?-ß Alkenyl isomerisation at diiron centres

?-Substituted alkenyl complexes [Fe2(CO)6(µ-RC=CH2)(µ-PPh2)] 1a-1e (R = Ph, Me, Prn, Bun or But) have been prepared via regioselective hydrodimetallation of primary alkynes by [Fe2H(CO)7(µ-PPh2)]. Thermolysis in toluene results in isomerisation to the ß-substituted complexes [Fe2(CO)6(µ-HC=CHR)(µ-PPh2)] 2a-2e. Isomerisation is accelerated in the presence of a range of tertiary phosphines yielding [Fe2(CO)5(PR'3)(µ-HC=CHR)(µ -PPh2)] 3a-3j in which the phosphine is coordinated to the ?-bound metal centre and lies trans to the phosphido-bridge. Addition of trimethyl phosphite to [Fe2(CO)6(µ-PhC=CH2)(µ-PPh2)] la at 60-70 °C affords mono- and di-substituted ?-alkenyl complexes [Fe2(CO)5{P(OMe)3}(µ-PhC=CH2)(µ-PPh2)] 4 and [Fe2(CO)4{P(OMe)3}2(µ-PhC =CH2)(µ-PPh2)] 5 respectively. Above 100 °C, conversion into the ß-substituted isomers [Fe2(CO)5{P(OMe)3}(µ-HC=CHPh)(µ-P Ph2)] 6 and [Fe2(CO)4{P(OMe)3}2(µ-HC= CHPh)(µ-PPh2)] 7 occurs cleanly. Crystallographic studies have been carried out on la, [Fe2(CO)6(µ-PrnC=CH2) (µ-PPh2)] 1c, [Fe2(CO)6(µ-HC=CHPh)(µ-PPh2)] 2a, [Fe2(CO)5(PPh3)(µ-HC=CHPh) (µ-PPh2)] 3a, 6 and 7 and compared with related structures. Different structural characteristics are seen between isomers, ß isomers being characterised by a longer Fe?-Cß interaction and a more obtuse Fe?-C?-Cß angle. The mechanisms of alkyne addition to [Fe2H(CO)7(µ-PPh2)] and alkenyl isomerisation have been probed using PhC2D. Hydrodimetallation results in an equal distribution of the deuterium over both ß sites in 1a, and since they do not in exchange on the NMR timescale suggests that a radical process is operating. Thermolysis of this mixture leads to a 3 : 1 mixture of [Fe2(CO)6(µ-DC=CHPh)(µ-PPh2)] 2a-d1? and [Fe2(CO)6(µ-HC=CDPh)(µ-PPh2)] 2a-d1ß. A mechanism for ?-ß alkenyl isomerisation is proposed in which oxidative additions of the trans (exo) and cis (endo) ß-protons to terminal sites on the diiron centre, giving a parallel alkyne intermediate, are in competition. © The Royal Society of Chemistry 2001.