Molybdenum

Molybdenum plays an important role in environmental processes. It is a trace element and has an unparalleled role in the lifecycle of living beings. It is a cofactor of many enzymes and hence is responsible for health and wellbeing of biological systems. Many interesting Mo complexes have been characterized since the publication of CCC-II in 2002. This chapter covers a comprehensive account of molybdenum complexes reported from 2002 to 2020. These complexes feature a variety of oxidation states of the central Mo atom ranging from VI to 0, which are considered as main criteria for their classification, and the sub-categories are based on the donor atoms or ligand classes. Complexes belonging to each category are exhaustively reviewed and mostly discussed based on synthesis, characterization and reactivity or application of their representative compounds. The ancillary ligands play key roles in stabilizing Mo complexes with various oxidation states and the electronic structures and steric effect of the ligands influence their reactivity. Proper design of ligands and their fine tuning by incorporating various functional groups, donor atoms and geometrical and steric factors led to the evolution of interesting systems. As such, a variety of ligands ranging from simple monodentate halido ligands, alkyl, or aryl groups to complex multidentate porphyrinato or phthalocyanine ligands are discussed in this chapter. Synthesis of Mo complexes varies from short time room temperature stirring of ligands and metal salts to multi-step procedures involving strictly temperature modulated, redox reactions involving air- and moisture-sensitive intermediates. Syntheses of these complexes were mainly driven by (1) purely fundamental thoughts like structural features or exploration of novel reactivities (2) fine-tuning and development of compounds for specific applications, such as catalytic epoxidation or bio-enzyme mimicking. In addition, a surge in low-valent Mo chemistry and the introduction of MoMo quintuple bonds are also reviewed in detail with reference to the design of ligands and structural analysis of the complexes. The reactivity and catalytic applications of these compounds are included as well.