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Binuclear alkyne manganese carbonyls of the type (RC≡CR')Mn₂(CO)_n(R and R'=methyl or dimethylamino;n=8, 7, 6) and their isomers related to the experimentally known (MeC₂NEt₂)Mn₂(CO)_n(n=8, 7) structures have been investigated by density functional theory. The alkyne ligand remains intact in the only low energy (Me₂N)₂C₂Mn₂(CO)₈isomer, which has a central Mn₂C₂tetrahedrane unit and is otherwise analogous to the well‐known (alkyne)Co₂(CO)₆derivatives except for one more CO group per metal atom. The low‐energy structures of the unsaturated (Me₂N)₂C₂Mn₂(CO)_n(n=7, 6) systems include isomers in which the nitrogen atom of one of the dimethylamino groups as well as the C≡C triple bond of the alkyne is coordinated to the central Mn₂unit. In other low‐energy (Me₂N)₂C₂Mn₂(CO)_n(n=7, 6) isomers the alkyne C≡C triple bond has broken completely to form two separate bridging dimethylaminocarbyne Me₂NC ligands analogous to the experimentally known iron carbonyl complex (Et₂NC)₂Fe₂(CO)₆. The (alkyne)Mn₂(CO)_n(n=8, 7, 6) systems of the alkynes MeC≡CMe and Me₂NC≡CMe with methyl substituents have significantly more complicated potential surfaces. In these systems the lowest energy isomers have bridging ligands derived from the alkyne in which one or two hydrogen atoms have migrated from a methyl group to one or both of the alkyne carbon atoms. These bridging ligands include allene, manganallyl, and vinylcarbene ligands, the first two of which have been realized experimentally in research by Adams and coworkers. Theoretical studies suggest that the mechanism for the conversion of the simple alkyne octacarbonyl (MeC₂NMe₂)Mn₂(CO)₈to the dimethylaminomanganaallyl complex Mn₂(CO)₇[μ‐η⁴‐C₃H₃Me₂] involves decarbonylation to the heptacarbonyl and the hexacarbonyl complexes. Subsequent hydrogen migrations then occur through intermediates with C−H−Mn agostic interactions to give the final product. Eight transition states for this mechanistic sequence have been identified with activation energies of ∼20 kcal/mol for the first hydrogen migration and ∼14 kcal/mol for the second hydrogen migration.

 

Theoretical methods show that the lowest energy bis(butadiene)metal structures (C 4 H 6 ) 2 M (M = Ti to Ni) have a perpendicular relative orientation of the two butadiene ligands corresponding to a tetrahedral coordination of the central metal atom to the four CC double bonds of the butadiene ligands. Distribution of the metal d electrons in the resulting tetrahedral ligand field rationalizes the predicted spin states increasing monotonically from singlet to quartet from nickel to manganese and back from quartet to singlet from manganese to titanium.