Octa-coordinate metal atoms

Structures incorporating octa-coordinate metal atoms. 

Hexacoordinated structures prevail in the case of complexonates of the triamine series [764]. However, a great variety of CN of metals is also observed. Moreover, many coordination modes are observed for the complexones of the triamine series, penta-, hexa-, hepta-, and octa-types with chelate and chelate-bridge function of mono-, di-, tri-, and tetra-deprotonated ligands. As a result, a number of metal-cycles are formed. Thus, maximum dentacity of diethylenetriami-noacetic acid 431 (m = n = p = q = 1), with respect to a metal atom, equals 8 (3N+50), leading to seven metal-cycles (two ethylenediamine and five glycine) being formed [764]. 

Several examples of direct ring phosphorus interaction with transition metals are now known [279-287]. For example, reaction of N3P3CI6 with Na2Fe2(CO)g affords a novel spirocyclic diiron octa carbonyl derivative (Scheme 25) [282, 283]. The diiron spiro derivative acts as a template for the construction of several transition metal clusters. Some of the other examples where ring phosphorus atom is involved in interaction with transition metals are summarized in Scheme 25. The ring phosphorus atoms in hydrido phos-phazenes, N3P3R4R H also coordinates to transition metals. This has been discussed in an earlier section (vide supra). 

Finally it should be noted that formation of the perfluorobicyclo[3.3.0]-octa-2,7-diene-4,6-diyl ligand allows pyramidalization of four fluorinated carbons and may reflect a thermodynamic preference for sp2-hybridized carbon atoms in coordinated OFCOT to undergo rehybridization to sp3, provided that the ancillary ligands present on the metal can support an increase in the formal oxidation state and that the constraints of the 18-electron rule are obeyed. The origins of this thermodynamic effect for uncoordinated fluorinated alkenes have been discussed in detail (2). Extensions to nickel, palladium, and platinum systems are described in Section IX. 

The capping atoms in the hexa-, octa-, dodeca-, and octadecanuclear structures listed in Table III all have the same disposition i.e., they are attached to a /u,4-oxo group of the central cubane(s) to complete a tetrahedral coordination geometry about this site. This contrasts with the additional metal sites in the decanuclear cages, where they are attached to ixq-oxo groups which have octahedral geometries. 

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