A theoretical study of the bonding and charge distribution in cationic group 8 metal borylene and alylene complexes: Consequences for complex stability and reactivity

Electronic, molecular structure and bonding analyses of the group 8 borylene and alylene complexes [(η5-C5H5)M(PMe3)2(EX)]+ and [(η5-C5H5)M(CO)2(AlX)]+ (E = B, Al; M = Fe, Ru, Os; X = Mes, NMe2) have been investigated by quantum chemical methods at the DFT/BP86/TZ2P level of theory. The calculated geometric parameters of the iron and ruthenium borylene complexes are in excellent agreement with the available experimental values. Moreover the effects, both geometric and in terms of the charge distribution, of substitution of ancillary carbonyl ligands by tertiary phosphines are accurately predicted. In particular, the suppressed electrophilicity of peripherally bis(phosphine) ligated borylene complexes is rationalized in terms of the reduced partial positive charge at boron. With regard to two-coordinate alylene systems – which have yet to be synthetically realized – lower M–Al bond orders (cf. to M–B) and higher partial positive charges at the group 13 centre are predicted computationally, with the latter being partially alleviated by carbonyl/phosphine substitution. In terms of the M–E(X) bonding contributions, the electrostatic interaction terms, ΔEelstat, are significantly larger in all the borylene and alylene complexes studied than the corresponding covalent bonding terms, ΔEorb. Thus, all of the M–E(X) bonds have a dominant degree of ionic character. In addition, ΔEσ is clearly the principal component of the orbital interaction, with ΔEπ representing no more than ca. 30% of ΔEorb. Moreover, in comparison with the metal borylene complexes, both the σ- and π-bonding contributions are much smaller in metal alylene complexes.

Electronic, molecular structure and bonding analyses of the group 8 borylene and alylene complexes [(η5-C5H5)M(PMe3)2(EX)]+ and [(η5-C5H5)M(CO)2(AlX)]+ (E = B, Al; M = Fe, Ru, Os; X = Mes, NMe2) have been investigated by quantum chemical methods at the DFT/BP86/TZ2P level of theory. With regard to the borylene systems, the effects, both geometric and in terms of the charge distribution, of the substitution of ancillary carbonyl ligands by tertiary phosphines are accurately predicted. Two-coordinate alylene systems have yet to be synthetically realized, and lower M–Al bond orders and higher partial positive charges at the group 13 centre are predicted computationally, with the latter being only partially alleviated by carbonyl/phosphine substitution.Figure optionsDownload as PowerPoint slide