Tridentate configuration

The tripyrazolylborate anion (RB(pz)3, 58) acts as a tridentate ligand, and frequently results in complexes which are analogous to cyclopentadienyl complexes. The first Rhin-RB(pz)3 complexes were prepared by the oxidative addition of I2 to [Rh2 RB(pz)3 (CO)3], and resulted in octahedral, monomeric complexes [Rh RB(pz)3 (CO)I2] (R = H, and, in low yield, for R = pz).892 For the tetrapyrazolyl species (R = pz) the NMR spectrum showed two, non-interconverting pyrazole environments in a 3/1 ratio, consistent with a tridentate configuration for the borate. The C—O stretch is very high (2090 cm-1), consistent with the formal Rhni designation.892... 

Monodentate (or tridentate) configuration. The association of one (or three) of the BF4" fluorine atoms with a cation lowers the anion Ti... 

In contrast with the Schiff base salen, salicylaldehyde oxime (79) (salox) complexes of Co have received comparatively little attention, but a series of bis-bidentate divalent complexes of the form iraiis-Co(sa 1 ox)2( D M SO)2 have been reported.343 The heterocyclic bidentate oxime violurate (lH,3H-pyrimidine-2,4,5,6-tetrone 5-oximate, Hvi) (80) and its /V-methyl (mvi) and /V,/V -dimethyl (dmvi) derivatives form high-spin divalent [Co(vi)]+ and Co(vi)2 complexes, whereas [Co(vi)3] is low spin.344 The mixed-ligand Co(dmvi)2(phen) complex is also low spin. The crystal structure of m-Co(pxo)2Br2 (pxo = 2-acetylpyridine-l-oxide oxime) is isostructural with the Ni11 relative.345 The dichloro complex also adopts a cis configuration. The tridentate dioximes 2,6-diformyl-4-methylphenol dioxime and 2,6-diacetyl-4-methylphenol dioxime (Hdampo) form binuclear complexes of the type (81a) and (81b) respectively.346 Cobalt oxide nanoparticles were prepared by... 

In addition, Zn2+ was shown to inhibit dopamine uptake in a mutant containing an engineered tridentate zinc site, in which the i-4 site from His3757.60, Met3717.56, was replaced with histidine, whereas the introduction of histidines at the i-2, i-3, and i-5 position did not increase Zn2+ affinity (29). In contrast, histidines at positions i+2, i+3, and i+4 all resulted in potent inhibition of dopamine uptake by Zn2+. The incorporation of these data in a model of secondary structure provides evidence for an a-helical configuration of the extracellular portion of TM7, as well as the absence of well-defined secondary structure between positions 3757.6o and 37 97.64 (Fig. IB), thereby suggesting an approximate boundary between the C-terminal end of the helix and the beginning of EL4 (29). 

Cornelius and Cleland have recently determined the absolute stereochemistry of the Mg2+ complex of ATP that is active with yeast hexokinase (43). MgATP can exist in solution as a pair of bidentate diastereomers (monodentate and tridentate complexes can also exist). The two possible bidentate isomers are shown schematically in Fig. 11. The two configurations have been labeled A and A (44). There are also four possible tridentate isomers. 

Table 9 Linear Tridentate Polyamine Ligand Configurations in M(triden-...

It has already been stated that chromium complexes of tridentate metallizable azo compounds occupy their position as the single most important class of metal complex dyestuffs because of their high stability. It should be noted, however, that in this context the term stability is not used in the thermodynamic sense but relates to the kinetic inertness of the complexes.25 Octahedral chromium(III) complexes have a tP electronic configuration and the ligand field stabilization energy associated with this is high.26 Ligand replacement reactions involve either a dissociative... 

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