Coordination mode and stability of the tetrahydroborate ligand in group 10 metal pincer complexes.

Abstract

The coordination mode of the BH4- ligand in transition metal tetrahydroborate complexes is mainly dominated by the nature of the metal centres. However, other factors can also play important roles sometimes. In order to rationalize the coordination modes and the stability of the BH4- ligand in group 10 metal tetrahydroborate pincer complexes, [2,6-(tBu2PO)2C6H3]Pt(η1-HBH3) and [C6H4-o-(NCH2PtBu2)2B]M(η2-H2BH2) (M = Ni, Pt) were prepared and characterized. A structural comparison of [2,6-(tBu2PCH2)2C6H3]Ni(BH4), [2,6-(tBu2PO)2C6H3]M(BH4) and [C6H4-o-(NCH2PtBu2)2B]M(BH4) (M = Ni, Pd, and Pt) indicates that the M-P bond length, the P-M-P bite angle and the trans-influence of the central atom in the pincer platform also affect the coordination mode of the BH4- ligand. The nickel complexes tend to adopt a monodentate coordination mode while the palladium and platinum complexes can adopt either the monodentate or the bidentate mode depending on the structural features of the pincer platforms. Longer M-P bonds and smaller P-M-P bite angles favour the bidentate mode. The stability of the BH4- ligand is influenced by both the coordination mode and the nature of the metal centre. The BH3 species is released more easily from complexes with less electron rich metal centres. Following the series of Ni, Pd, and Pt, complexes with the same pincer ligand more easily lose a BH3 moiety.