Binuclear nickel carbonyls with the small bite chelating diphosphine ligands methylaminobis(difluorophosphine) and methylenebis(dimethylphosphine): formation of Ni=Ni double bonds in preference to ligand cleavage

摘要

The structures and thermochemistry of the triads of binuclear nickel carbonyl complexes (bid) Ni-2(CO)(n) (n = 6, 5, 4) and (bid)(2)Ni(CO)(n) (n = 4, 3, 2) of the small-bite bidentate chelating diphosphines CH3N(PF2)(2) and (Me2P)(2)CH2 have been investigated using density functional theory. The lowest-energy structures of the carbonyl-richest (bid) Ni-2(CO)(6) and (bid)(2)Ni-2(CO)(4) structures have long Ni...Ni distances indicating the lack of direct nickel bonds. Similarly, the lowest energy structures of the intermediate (bid) Ni-2(CO)(5) and (bid)(2)Ni-2(CO)(3) systems have Ni-Ni distances of similar to 2.7 angstrom and intact diphosphine ligands. Furthermore, the lowest energy structures of the carbonyl-poorest (bid)Ni-2(CO)(4) and (bid)(2)Ni-2(CO)(2) systems have shorter Ni=Ni distances of similar to 2.5 angstrom suggesting formal double bonds and retain the intact diphosphine ligands. This contrasts with the previously studied binuclear iron carbonyls [CH3N(PF2)(2)]Fe-2(CO)(6) and [CH3N(PF2)(2)](2)Fe-2(CO)(4) for which ligand cleavage to separate CH3NPF2 and PF2 units rather than Fe=Fe double bond formation occurs in the lowest energy structures. The experimental [(Me2P)(2)CH2](2)Ni-2(CO)(4) structure with the boat form of the NiPCPNiPCP eight-membered lies similar to 0.5 kcal mol(-1) in energy below the higher energy isomer with the chair form of the NiPCPNiPCP ring at the M06-L/TZP level of theory.