Electronic effects on the stability of isomeric alkyl transition metal compounds

Density functional calculations on a variety of alkyl-transition metal complexes R-M are reported. Specifically, the following compounds have been studied: zirconocene complexes [Cp2Zr(H)R], [Cp2Zr(Cl)R], [Cp2ZrR](+), and [CP*Zr-2(Cl)R]; iron compounds [CpFe(CO)(2)R] and [CpFe(CO){P(CH3)(3)}]; dimethylamino-dithiocyanato-palladium complexes [{(CH3)(2)NCS2}Pd{P(CH3)(3)}R]; and cationic diimino palladium complexes [{NN}Pd(L)R]+ (with {NN} = HN=CH-CH=NH or N,N-(o,o'-bis-diisopropylphenyl)diiminoacenaphthalene, and L = nothing, Cl-, (CH3)(2)O, (CH3)(2)S, C2H4, or CH3CN). The R groups considered are methyl, n-propyl, isobutyl, isopropyl, and tert-butyl, thus covering the whole range from primary groups to tertiary ones. It is shown that primary alkyl complexes are usually more stable than secondary and tertiary ones and that this is an electronic effect, due to the partial carbanionic character of the alkyl group. Steric effects, which are usually invoked in the literature whenever this issue is considered, are shown to play only a minor role in many cases. Notable exceptions occur in the case of extremely bulky compounds or for systems in which the metal-carbon bond is less polar.