Transition metal complexes of the naked pnictide elements

Even though transition metal compounds containing "naked" pnictide elements (E) have been known for many decades now, the chemistry of this interesting class of compounds continues to provide surprises and intellectual challenges in describing new structures and interesting bonding patterns. The number of ligand sets that now support naked pnictide ligands has expanded from the traditional organometallic ligand sets to include metal alkoxides and N-heterocyclic carbene species, and complexes are known from the early transition metals to the end of the d-block. In this review, complexes are presented based on their structural similarities starting with metal-rich compounds containing isolated E atoms in a variety of coordination environments presented in order of increasing metal connectivity. Subsequently, compounds possessing Ex fragments with x ranging from 2 to 24 are presented in increasing nuclearity. Finally, structures with low ligand ratios and metal to pnictide ratios close to 1:1 are discussed, followed by compounds that have no supporting ligands and resemble metal alloy fragments. Where appropriate, electron counting rules have been employed to explain observed structures and reactivity patterns. A particularly intense area of study has been the creation of inorganic-organometallic-organic polymers and frameworks based on discrete clusters with Ex fragments (2 ≤ x ≤ 6) where the Ex fragment provides a node for structure growth in combination with transition metal coordination frameworks based on copper and silver cations and suitable organic ligands. Simple oligomers, 1-dimensional, 2-dimensional and 3-dimensional polymeric structures have all been prepared and structurally characterized. In addition to the structural data, synthetic methods are described.