Synthesis, Structure, Bonding, and Reactivity of Metal Complexes Comprising Diborane(4) and Diborene(2): [{Cp*Mo(CO)<inf>2</inf>}<inf>2</inf>{μ-η²:η²-B<inf>2</inf>H<inf>4</inf>}] and [{Cp*M(CO)<inf>2</inf>}<inf>2</inf>B<inf>2</inf>H<inf>2</inf>M(CO)<inf>4</inf>], M=Mo,W

The reaction of [(Cp*Mo)2(μ-Cl)2B2H6] (1) with CO at room temperature led to the formation of the highly fluxional species [{Cp*Mo(CO)2}2{μ-η2:η2-B2H4}] (2). Compound 2, to the best of our knowledge, is the first example of a bimetallic diborane(4) conforming to a singly bridged Cs structure. Theoretical studies show that 2 mimics the Cotton dimolybdenum–alkyne complex [{CpMo(CO)2}2C2H2]. In an attempt to replace two hydrogen atoms of diborane(4) in 2 with a 2e [W(CO)4] fragment, [{Cp*Mo(CO)2}2 B2H2W(CO)4] (3) was isolated upon treatment with [W(CO)5⋅thf]. Compound 3 shows the intriguing presence of [B2H2] with a short B−B length of 1.624(4) Å. We isolated the tungsten analogues of 3, [{Cp*W(CO)2}2B2H2W(CO)4] (4) and [{Cp*W(CO)2}2B2H2Mo(CO)4] (5), which provided direct proof of the existence of the tungsten analogue of 2.