Trident

Eig. 1. Types of chelates where (1) represents a tetracoordinate metal having the bidentate chelant ethylenediamine and monodentate water (2), a hexacoordinate metal bound to two diethylenetriamines, tridentate chelants (3), a hexacoordinate metal having triethylenetetramine, a tetradentate chelant, and monodentate water and (4), a porphine chelate. The dashed lines iadicate coordinate bonds. 

Coordination of the dianion [S4N4] as a tridentate ligand has been established in a few cases. The complexes IrCl(CO)(S4N4)PPh3 and... 

Vinylogous amides, which have an enamine function in conjugation with a carbonyl group, constitute tridentate systems and thus open the possibility of alkylation on carbon, nitrogen, or oxygen. It has been found that the pyrrolidine enamine of acetylacetone gives rise to a carbon mcthylation but an oxygen ethylation product 41). The alkylation of cyclic 1,3-diketone-derived enamines has been studied 41,283). O Alkylation was found in alcohol solvents and predominant C alkylation in nonprotonic solvents. 

The complex CUCN.NH3 provides an unusual example of CN aeting as a bridging ligand at C, a mode which is common in p,-CO complexes (p. 928) indeed, the complex is unique in featuring tridentate CN groups which link the metal atoms into plane nets via the Cu... 

Figure 8.24 Schcmaiic diagram of the layer structure of CuCN.NH showing the tridentate CN groups each Cu is also bonded to 1 NH molecule at 207 pm. Note also the unusual 5-coordination of Cu including one near neighbour Cu at 242 pm (13 pm closer than Cu-Cu in the metal). The lines in the diagram delineate the geometry and do not represent pairs of electrons.

Figure 13.19 Proposed structures for (a) the tridentate cycZo-polyarsane complex [Cr(CO)3(As5Mes)], and (b) the bismonodentate binuclear complex [ Cr(CO)sl2(As5Me5)].

MeCN. The structure of the anion (Fig 24.4b) can be envisaged as a tridentate [ReHg] ligand coordinated to ReCtriphos)" ", and, since the metal atoms are only 259.4 pm apart, is said to involve an Re=Re triple bond (in which case the [ReHg] should be regarded as tetradentate and its Re atom as 10-coordinated). 

Cationic complexes of rran.s-chelating tridentate ligand, (/ ,/ )-4,6-dibenzo-furandiyl-2,2 -bis(4-phenyloxazoline), with transition metal(II) perchlorates as effective catalysts for asymmetric cycloaddition of nitrones 98YGK368. 

Bis(3,5-dimethylpyrazol-l-yl)cyclotriphosphazene containing phenyl substituents at the phosphorus atoms (R) reacts with molybdenum and tungsten hexacar-bonylstogive34(R = Ph M = Mo, W) [94JCS(D)63]. IfRj = N(Me)(CH2)20, compounds 34 (R2 = N(Me)(CH2)20 M = Mo, W) having better solubility in organic solvents are produced. Their structures show the novel tridentate NNN feature of the ligands. 

L = C3H3, C H ) and then [Rh(acac)(CO),] to yield the tetranuclear species 180 (85ICA(i00)L5), where the heterocyclic ligands are tridentate. The product reacts with the rhodium(I) dimer [Rh(CO)2Cl]3 to give the trinuclear complex 181. In the solid state, the molecules of this complex form the intermolecular stacks along the z-axis. 

Jacobsen et al. took an important step towards the development of a more general catalytic enantioselective cycloaddition reaction of carbonyl compounds by introducing chiral tridentate Schiff base chromium(III) complexes 15 (Scheme 4.15)... 

The major developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds with conjugated dienes have been presented. A variety of chiral catalysts is available for the different types of carbonyl compound. For unactivated aldehydes chiral catalysts such as BINOL-aluminum(III), BINOL-tita-nium(IV), acyloxylborane(III), and tridentate Schiff base chromium(III) complexes can catalyze highly diastereo- and enantioselective cycloaddition reactions. The mechanism of these reactions can be a stepwise pathway via a Mukaiyama aldol intermediate or a concerted mechanism. For a-dicarbonyl compounds, which can coordinate to the chiral catalyst in a bidentate fashion, the chiral BOX-copper(II)... 

The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

An enbancenient in die solubdity of odierwise almost insoluble, and most probably polymeric, pbenylcopper species bas been adiieved by treatment widi a tridentate pbospbine. Hie resulting soluble pbenylcopper compoimd was fiidy strucuirally diaracterized fsee Sdienie 1.9) [53]. 

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