Covalent d-Block Organometallics: Teaching Lewis Structures and sd/sd² Hybridization Gives Students Additional Explanations and Powerful Predictive Tools

Despite tremendous efforts by instructors and textbook authors, students find it difficult to develop useful chemical intuitions about structures and key properties of the important d-block organometallic species that have a d⁶, d⁸, or d¹⁰ d-electron count. A full molecular orbital analysis is not always practical, and crystal field theory, while generally useful, is too limited here. It would be helpful to give students of organometallic chemistry an additional toolkit for understanding highly covalent d-block compounds. Hybridization arguments involving s and d orbitals (such as sd/sd² hybridization for d⁸/d⁶ systems) provide useful insight, as is known from the research literature but rarely taught in undergraduate courses. This article makes descriptions of bonding that are based on s,d-hybridized orbitals more accessible, targeted toward undergraduate teaching. Geometries of unusual low-coordinate structures can be predicted. An in-depth physical explanation for the trans influence is provided. A clear explanation is given for the higher stability of the cis isomers versus the trans isomers in square-planar d⁸ complexes MR₂L₂ (R = alkyl/aryl, L = relatively weakly bonded neutral ligand) and for the fac versus mer isomers in octahedral d⁶ complexes MR₃L₃. Relevant to catalysis, it is explained why strongly donating ligands do not always facilitate oxidative addition and why 12-electron and 14-electron Pd(0) species are thermodynamically much more accessible than expected. The method capitalizes on first year knowledge, namely, the ability to write Lewis structures and to use hybridization arguments. It ties into the upper-year experience, including graduate school, where covalent d-block complexes may be encountered in research and where hybridization schemes will naturally emerge from using the natural bond orbital (NBO) formalism. It is discussed where the method might fit into the inorganic curriculum.