Side-On versus End-On Binding Modes between Metal Cations and (NHC)AlAl(NHC)

Group 13 or 14 analogues
of alkene or alkyne are characterized
with unusual bonding, high reactivity, and potential applications
in catalysis, but few studies have ever addressed their bonding modes
with metal cations. The unique behaviors of heavier group 13 or 14
ethyne analogues inspire us to hypothesize the existence of their
end-on complexation with metal cations. Here we computationally studied
the interactions of the lightest metal alkyne analogue in group 13,
(NHC)­AlAl­(NHC), with alkali metal cations Li+, Na+, and K+ and transition metal cations Cu+,
Ag+, and Au+. The quantum chemical bonding analysis
shows that only the side-on binding mode is feasible in complexes
with alkali metal cations. In contrast, complexes with transition
metal cations prefer the end-on bonding mode, where pronounced covalent
(dative) character is identified based on the bond distances, charge
transfer, and topological properties at the bond critical points (BCPs).
It can be interpreted with the electron promotion (excitation) and
the subsequent σ donation from the lone pairs of Al to the s
orbitals of transition metal cations. Further studies show that replacing
the substituent NHC by phenyl anion has little impact on the end-on
binding mode. Similar results can be found for disilynes.