Activation of alkynes by diphosphine- and µ-phosphido-spanned heterobimetallic complexes

This review deals with the coordination of alkynes as reactive unsaturated substrates on heterobimetallic transition metal complexes LnM-M′Ln in which two transition metal centers interact through a metal-metal bond spanned by a diphosphane ligand such as bis(diphenylphoshino)methane or a bridging phosphido group of the type μ-PR2. Presented are few examples featuring simple π-coordination of the alkyne on M or M′, but the majority of examples involve bimetallic activation associated with synergetic effects of the two adjacent metal centers giving rise to dimetallatetrahedranes, dimetallacyclobutenes, dimetallated olefins, μ-vinylidenes and acetylides species. Covered are also heterobimetallic systems, in which carbon-carbon or carbon-phosphorus couplings have occurred between two alkyne ligands or with carbonyl groups and P-donor ligands present in the coordination sphere of the heterodinuclear scaffold. Furthermore, hydrometallation reactions leading to vinyl-species through insertion of R-CC-H or R-CC-R into a terminal or bridging M-H bond are presented. Included in this review are also selected examples of reactivity studies and transformation of the initially formed heterobimetallic alkyne species, such as addition of electrophiles (H+, R+), generating heterometallic alkenyl and µ-η1:η2-allenyl salts.