Nature of beryllium, magnesium, and zinc bonds in carbene ⋯MX2⋯MX_2 (M = Be, Mg, Zn; X = H, Br) dimers revealed by the IQA, ETS-NOCV and LED methods

The nature of beryllium–, magnesium– and zinc–carbene bonds in the cyclopropenylidene⋯MX [Formula: see text] (M = Be, Mg, Zn; X = H, Br) and imidazol-2-ylidene⋯MBr [Formula: see text] dimers is investigated by the joint use of the topological QTAIM-based IQA decomposition scheme, the molecular orbital-based ETS-NOCV charge and energy decomposition method, and the LED energy decomposition approach based on the state-of-the-art DLPNO-CCSD(T) method. All these methods show that the C⋯M bond strengthens according to the following order: Zn < Mg [Formula: see text] Be. Electrostatics is proved to be the dominant bond component, whereas the orbital component is far less important. It is shown that QTAIM/IQA underestimates electrostatic contribution for zinc bonds with respect to both ETS-NOCV and LED schemes. The [Formula: see text] carbene→MX [Formula: see text] donation appears to be much more important than the MX [Formula: see text] carbene back-donation of [Formula: see text] symmetry. The substitution of hydrogen atoms by bromine (X in MX [Formula: see text]) strengthens the metal–carbene bond in all cases. The physical origin of rotational barriers has been unveiled by the ETS-NOCV approach