Hydroboration reactions of Fischer carbyne metal complexes [M(
CR)X(CO)2] (X = η-C5R5 or tpb′; tpb′ = tris(3,5-dimethylpyrazolyl)hydroborate) were studied. Depending on R and X two types of hydroboration products were formed, namely the α-boryl-η3-benzyl metal complexes [M(C5R5)(CO)2{(BEt2)C(H)(C6H4Me-4)}] (M = W, R5 = Me4Et; M = Mo, R = Me) and the novel borylmetal complexes [M(tpb′)(CO)2{η2-B(R′)CH2R}] (M = W, R′ = Et, R = C6H4Me-4 or Me; M = Mo, R′ = Et, R = C6H4Me-4; M = W, R′ = Ph, R = C6H4Me-4 or Me). β-Agostic CHM interactions are present between the metal and the boryl ligands. Labelling experiments involving “DBEt2” showed selective transfer of deuterium to the former carbyne carbon, with only deuterium participating in the agostic interaction. From [W(
CR)(C5Me5)(CO)2] 2 (R = Me or SiPh3) and “HBEt2” the complex trans-[W(C5Me5)(CO)2(H)(C2H4)] was obtained as the sole organometallic compound. Deuterium labelling experiments confirmed transfer of hydrogen from the borane to the carbyne complex. In polar solvents, [W(η-C5Me5)(CO)2{(BEt2)C(H)C6H4Me-4}] and the η-C5Me4Et analogues were at least partially (30–100%) converted into [W(η-C5R′5)(CO)2(H)(C2H4)] (R′ = Me, R′5 = Me4Et) and unknown boron containing products. From an acetone solution of the α-borylbenzyl η-C5Me4Et complex crystals of a new product were isolated. Its unusual structure results from disruption of the boryl group, incorporation of two molecules of acetone to give a 1,3-dioxa-2-boracyclohexane ring system, conversion of the benzyl ligand into an η4-2-methyl-5-exo-methylenehexa-1,3-diene ligand and the formation of an acetyl group on the tungsten.