Structure of (2,2'-Biallylene)hexacarbonyldiiron

Diverse bonding modes of the tetramethyleneethane ligand in binuclear iron carbonyl derivatives

“…The global minimum 6S-1 is predicted to be the C 2 singlet structure cis-(Z 3 ,Z 3 -TME)Fe 2 (CO) 6 , which has been synthesized and structurally characterized by X-ray crystallography. 2 In 6S-1 each Fe(CO) 3 moiety is coordinated to an Z 3 -allylic subunit of the TME ligand. The predicted Fe-C distances to the Z 3 -allylic subunits by either the B3LYP or BP86 methods fall within 0.02 Å of the experimental values (Table 2).…”

“…This rotation converts the C 2 global minimum structure 6S-1 to an equivalent C 2 structure through this intermediate C 2v transition state, consistent with experimental variable-temperature NMR studies. 2 The barrier for this conversion is estimated at only 0.3 kcal mol À1 (B3LYP) or 0.5 kcal mol À1 (BP86).…”

“…This is consistent with the observation that reaction of allene with Fe 2 (CO) 9 , even under relatively mild conditions, gives (TME)Fe 2 (CO) 6 rather than (TME) 2 Fe 2 (CO) 7 . 1,2 Note that (TME)-Fe 2 (CO) 7 is thermodynamically viable towards disproportionation into (TME)Fe 2 (CO) 8 + (TME)Fe 2 (CO) 6 , since this process is endothermic, requiring B11 kcal mol À1 (Table 5). However, (TME)Fe 2 (CO) 6 is clearly much more thermodynamically viable towards disproportionation into (TME)Fe 2 (CO) 7 + (TME)-Fe 2 (CO) 5 , with a large disproportionation energy of B30 kcal mol À1 (Table 5).…”

“…1). 2 The synthesis of tetramethyleneethane metal carbonyl complexes from metal carbonyls and allene has limitations because of low yields. Besides cis-(Z 3 ,Z 3 -TME)[Fe(CO) 3 ] 2 , only the manganese carbonyl complex trans-(Z 3 ,Z 3 -TME)[Mn(CO) 4 ] 2 , reported in 1986, was synthesized by this method.…”

“…In such Z 3 ,Z 3 -TME complexes the TME ligand donates a total of six electrons to the dimetal unit. This mode of TME bonding occurs both in the known iron complex 1,2 cis-(Z 3 ,Z 3 -TME)[Fe(CO) 3 ] 2 and the known cobalt complex 4 trans-(Z 3 ,Z 3 -TME)[Co(CO) 3 ] 2 . The difference between the two complexes is that the iron complex requires an Fe-Fe bond between the two Fe(CO) 3 units for each iron to have the favored 18-electron configuration.…”