| Achoimre: | The phosphido-bridged diiron allenyl complex [Fe2(CO)6(μ-PPh2){μi-η 1:ηαβ2-(H)C α=Cβ=CγH2}] (1) reacts with dppm to afford, in the first instance, [Fe2(CO)6(μ-PPh2){η 1(P):η2(C)-Ph2PCH2-PPh 2(H)C=C=CH2}] (3), which contains a novel η1(P):η2(C)-dppm-functionalized allene. A single-crystal X-ray study reveals 3 to be derived from 1 by nucleophilic attack of dppm at Cα with metal-metal bond cleavage and internuclear migration of CO. Upon standing in toluene [Fe2(CO)6(μ-PPh2){η 1(P):η2(C)-Ph2PCH2PPh 2(H)C=C=CH2}] (3) slowly decarbonylates to afford the unusual σ-π-alkenyl complex [Fe2(CO)5(μ-PPh2){μ-η 1(P):η1(C):η2(C)-Ph 2PCHPPh2-(H)C=CCH3}] (4), containing a metal- and carbon-coordinated bis(diphenylphosphino)-methanide ligand. Overall, the transformation of 3 into 4 requires activation of a dppm methylene C-H bond, hydrogen migration to Cγ together with metal - metal bond formation, and loss of CO. Surprisingly, [Fe2(CO)6(μ;-StBu){μ-η 1:ηα,β2-(H)C α=Cβ=CγH2}] and dppm react to afford high yields of the isomeric alkenyl complexes [Fe2(CO)5(μ-StBu){μ-η 1(P):η1(C):η2(C)-Ph 2PCHPPh2(H)C=CCH3}] (5a; 90%) and [Fe2(CO)5(μ-StBu){μ-η 1(P):η1(C):η2(C)-Ph 2PCHPPh2-CH2C=CH2}] (5b; 10%), the former of which is a structural analogue of [Fe2(CO)5(μ-PPh2){μ-η 1(P):η1(C):η2(C)-Ph 2PCHPPh2(H)C=CCH3}] (4). We tentatively suggest that 5a and 5b form via C-H activation and hydrogen migration in an η2-allene intermediate similar to 3 with the final isomeric distribution depending upon the regiochemistry of the hydrogen migration step, namely to Cα or Cγ. Full structural details of 3, 4, 5a, and 5b are reported.
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