Chelating ligands cations

Phosphorus Donors. Phosphine coordination complexes of thorium are rare because the hard Th(IV) cation favors harder ligand donor types. The only stable thorium—phosphine coordination complexes isolated as of the mid-1990s contain the chelating ligand,... 

The chiral information of stereogenic centers in the allyl moiety of the precursor is destroyed on deprotonation. While an i/3-bound ion pair with a planar carbon frame is a chiral compound, usually rapid racemization takes place by intra- or intermolecular migration of the cation from one face to the opposite one. The sole exceptions known at present are secondary 2-alkenyl carbamates with X = dialkylaminocarbonyloxy21, in which the cation is tied by the chelating ligand, see Section 1.3.3.3.1.2. 

The bis-phosphonium salt 56 in presence of Pd(OAc)2 leads to the formation of the neutral bis-ylide 57 which reacts with TICIO4 to give the dinuclear cationic complex 58 (Scheme 22) [89,90]. The bis-ylide part, which has potentially two carbons and one oxygen donor atoms, acts as a C,C-chelating ligand through its two soft ylidic carbons. 

In 1994, Kiindig et al. described a related catalyst, in which both CO ligands have been replaced by a chiral F,f -chelate ligand providing evidence that structurally defined cationic Fe sandwich complexes are indeed efficient catalysts for Diels-Alder reactions [33]. 

Liaw et al. reported that conversions between the neutral sparteine [Fe(NO)2] complex 133 and the anionic Fe(NO)2 [Fe(NO)2(S2C3Hg)] 137 proceed via the cationic sparteine Fe(NO)2 -complex 135 through oxidation by NO" " and transfer of the [Fe(NO)2] -unit to the chelating ligand S-(CH2)3-S 136 (Scheme 35). The resulting anionic complex 137 is stable in contrast to the cationic complex 135. The cationic complex 135 also acts as a [Fe(NO)2] donor in the presence of the DNIC [(PhS)2Fe(NO)2] 138 to yield Roussin s red ester 139. The bidentate thiol ligand S-(CH2)3-S 136 promotes the stability of the anionic DNIC Fe(NO)2 ... 

Chelating ligands bind much more tightly to their metal cations than do ligands that possess only one donor atom. A good example is the ethylenediamine complex with. The ethylenediamine complex is much more stable than the analogous ammonia complex ... 

Novel cationic methyl and neutral methyl, chloro Pd11 complexes have been synthesized with various functionalized imidazole ligands. For the first time, structurally characterized examples of this type of Pd complex could be obtained. The chelate ligand is shown to adopt a boat conformation. Inversion of the chelate ring was established by NMR spectroscopy. Depending on electronic and steric features of the ligand, the complexes can act as catalysts in the CO/ethylene as well as the Fleck reaction.186... 

Towards Cd the cz s-[Cr(OH)2(en)2]+ unit acts as a chelating ligand and yields heterometallic Cd(/i-OH)2Cr moieties as components in structures. Examples are the straight cationic chain... 

The five-membered ring chelate ligands (CHIRAPHOS (7) and DIPAMP (9)) showed poor activity. DIOP (5) was found to be more effective than BINAP (6), while no real improvements in the levels of asymmetric induction were found by using cationic complexes [Rh(COD)(L-L)]+ instead of neutral systems. 

From the method of preparation of [BeCl(12-crown-4)]+ (179), it is known that the Cl ligand can be substituted by a solvent molecule. We applied our most common test solvents water and NH3 to a Be2+ cation, where most coordination sites are occupied by a chelating ligand, in this case the crown ether 12-crown-4. In contrast to the tetrahedral [Be(solvent)4]2+ solvated complexes, the precursor complexes [Be(solvent)(12-crown-4)]2+ are quadratic pyramidal, where four oxygen donor atoms of the crown ether form the quadratic basis, while Cl- or a coordinating solvent molecule occupies the apical position. Addition of one water or ammonia molecule to [Be(12-crown-4)]2+ is exothermic (see Table IX). 

Chelating ligands have a much lower propensity for dissociation. Yet detailed kinetic studies on similar systems with bidentate phosphine ligands, /ac-(L2)PtMe3X (L2 = dppe (bis(diphenylphosphino)ethane), dppbz (bis(diphenylphosphinobenzene) X = I, OAc, OPh) also showed that ligand dissociation was required prior to any C-C coupling (48-51). In this case, however, the X- group rather than the phosphine was lost to form a five-coordinate intermediate, as shown in Scheme 11. A competitive C-X reductive elimination also occurs from these complexes and involves the same five-coordinate cation (Section V. A). 

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