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

摘要

Electronic, molecular structure and bonding analyses of the group 8 borylene and alylene complexes [(η ~5-C _5H _5)M(PMe _3) _2(EX)] ~+ and [(η ~5-C _5H _5)M(CO) _2(AlX)] ~+ (E = B, Al; M = Fe, Ru, Os; X = Mes, NMe _2) 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, ΔE _(elstat), are significantly larger in all the borylene and alylene complexes studied than the corresponding covalent bonding terms, ΔE _(orb). Thus, all of the M-E(X) bonds have a dominant degree of ionic character. In addition, ΔE _σ is clearly the principal component of the orbital interaction, with ΔE _π representing no more than ca. 30% of ΔE _(orb). Moreover, in comparison with the metal borylene complexes, both the σ- and π-bonding contributions are much smaller in metal alylene complexes.