Structure and bonding in binuclear metal carbonyls. Classical paradigms vs. insights from modern theoretical calculations

The nature of the bonding interactions in a series of bridged and unbridged binuclear metal carbonyls has been analyzed using the visual insights provided by the analysis of domain averaged Fermi holes (DAFH). Picture of the bonding emerging from this analysis has been confronted with the predictions anticipated in each particular case by the traditional bonding paradigms exemplified by the 18-electron rule and isolobal analogy. Such a confrontation has shown that the frequently observed inconsistencies, especially in what concerns the eventual existence and/or multiplicity of direct metal–metal bonds, are due to simplistic interpretation of 18-electron rule that primarily relies on formal electron count and qualitative consideration of metal–metal distances without taking into account the actual bonding capabilities of individual M(CO)n fragments. The systematic scrutiny of the results of DAFH analysis for the wide series of studied carbonyls allowed to reveal the shortcomings of the original simplistic interpretation and to propose a slight reinterpretation of 18-electron rule that makes its predictions consistent with the insights of moderns theoretical tools.