Nickel complexes, paramagnetism

TC-Cyclopentadienyl Nickel Complexes. Nickel bromide dimethoxyethane [29823-39-9] forms bis(cydopentadienyl)nickel [1271 -28-9] upon reaction with sodium cyclopentadienide (63). This complex, known as nickelocene, 7T-(C3H3)2Ni, is an emerald-green crystalline sandwich compound, mp 173°C, density 1.47 g/cm. It is paramagnetic and slowly oxidi2es in air. A number of derivatives of nickelocene are known, eg, methylnickelocene [1292-95-4], which is green and has mp 37°C, and bis( 7t-indenyl)nickel [52409-46-8], which is red, mp 150°C (87,88). 

A cyclization reaction involving a half-formed bridge in which alkyl halide functions interact with (initially) coordinated oxygen atoms is illustrated by [2.9] (Kluiber Sasso, 1970). The X-ray structure of the red paramagnetic nickel complex (65) indicates that the macrocycle coordi-... 

The reaction of bis(salicylaldehydato)nickel(II)-2H20 with /w-xylenebis-2-(l,3-propanediamine) in EtOH results in the formation of the dinuclear complex (365).2621 This complex undergoes a quasi-reversible two-electron reduction at -1.47 V vs. SCE attributable to the formation of the corresponding dinuclear Ni1 complex. EPR measurements do not indicate any interaction between the two nickel(I) paramagnetic centres. The dinuclear nickel(I) complex forms adducts with CO and MeCN. 

A minor change in the nature of the ligands sometimes produces a great change in the nature of a complex. Thus, if R" is —(CH2)2— in the branched quadridentate ligand (6), the nickel complex is a dimer in which each half is planar and diamagnetic, but if R" is —(CH2)3— the complex is polymeric and paramagnetic each nickel atom is at the center of a tetrahedron.23... 

The nickel complexes, like those of monothio-jS-diketones, are diamagnetic. The Co11 complex of dithioacetylacetone is low-spin Ip., 2.35 BM) and square-planar the paramagnetic anisotropy and ESR spectrum indicate the ground-state configuration (dxy f (dxz)2 )2(dz2). 249,250 The complexes... 

Thus, the historical development of the chemistry of metallocorrolates until 1980 includes complexes with Cu2+, Ni2+, Pd2+, Fe3+, Co3+, Rh+, Mo5+ and Cr5+. The palladium complex has been isolated as its pyridinium salt since the neutral species was too unstable to be isolated or spectroscopically characterized [19]. The nickel complex was non-aromatic, with one of the potentially tautomeric hydrogens displaced from nitrogen to carbon in such a way as to interrupt the chromophore. In contrast the electronic spectrum of the paramagnetic copper complex is similar to those of the fully conjugated lV(21)-methyl derivatives [11],... 

Many spectroscopic models have focussed on reproducing the unusual EPR properties of the different paramagnetic states of the active sites of the hydrogenase enzymes. Model chemistry should be instrumental in determining the coordination geometry and oxidation state of the paramagnetic center. As we mentioned above, the nickel ion has been implicated as the source of the EPR signals of the active site and model ehemistry has focussed on monomeric nickel complexes. 

Synthesis of macrobicyclic MDma(BR)2 complexes (where M is nickel (paramagnetic), iron and cobalt ions R is CeHs and re-C4H9) was reported by Umland and coworkers [38]. However, the formation of nickel complexes of this type was not further confirmed. 

Today, a number of one- and two-dimensional NMR experiments are available for the detection of homonuclear Li, Li and Li, Li couplings. Aside from the COSY experiment, the double quantum filtered COSY (COSY-DQF), the TOCSY, and the ID and 2D INADEQUATE experiments [24] have been successfully employed. An attractive feature of all these experiments is their sensitivity for small scalar interactions which give rise to crosspeaks even if line splittings in the corresponding ID spectra are not resolved. This was first demonstrated with COSY experiments for a paramagnetic nickel complex [82] and for quadrupolar nuclei in the case of boron-11 [83]. 

In a discrete molecule in solution, the unpaired electrons act totally independently and are randomly oriented. For example, divalent nickel in both an octahedral and square-planar ligand field is shown in Figure 5.18. Random orientation of the unpaired electrons and their associated magnetic fields, as in the case of octahedral nickel, is known as paramagnetism When all the electrons are paired, as in the case of the square-planar nickel complex, the material is called diamagnetic. 

Owen and Thomley (547) have reviewed covalency in transition metal ions and, in particular, in nickel complexes. The NMR contact shift method has been used by Eaton et al. (195) to determine spin densities on organic ligands of paramagnetic molecules. In particular (194), a series of nickel aminotroponeiminates have been studied whereby conjugative and hyperconjugative effects within the molecule may be monitored. Similar studies of contact shifts have been carried out on Co and Ni pyrromethenes and porphyrins (196) and on many transition metal acetylacetonates (193) by Eaton et al. 

Nickel complexes of this ligand have been prepared. The complexes (152) have been prepared. When R = Bu , the complex is paramagnetic and tetrahedral, the others being square-planar. Schiff bases derived from hydrazine-... 

In the late 1950 s evidence began to appear that called into question the presumed tetrahedral structure of Ni(acac)2. About 1959 Cotton and his associates 37) found or noted in the literature that the magnetic moments of Ni(acac) 2 and its dihydrate adduct were virtually identical that the visible spectra of Ni(acac)2 and Ni(acac)2 -2 H2O were similar 38) and that the spectral and magnetic data did not correlate with data for known tetrahedral nickel complexes. Thus, they concluded 37) that it is clear that the latter (i. e., tetrahedrally coordinated nickel) does not occur in Ni(acac)z under any known conditions. About this same time Bullen, Mason, and Pauling 39) reported the X-ray crystal structure of Ni(acac) 2. A trimeric structure was found and is illustrated in Fig. 1J. Thus Ni(acac) 2 is octahedral and paramagnetic which explains its spectral and magnetic similarities to the dihydrate known to be octahedrally coordinated by isomorphism with Co(acac)2 2 HaO 40) whose octahedral structmre was determined by single crystal X-ray analysis 41). The trimeric structure was also found to exist in benzene solution from cryoscopic measurements 42). 

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