Tridentate anionic ligands

The first complexes of a-keto ylides and group 5 early transition metals have only recently been obtained by reaction of Nb(III) derivatives [[NbCl3(dme) (R C=CR")] with 25 (R = thiazolyl) (Scheme 16). The chelation of the ylide occurs through an N,0-coordination to the metal center and in presence of MeLi a deprotonation of a phenyl ring takes place with the loss of alkyne, leading to the formation of a new orfho-metallated binuclear compound 32. The two ylides involved in the complexation behave as tridentate anionic ligands and are mutually in a trans disposition in order to minimize the steric hindrance [71,72]. Another binuclear niobium complex 33 has been obtained from 25 (R = Me, Ph) with this time an 0-coordinated a-keto ylide [68]. 

Because these dehydrogenation reactions occur at high temperatures, catalyst stability limits turnover numbers. The development of so-called pincer ligands, which are tridentate anionic ligands that give rise to thermally stable complexes, has led to the... 

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). 

Iron(III) complexes of 2-acetylpyridine Af-oxide iV-methyl- and 3-azabicyclo[3.2.2.]nonylthiosemicarbazone, 24 and 25, respectively, have been isolated from both iron(III) perchlorate and chloride [117], The perchlorate salt yields low spin, octahedral, monovalent, cationic complexes involving two deprotonated, tridentate thiosemicarbazone ligands coordinated via the N-oxide oxygen, azomethine nitrogen and thiol sulfur based on infrared spectral studies. Their powder ESR g-values are included in Table 1 and indicate that bonding is less covalent than for the analogous thiosemicarbazones prepared from 2-acetylpyridine, 3a and 4. Starting with iron (III) chloride, compounds with the same cations, but with tetrachloroferrate(III) anions, were isolated. 

Dimethyl and other sulfoxide complexes have cytotoxic activity.46 Other complexes include catecholate and quinone compounds47 such as Ruin(3,5-DBQ)3 the tridentate PiO ligand gives anions such as [(C6H6)Ru09P3] .48... 

Bis(pyridyl)urea can bind as an A, (9-chelate with intramolecular hydrogen bonding toward various 3d M(II) transition metal ions (48), and as a tridentate anionic A,A, A"-chelate toward Co(III) (49) (109). The v(C=0) stretches in the complexes (1642-1672cm ) appear at lower frequency than in the free ligand (1700 cm ). 

The reactions of these trihalo-tetracarbonyl anions with monodentate, bidentate/ and tridentate donor ligands also have been investigated. 

Fig. 28. Diorganotin dihauaes containing a tridentate bis-anionic ligand resulting in a trigonal bipyramidal coordination geometry at the tin centers.

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