Reactions of the metallabromoalkane [Mo{(CH2)3Br}(CO)3(η-C5H5)] and the carbene complexes trans-[MoI{C(CH2)3O}(CO)2(η-C5H4R)](R = H or Me) with Li(BEt3H). The synthesis (R = H or Me) and X-ray structure (R = Me) of [Mo{CH(CH2)3O}(CO)2(η-C5H4R)] containing an unusual oxygen–molybdenum donor interaction