Assessing the orbital contribution in the “spodium bond” by natural orbital for chemical valence–charge displacement analysis

The term "spodium bond"(SpB) has been recently proposed to describe the noncoordinative interaction that can be established between a polarized group 12 metal and a mild Lewis base (LB). Most of the systems showing short metal-donor distances compatible with SpB are characterized by the coexistence of multiple weak interactions, including hydrogen and halogen bonding, making the assessment of real importance of SpB difficult. Here, we show that the relative importance of each contribution can be probed by dissecting the orbital component of the interaction through the extended transition state-natural orbital for chemical valence-charge displacement analysis (ETS-NOCV-CD). The latter gives useful information about relative energies and electrons involved, for model systems ([(thiourea)2MX2]···LB; M = Zn, Cd, and Hg; X = Cl and I; and LB = CH2S, CH2O, CH3CN, and CO) and a variety of structures extracted from experimentally characterized adducts, allowing us to demonstrate the lack of a direct correlation between a favorable metal-base distance and the presence of an orbital contribution for the SpB.