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

The structures and thermochemistry of the triads of binuclear nickel carbonyl complexes (bid)Ni₂(CO)_n (n = 6, 5, 4) and (bid)₂Ni(CO)_n (n = 4, 3, 2) of the small-bite bidentate chelating diphosphines CH₃N(PF₂)₂ and (Me₂P)₂CH₂ have been investigated using density functional theory. The lowest-energy structures of the carbonyl-richest (bid)Ni₂(CO)₆ and (bid)₂Ni₂(CO)₄ structures have long Ni···Ni distances indicating the lack of direct nickel bonds. Similarly, the lowest energy structures of the intermediate (bid)Ni₂(CO)₅ and (bid)₂Ni₂(CO)₃ systems have Ni-Ni distances of ∼2.7 Å and intact diphosphine ligands. Furthermore, the lowest energy structures of the carbonyl-poorest (bid)Ni₂(CO)₄ and (bid)₂Ni₂(CO)₂ systems have shorter Ni=Ni distances of ∼2.5 Å suggesting formal double bonds and retain the intact diphosphine ligands. This contrasts with the previously studied binuclear iron carbonyls [CH₃N(PF₂)₂]Fe₂(CO)₆ and [CH₃N(PF₂)₂]₂Fe₂(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)₂CH₂]₂Ni₂(CO)₄ structure with the boat form of the NiPCPNiPCP eight-membered lies ∼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.