Coordination number complexes having unusually high

Numerous phosphine and arsine complexes have been synthesized and characterized predominately with osmium in the +2, + 3 or +4 oxidation states. Examples include [OsCl2(dppm)2] [108341-10-2], [OsC13(P(CH3)2(C6H5)3] [20500-70-3], [0s2Cl6(dppm)2(0)] [87883-12-3], and [Os(AsC2Hb(C6Hb)2)4H2] [27498-19-7]. An example of an unusually low oxidation state is the Os(—2) complex K2[Os(PF3)4] [26876-74-4]. High coordination numbers and formal oxidation states are found in the phosphine hydrides, eg, [Os(P(CH3)(C(5HB)2)H6] [25895-55-0] and... 

The calcium, strontium, barium, and lead 80) complexes of 160 and 161 have also been reported. In these two ligands the six donor atoms are essentially confined in a plane these complexes thus permit study of unusual coordination geometries in species of high coordination number. Attempts to form alkali metal complexes with 160 and 161 under the same conditions as employed for the alkaline earth metal complexes have failed. The successful syntheses of complexes of the latter type indicate that the higher charge to radius ratio is of consequence when spherically charged cations are employed. Such metal ions have no apparent coordinative discrimination as the template ion 87). 

Macrocyclic ligands and their open-chain analogues have added a new dimension to the coordination chemistry of lanthanide ions. In particular, they allowed the study of high and unusual coordination numbers, e.g. 11-coordination. Basic thermodynamic data are still needed to get a better understanding of the complexation process and of both macrocyclic and macrobicyclic effects. Moreover, kinetic data on ligand exchange remain scarce and studies in this field are certainly desirable. 

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