COORDINATION-COMPOUNDS FORMED USING 3-CENTER HYDROGEN BRIDGE BONDS - AN EXTENSION OF THE LEWIS DONOR-ACCEPTOR COORDINATE BOND

In 1959 it was suggested by Parry and Edwards that the concept of a co ordinate covalent bond could be extended to include three-center bridg e bonds. Although the [GRAPHICS] bond was soon seen in metal borohydri des and salts of metals with other boron hydride anions, questions abo ut ionic contributions to bonding in these structures rendered questio nable the concept of valid coordination to a metal through a [boron-hy drogen-metal] three-center bond. Now complexes have been characterized with formulas of ZnCl2 . L, (CuClL)2, CuPphi3LCl, Ni(CO)2L, Cr(CO)4L, Mo(CO)4L, and W(CO)4L where L is the bidentate, neutral, sigma-bondin g ligand which can be compared in some sense to ethylenediamine: [GRAP HICS] In these compounds, as in some of the boron hydrides such as B2H 6, the [GRAPHICS] linkage is clearly and unequivocally the basis for c ompound stability. Relatively unstable complexes of the formula M(CO), [B2H4 . 2P(CH3)3] have also been characterized where M is Cr and W, a nd where the [B2H4 . 2P(CH3)3] is a monodentate ligand. Even less stab le monodentate complexes of formula Cr(CO)5(H3B . Base) and W(CO),(H3B . Base) have been characterized where the bases fastened to BH, are P (CH3)3, N(CH3), and P(C6H5)3. These borane ligands are excellent leavi ng groups.