‘Supine’ or ‘prone’ ligands: geometric preference of conjugated diene, 1-azadiene, and 1,4-diazadiene ligands on half-metallocene complexes of early transition metals

The two terms defined originally, supine and prone, describing the orientation of an s-cis-1,3-diene ligand bound to half-metallocene fragments of niobium and tantalum are considered to include various new examples of diene complexes. Typically, bis(diene) complexes of these fragments predominantly prefer one supine and one prone diene ligands. The supine/prone preference of various mono- and bis(diene), bis(allyl) and (allyl)(diene) complexes together with 1-azadiene and 1,4-diazadiene complexes is also described. Structural features and unique reactivity of these two different diene coordination modes are discussed.