Synthesis, Structure, Bonding, and Reactivity of Metal Complexes Comprising Diborane(4) and Diborene(2): [{Cp*Mo(CO)2}2{μ-η2:η2-B2H4}] and [{Cp*M(CO)2}2B2H2M(CO)4], M=Mo,W

The reaction of [(Cp*Mo)(2)(mu-Cl)(2)B2H6] (1) with CO at room temperature led to the formation of the highly fluxional species [{Cp*Mo(CO)(2)}(2){mu-eta(2):eta(2)-B2H4}] (2). Compound 2, to the best of our knowledge, is the first example of a bimetallic diborane(4) conforming to a singly bridged C-s structure. Theoretical studies show that 2 mimics the Cotton dimolybdenum-alkyne complex [{CpMo(CO)(2)}(2)C2H2]. 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) angstrom. We isolated the tungsten analogues of 3, [{Cp*W(CO)(2)}(2)B2H2W(CO)(4)] (4) and [{Cp*W(CO)(2)}(2)B2H2Mo(CO)(4)] (5), which provided direct proof of the existence of the tungsten analogue of 2.