Molybdenum(VI) Bis(imido) Complexes: From Frustrated Lewis Pairs to Weakly Coordinating Cations

Molybdenum(VI) bis(imido) complexes [Mo(NtBu)(2)(L(R))(2)] (R=H 1 a; R=CF(3) 1 b) combined with B(C(6)F(5))(3) (1 a/B(C(6)F(5))(3), 1 b/B(C(6)F(5))(3)) exhibit a frustrated Lewis pair (FLP) character that can heterolytically split H−H, Si−H and O−H bonds. Cleavage of H(2) and Et(3)SiH affords ion pairs [Mo(NtBu)(NHtBu)(L(R))(2)][HB(C(6)F(5))(3)] (R=H 2 a; R=CF(3) 2 b) composed of a Mo(VI) amido imido cation and a hydridoborate anion, while reaction with H(2)O leads to [Mo(NtBu)(NHtBu)(L(R))(2)][(HO)B(C(6)F(5))(3)] (R=H 3 a; R=CF(3) 3 b). Ion pairs 2 a and 2 b are catalysts for the hydrosilylation of aldehydes with triethylsilane, with 2 b being more active than 2 a. Mechanistic elucidation revealed insertion of the aldehyde into the B−H bond of [HB(C(6)F(5))(3)](−). We were able to isolate and fully characterize, including by single‐crystal X‐ray diffraction analysis, the inserted products Mo(NtBu)(NHtBu)(L(R))(2)][{PhCH(2)O}B(C(6)F(5))(3)] (R=H 4 a; R=CF(3) 4 b). Catalysis occurs at [HB(C(6)F(5))(3)](−) while [Mo(NtBu)(NHtBu)(L(R))(2)](+) (R=H or CF(3)) act as the cationic counterions. However, the striking difference in reactivity gives ample evidence that molybdenum cations behave as weakly coordinating cations (WCC).