DFT Study on Alkyl- and Haloborylene Complexes of Manganese and Rhenium: Structure and Bonding Energy Analysis in [(η5-C5H5)(CO)2M(BR)] and [(η5-C5H5)(CO)2M(BX)] ( M = Mn, Re; R = Me, Et, iPr, tBu; X = F, Cl, Br, I)

Electronic, molecular structures, and bonding analysis of the terminal neutral alkylborylene and haloborylene complexes of manganese and rhenium [(eta(5)-C5H5)(CO)(2)M(BR)] and [(eta(C5H5)-C-5)(CO)(2)M(BX)] (M = Mn, Re; R = Me, Et, iPr, tBu; X = F, Cl, Br, I) were investigated at the DFT/BP86/TZ2P level of theory. The calculated geometry of the manganese alkylborylene complex [(eta(5)-C5H5)(CO)(2)Mn(BtBu)] is in excellent agreement with the experimentally derived structural data. Pauling bond order of the optimized structures shows that the M-B bonds in these complexes are almost M=B double bonds, which is also supported by the performed energy decomposition analysis. The orbital interactions between the metal and boron arise mainly from M <- BR sigma donation, while the pi bonding contribution is relatively small (22.6-25.8% of total orbital contributions). The M-B pi bond orbitals are highly polarized towards the metal atom and the contributions of boron are very small. In the BX ligands, sigma bonding, interaction energies, electrostatic interactions and bond dissociation energies are smaller than those in the BR ligands. The contributions of the electrostatic interactions Delta E-elstat are significantly larger in all borylene complexes than the covalent bonding Delta E-orb: the [M]=BR bonding in the alkylborylene and haloborylene complexes has a greater degree of ionic character (ca. 61.7% for the complexes I-IV, ca. 65.7% for the complexes V-VIII, 53.9-56.2% for the complexes IX-XII and 58.5-60.4% for the complexes XIII-XVI). The chloroborylene complexes [(eta(5)-C5H5)(CO)(2)M(BCl)] possess the highest electrostatic interactions, Delta E-elstat of all haloborylene complexes [(eta(5)-C5H5)(CO)(2)M(BX)], whereas the iodoborylene complexes [(eta(5)-C5H5)(CO)(2)M(BI)] display the highest orbital interactions, Delta E-orb. The dimerization of the terminal borylene complex [(eta(5)-C5H5)(CO)(2)Mn(BCl)] to [{(eta(5)-C5H5)(CO)(2)Mn}(2)(eta:eta:mu-B2Cl2)] is a strongly exothermic process (21.04 kcal/mol below the starting monomers) and the energies are in favor of a dimerization reaction.