The diphosphacyclobutadiene sandwich compounds (¦Ç4-But2C2P2)2Mz (M = Ni, z = 0; M = Co, z = ?1, 0; M = Fe, z = ?1, 0) as well as the iron carbonyl derivative (¦Ç4-But2C2P2)Fe(CO)3 have recently (2008) been synthesized by Lammertsma and co-workers using the dimerization of the phosphaalkyne ButCP: on suitable reactive transition metal sites. The structures and energetics of the closely related dimethyl derivatives (Me2C2P2)Fe(CO)n (n = 3, 2, 1) and (Me2C2P2)2Fe2(CO)n (n = 5, 4, 3, 2) have now been investigated by density functional theory. For (Me2C2P2)2Fe2(CO)5 a structure with a bridging ¦Ç1,¦Ç4-Me2C2P2 ring and no iron-iron bond is energetically preferred by more than 24 kcal/mol over an alternative structure with only terminal ¦Ç4-Me2C2P2 rings and an iron-iron single bond. The lowest energy singlet and triplet (Me2C2P2)2Fe2(CO)4 structures have at least one bridging ¦Ç1,¦Ç4 ligand, which in one case is in the form of an (¦Ç4-Me2C2P2)2Fe(CO) sandwich bidentate ligand chelating to an Fe(CO)3 unit through two ring phosphorus atoms. In contrast to (Me2C2P2)2Fe2(CO)n (n = 5, 4), the lowest energy structures for (Me2C2P2)2Fe2(CO)3 have only terminal ¦Ç4-Me2C2P2 ligands, three carbonyl groups, and an FeFe triple bond. Interesting structures for the dicarbonyl (Me2C2P2)2Fe2(CO)2 include a structure with a (¦Ç4-Me2C2P2)2Fe sandwich ligand chelating to an Fe(CO)2 group and a structure with exclusively terminal ¦Ç4-Me2C2P2 ligands and a short Fe-Fe distance of ¡«2.16 ?, suggesting a formal quadruple bond.
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