Homo-bridged ligand

The cyclo-P3 unit can also behave as a three electron bridging ligand (45) to give a variety of homo and heterometallic triple-decker sandwich complexes which are treated in a recent review (100). 

Bis(2-pyridyl)pyrazine (2,3-dpp = (88)), 2,3-bis(2-pyridyl)quinoxaline and 2,3-bis(2-pyridyl) benzoquinoxaline (91) find numerous applieations as bridging ligands in homo- and heterome-tallic complexes. At this point, we consider some dinuelear species complexes eontaining >3 metal centers are covered in Section 5.5.3.1.5. By using the complexes in the series [(bpy)2M-(/u-L)M(bpy)2]" " (M-M = Ru-Ru, Os-Os, Ru-Os L = (88), 2,3-bis(2-pyridyl)quinoxaline or (91))... 

Chisholm and Cotton have summarized their successful efforts characterizing metal-metal triple bonds in molybdenum(III) and tungsten(III) compounds containing no bridging ligands (46). This realm of strong homo-nuclear metal-metal bonds was unknown prior to the 1970s, and the rapid development and exploitation of the chemistry of these dimers provide an... 

Tanase, T., Inukai, FI., Onaka, T., Kato, M., Yano, S., and Lippard, S.J., Trinuclear Zn(II) and Cu(II) homo and heterotrimetallic complexes involving D-glucopyranosyl and biscarboxylate bridging ligands. A substrate binding model of Xylose isomerases, Inorg. Chem. 40 (2001) 3943-3953. 

Scheme 6.9 Preparative route to homo- and hetero-metallic triple-decker complexes 20-24 with cyclic carboranes as bridging ligands (the substituents at the boron and carbon atoms of the middle deck are alkyl groups)...

If Ru orbitals closely match the energy of the HOMO or LUMO of the bridging ligand, covalency is approached and the distinction between formal oxidation states becomes blurred, resulting in the extreme case of a radical bridging ligand. The situation is illustrated below ... 

Monodentate bridging ligands snch as SO2, PPh2, and CO usually lead to nuclearities higher than two. Several examples of homo- and heteronuclear, three or four-member palladium clusters that adopt a variety of geometries are known. 

Drago and Elias s idea [46] to employ tridentate amines for protection of one of the two triangular bases of hexacoordinate d-metal ions coordination polyhedron (in case the latter act as capping agents) from polymerization has successfully been developed by P. Chaudhuri and K. Wieghardt for the synthesis of linear homo-and heterotrinuclear macrobicyclic complexes with tris-dioximate bridging ligands. 

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