A transition metal borylene
complex stabilized by a non-
carbonyl ligand set: formation
by spontaneous halide loss to
give an extremely short metal-
-boron bond (video).
(work by David Addy)
Multiply Bonded Systems
Transition metal complexes containing multiply-bonded ligands from groups 14 and 15 are well known. Thus, for example, metal complexes containing carbene (M=C), carbyne (MºC) or imido (MºN) ligands are now found in undergraduate textbooks. The electronic structures and modes of reactivity of these systems are well understood, and their applications in synthesis have been widely exploited.
By contrast, multiple bonding involving the group 13 elements is a very new, relatively poorly understood and sometimes controversial area. We have been working to develop synthetic routes to compounds offering the potential for multiple bonds between group 13 elements (chiefly B and Ga) and transition metals. A range of techniques (spectroscopic, crystallographic and DFT computation) are used to probe the nature of such bonds, which are found to have similarities (in terms of covalent bonding character) with classical organometallic systems such as Fischer carbenes (LnM=CR2), vinylidenes (LnM=C=CR2) and carbonyls (LnMCO), but to be significantly more polar.
A further goal of this project is the delineation of fundamental patterns of reactivity for these new ligand systems. We have recently demonstrated metathesis chemistry for M=B bonds (which proceeds via a different mechanism to metathesis involving M=C systems), together with the first examples of [4+1] cycloaddition, insertion and hydride transfer reactions for such compounds (see scheme). Ongoing research efforts are exploiting this reactivity in synthetically useful transformations.
Multiply bonded systems
(top, l to r) Crystal structure of a
cationic ruthenium phosphine complex
containing a Ru=B double bond;
DFT-calculated p* orbital for the M=B
double bond; product of insertion
chemistry with a heteroallene.
(bottom) Fundamental modes of
reactivity of M=B double bonds.