Coordination compounds formed using three-center hydrogen bridge bonds: An extension of the Lewis donor—acceptor coordinate bond

Abstract

In 1959 it was suggested by Parry and Edwards that the concept of a coordinate covalent bond could be extended to include three-center bridge bonds. Although the ▪ bond was soon seen in metal borohydrides and salts of metals with other boron hydride anions, questions about ionic contributions to bonding in these structures rendered questionable the concept of valid coordination to a metal through a [boron-hydrogen-metal] three-center bond. Now complexes have been characterized with formulas of ZnCl 2·L, (CuClL) 2, CuPφ 3LCl, Ni(CO) 2L, Cr(CO) 4L, Mo(CO) 4L, and W(CO) 4L where L is the bidentate, neutral, σ-bonding ligand which can be compared in some sense to ethylenediamine: ▪ ▪ In these compounds, as in some of the boron hydrides such as B 2H 6, the ▪ linkage is clearly and unequivocally the basis for compound stability. Relatively unstable complexes of the formula M(CO) 5[B 2H 4 · 2P(CH 3) 3] have also been characterized where M is Cr and W, and where the [B 2H 4 · 2P(CH 3) 3] is a monodentate ligand. Even less stable monodentate complexes of formula Cr(CO) 5(H 3B · Base) and W(CO) 5(H 3B · Base) have been characterized where the bases fastened to BH 3 are P(CH 3) 3, N(CH 3), and P(C 6H 5) 3. These borane ligands are excellent leaving groups.