Assessing the Strength of Metal-Metal Interactions

A proper comparison between bond strengths of different atom pairs is relevant only for the same formal bond order (BO) of atomic interactions, e.g., for single bonds, because it is clear that the higher is the BO or the number of the electron pairs responsible of bonding, the stronger is the bond. For the metal-metal interactions, such a comparison of the bond strengths is especially problematic because formal BOs may differ from the effective ones by more than 1 v.u. (valence units). In this paper, we investigated the strength of bonding and its correlation to structural parameters (bond length/BO) for 24 metal-metal pairs. A simple way of the BO and bond-strength analysis was proposed and verified on the transition metal dimers. In contrast to the previous studies, the effective BOs were not calculated from spectroscopic data or related to reference compounds, but determined independently based on the Pauling model and bond valence parameters obtained from recent quantum chemistry data. To characterize the bond strength, we used the force constants based on the available experimental or DFT data of stretching frequencies. A linear correlation between the effective BOs and force constants of the metal-metal bonds, which was confirmed in this work, allows for prediction of the stretching frequencies according to the effective BOs (and/or the bond lengths) and vice versa.