The metallacyclopentanes [[graphic omitted]H₂}(η⁵-C₅Me₅)(PPh₃)][(1b) M = Rh, R = Me, R′= H; (1c) M = Rh, R = R′= Me; (1e) M = Ir, R = Me, R′= H; (1f) M = Ir, R = R′= Me; (1g) M = Ir, R = Prⁱ, R′= H] were prepared by reacting the appropriate alkylating reagent M′[CH₂CH(R)CH(R′)CH₂]M′(M′= Li or MgCl) with the dichlorides of formula [MCl₂(η⁵-C₅Me₅)(PPh₃)]. Complexes (1) were characterized by elemental analysis, ¹H n.m.r., and mass spectrometry. The X-ray crystal structures of (1c), (1e), and (1f) have been determined. (1c) and (1f) give isostructural P2₁/c crystals, while (1e) crystallizes in the space group C2/c. The cell constants are: (1c), a= 14.982(9), b= 11.058(6), c= 18.421(10)Å, β= 101.13(2)°, Z= 4, R(3 964 reflections, 493 parameters)= 0.0443; (1e), a= 33.63(4), b= 9.29(1), c= 20.56(2)Å, β= 116.6(2)°, Z= 8, R(3 196, 476)= 0.0472; (1f), a= 15.01(2), b= 11.10(1), c= 18.44(2)Å, β= 100.90(4)°, Z= 4, R(4 136, 446)= 0.0334. All compounds possess a structure of the ‘three-legged piano stool’ type, the major differences lying in the puckering of the metallacyclopentane rings which is larger in the dimethylated derivatives (1c) and (1f) than it is in (1e), where the metallacyclic moiety is rather flattened. The endocyclic C_β–C_β, bond in (1e) is 1.34(3)Å, while it is 1.51(1) and 1.52(1)Å in (1c) and (1f), respectively. The rhoda- and irida-cyclopentanes (1a)(M = Rh, R = R′= H), (1b), (1c), (1d)(M = Ir, R = R′= H), (1e), and (1f) react under very mild conditions, with BF₃·Buⁿ₂O and with [CPh₃][PF₆] to give the η³-allyl complexes [M{η³-CH₂C(R)C(R′)Me}(η⁵-C₅Me₅)(PPh₃)][BF₄](2) and [M{η³-CH₂C(R)C(R′)Me}(η⁵-C₅Me₅)(PPh₃)][PF₆](3)[(a) M = Rh, R = R′= H; (b) M = Rh, R = Me, R′= H; (c) M = Rh, R = R′= Me; (d) M = Ir, R = R′= H; (e) M = Ir, R = Me, R′= H; (f) M = Ir, R = R′= Me], respectively, via the regiospecific hydrogen abstraction from the alkyl-substituted β-carbon atom by Lewis acids. Complexes (2) and (3) were characterized by elemental analysis, ¹H n.m.r., and fast atom bombardment mass spectrometry.