrahydroborate ligand is q2, being known as at least one example of q2 - BH4 for all the transition metals of this row. Although less abundant, several q1 and q3 complexes have been characterised experimentally, though only for a limited number of metals. In particular, most of the monodentate complexes are Cu compounds and most of the tridentate are Ti compounds. 

Numerous examples of mono(tetrahydroborate) complexes illustrate this simple relationship between the BH4" coordination mode and the 18-electron rule. A nice case can be found in the series of Cu-borohydride complexes. The tetrahydroborate ligand is bound in an r]1 fashion in the Cu(BH4)(PMePh2)3 complex, in which the total number of electrons is 10(Cu) + 3 x 2(phosphines) + 2(BH4 ). The loss of a phosphine group in Cu(BH4)(PPh3)2 entails a change in the coordination mode of BFU" 1 2) in order to keep the 18-electron count. 

Zirconium(IV) and hafnium(IV) tetrakis(tetrahydroborates) M(BH4)4 are of interest as extremely volatile, covalent complexes that contain tridentate BH ligands and exhibit rapid intramolecular exchange of bridging and terminal hydrogen atoms. 4,615 These compounds were prepared initially from NaMF5 (M = Zr or Hf) and excess A1(BH4)3 (equation 53),616 but they are obtained more conveniently from the reaction of the anhydrous metal tetrachloride with excess lithium tetrahydroborate (equation 54), either in the solid state617,618 or in the presence of a small amount of diethyl ether.619... 

A series of diheteronuclear complexes was isolated (3.218) on the basis of metal bis-phospholyldichlorides (tetrahydroborates) [486], They are bidentate P-donor chelating ligands and form metal-cyclic structures. The chelates of the type 772 [486] are typical examples of such compounds ... 

The preparations described here are developed from published work by Malatesta et al.5 and from more recent studies in the contributors own laboratory.2 The cobalt and nickel complexes are prepared by reduction of the corresponding metal nitrates with sodium tetrahydroborate in the presence of excess ligand, whereas the syntheses of the rhodium and platinum complexes involve simple ligand exchange processes. The preparative routes are suitable for use with triphenyl- or p-substituted triphenyl phosphites reactions involving o- or m-substituted triphenyl phosphites give much reduced yields of products which are difficult to crystallize and are very air-sensitive. These features probably reflect the unfavorable stereochemistry of the o- and m-substituted ligands. 

In tetrahydroborate and borane complexes the effective ligand is hydrogen so we deal with these small sections here. 

There are exceptions involving coordination of the tetrahydroborate ligand in certain transition-metal complexes. See, for example, Wink, D. J., and Cooper, N. J., J. Chem. Soc., Dalton Trans., 1257 (1984). 

Treatment of 5-thioxo-4,5-dihydro-3,4-dimethyl-l,2,4-triazole with sodium tetrahydroborate at 210 °C provides the new N3S3-donor tripod ligand Tt as its sodium complex salt [Na(H20)6] [Na(Tt)2]. Bailey also describes some Tt bismuth(III), tin(IV) and manganese complexes, which illustrated the flexible Tt offering a combination of the properties ot the Tp and Tm ligands.163... 

Tris(tri-o-tolyl phosphite)nickel(O),4 likewise a useful catalyst,5 is conveniently prepared by displacement of ethylene from the olefin complex by a mole of phosphite ligand as described in Sec. B. Alternatively, it can be prepared directly by reduction of hydrated nickel nitrate with sodium tetrahydroborate in acetonitrile (Sec. C). This method avoids the need to handle spontaneously flammable triethylaluminum. 

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