To bend or not to bend, the dilemma of multiple bonds

Beyond the second row of the periodic table, the nature of the multiple bonds between the elements of the main groups remains yet elusive, and "non-classical" bonding schemes are often invoked for their description. Here, focusing on group 14, we have performed an accurate modeling of the Si-Si and C-C double bonds, including electron correlation effects. We have shown that SiSi bonds are "classical" and closely resemble CC ones, being similarly subjected to a sort of tug of war in which the sigma bond favors distortion and the pi bond opposes it. The essential difference between Si and C boils down to the sizes of their valence shells, which determine the pi-bending stiffness. In carbon, such a stiffness is large because, upon bending, the atomic s orbitals interfere destructively with the p ones. In silicon, the s shell is smaller than the p one, the bending stiffness is reduced and the pi bonds typically succumb, distort, and weaken. Electron correlation plays a major role in this context, since pi bonds are far from their molecular orbital limit. Hence, we have further shown that upon weakening the effective repulsion between pi electrons one may remove any structural instability, strengthen the pi bonds and turn Si into a closer relative of C than it used to be.