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Paramagnetic complexes bonds

More recently [147], five-coordinate, paramagnetic complexes of stoichiometry [NiLAj] (A = Cl, Br, I, NO3, NCS, and NCSe) have been prepared from both 2-formylquinoline and 1-formylisoquinoline thiosemicarbazone, 20 and 21, respectively. Bonding of the neutral ligands is considered to be NNS. [Pg.40]

The third major mechanism is based on homolytic cleavage of the dihydrogen molecule by metal-metal (Co) bonded species or by a paramagnetic complex (equations 3 and 4)15. [Pg.993]

The case of the hexa(pyridine) nickel(II) complex has been extensively debated in the early literature of NMR of paramagnetic complexes [17-21]. The shift pattern with a-H > y-H > (i-H (Fig. 2.11 and Table 2.3) was soon recognized to be predominantly of a-type. The ligand has a a MO system which has the correct symmetry to overlap with the dx2 y2 and dz2 orbitals. However, spin polarization can induce V2 spin density in the it system. Once some unpaired spin density is in a p orbital, it spin-polarizes the electrons of the C—H a bond, thus producing a further mechanism for transferring spin density on the proton. The proton A/h value from this mechanism is proportional to the spin density on the carbon pz orbital, p , through a proportionality constant >ch ... [Pg.47]

TABLE 2.1. Values of the attenuation parameter of individual groups (yx), van der Waals contact (yv) and hydrogen bond (yi,b) for spin exchange in biradicals and paramagnetic complexes of transition metals with nitroxide ligands (see text) (Likhtenshtein 1996) Reproduced with permission. [Pg.49]

The corresponding paramagnetic complex (cod)2Fe decomposes at temperatures above -30 °C. Nevertheless, this weakly bonded Fe complex is an excellent precursor for ligand-exchange reactions. Treatment at low temperatures with a variety of tt- and a-binding hgands leads to partial or complete displacement of the cod units ... [Pg.2626]

Here is an example in which the MO model has a distinct advantage over the Lewis dot picture. B2 is found only in the gas phase solid boron is found in several very hard forms with complex bonding, primarily involving B12 icosahedra. B2 is paramagnetic. This behavior can be explained if its two highest energy electrons occupy separate it orbitals as shown. The Lewis dot model cannot account for the paramagnetic behavior of this molecule. [Pg.127]

Extensive electrochemical studies of complexes of Type I were also reported (164-166). It was ascertained that [Rh2]2+ di-isocyanide complexes are more susceptible to oxidation than their dicarbonyl analogues, and that one-electron oxidations of the di-isocyanide complexes produce stable and isolable paramagnetic [Rh2]3+ species (165). In particular, paramagnetic complexes of the type [Rh2(/i-EE )2(f-BuNC)2(p-pz )](PF6)2 (EE = dppm, dapm, or dpam Hpz = Hpz, 4-MepzH, Hdmpz, 4-BrdmpzH, or 3,4,5-BrpzH) containing the [Rh2]3 + core were obtained via controlled potential electrolysis of the parent [Rh2]2+ species [Rh20i-EE )2(f-BuNC)2(/ -pz )](PF6). The X-ray crystal structure of [Rh2(/j.-dppm)2(r-BuNC)2(/x-dmpz)](PF6)2 showed a Rh- -Rh distance of about 2.83 A, which is consistent with a bonding order of 0.5 for the [Rh2]3 + core (166). [Pg.207]

The relatively weak Ar-Br and especially Ar-I bonds would readily dissociate, giving rise to the Ni(I) paramagnetic complex and free aryl radical. This decomposition path is normally disfavored for aryl chlorides with considerably stronger Ar-Cl bonds. As a result, no Ni(I) species formed in the reactions of all chlo-roarenes studied, the only exception being [p-Me3NC6H4Cl]+. The p value of 5.4 obtained by Tsou and Kochi [33] is close to that (8.8 see above) previously reported by Foa and Cassar [32], suggesting that SET (Scheme 1) may play a certain role in some of the reactions of triphenylphosphine nickel(O) complexes with chloroarenes. It is still unclear if every reaction between any chloroarene and Ni(0) always involves the SET step. However, the excellent selectivity of the o-aryl Ni(II) complex formation from ArCl and highly reactive Ni(0) makes chloroarenes especially attractive substrates for various arylation reactions catalyzed by Ni complexes. [Pg.198]


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See also in sourсe #XX -- [ Pg.309 , Pg.310 ]




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