Ligands dianionic

Two main synthetic routes are available for the preparation of dithiolenes in the first and most frequently used method, either the free ethylenedithiol or an appropriate salt of the ethylene-dithiolato ligand dianion is reacted with a metal salt to produce anions of the dithiolenes, which may or may not be subsequently oxidized to the neutral species the second one, applied so far only to transition metal dithiolenes, converts vicinal diketones into dithiodiketones and reacts these either with zerovalent metals to form dithiolenes directly or uses metal salts to arrive at cationic species, which are reduced to the neutral dithiolenes either during the reaction or in a subsequent step. 

Complexes with chloro and methyl ligands 46-49 are prepared by using monodentate and bidentate phosphine ligands.Dianionic stanna-< /(9j-(9-dodecaborate, Sn(BiiHn), forms the methyl complex 50. " ... 

Co(C2sH2oN4) CH3CN 8 2) room 2.84 Farad suggested to contain Co" and ligand dianion 74T2... 

Reaction of free-base porphyrin compounds with iton(II) salts in an appropriate solvent results in loss of the two N—H protons and insertion of iron into the tetradentate porphyrin dianion ligand. Five-coordinate iton(III) porphyrin complexes (hemins), which usually have the anion of the iton(II) salt for the fifth or axial ligand, ate isolated if the reaction is carried out in the presence of air. Iron(II) porphyrin complexes (hemes) can be isolated if the reaction and workup is conducted under rigorously anaerobic conditions. Typically, however, iton(II) complexes are obtained from iton(III) porphyrin complexes by reduction with dithionite, thiolate, borohydtide, chromous ion, or other reducing agents. 

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

The combination of hard (A) and soft (5) coordination in the 1,5-P2N4S2 ring system leads to a diversity of coordination modes in complexes with transition metals (Lig. 13.1). In some cases these complexes may be prepared by the reaction of the dianion [Ph4P2N4S2] with a metal halide complex, but these reactions frequently result in redox to regenerate 13.3 (L = S, R = Ph). A more versatile approach is the oxidative addition of the neutral ligand 13.3 (L = S) to the metal centre. 

Reaction between [W(RC=C)Cl(CO)2(py)2] (R = Ph, Me) with the anionic chelating Schiff base pyrrole-2-carboxaldehyde methylimine yields the cationic complexes [NEt4][W(RCCO)(NN)2(CO)] (where NN is the dianion of the pyrrole ligand). These complexes react with methyltriflate, forming the neutral acetylenic complexes [W(NN)2(CO)(RC=COMe)] (87OM1503). One of the pyrrolic Schiff bases is coordinated via the pyrrole and imino nitrogen atoms, and another one only via the imino nitrogen atom. 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

A benzannulation reaction yielding the naphthoquinone 61 could also be performed with the ruthenium carborane-stabilised carbene 60 and 1-hexyne [56] (Scheme 36). The ruthenium carbene unit can be regarded as an 18-electron fragment containing a formal Ru(II) centre coordinated to a dianionic six-electron-donor cobaltacarborane ligand. 

Unique examples of N,A/ , N "-tri(isopropyl)guanidinate complexes of Nb(V) and Ta(V) bearing additional terminal imido ligands have been obtained as depicted in Scheme 116. Depending on the reaction conditions, these reactions may also lead to the formation of compounds containing dianionic guanidinate... 

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