Correlation nickel complexes

The IR bands in a number of nickel complexes of triaryl formazans have been assigned by Arnold and Schiele.415 A similar assignment of the electronic bands has been carried out.414 LCAO-MO calculations correlate well with these assignments417 and have been extended to include both inner ligand transitions as well as charge transfer bands and d—d transitions.418 EPR spectra have been used to study the nature of bonding in copper complexes of heterocyclic-containing formazans.419 Metal formazan complexes have also been studied by electrochemistry.283,398 420-422... 

Dale Margerum Ralph Wilkins has mentioned the interesting effect of terpyridine on the subsequent substitution reaction of the nickel complex. I would like to discuss this point—namely the effect of coordination of other ligands on the rate of substitution of the remaining coordinated water. However, before proceeding we should first focus attention on the main point of this paper-which is that a tremendous amount of kinetic data for the rate of formation of all kinds of metal complexes can be correlated with the rate of water substitution of the simple aquo metal ion. This also means that dissociation rate constants of metal complexes can be predicted from the stability constants of the complexes and the rate constant of water exchange. The data from the paper are so convincing that we can proceed to other points of discussion. 

The observed spectra of some duroquinone-nickel complexes with olefins have been correlated by means of semiquantitative molecular-orbital theory by Schrauzer and Thy ret (48). In the case of n complexes of polynuclear hydrocarbons, such as naphthalene and anthracene, although their spectra are recorded, no conclusions have been drawn with regard to structure nor has any theoretical work been reported. Similar remarks apply to complexes of nonalternant hydrocarbons such as azulene. Although innumerable complexes of olefins with various transition metals are known and admirably reviewed (84), no theoretical discussion of even a qualitative nature has been provided of their electronic spectra. A recent qualitative account of the electronic spectra of a series of cyclopentadienone, quinone, and thiophene dioxide complexes has been given by Schrauzer and Kratel (85). 

The observation (112-115) that neutral aqueous solutions of [Ni11H 3G4]2 consume molecular oxygen with the appearance of a strongly absorbing transient at 350 nm lead to detailed investigations and discovery of nickel(III)-peptide complexes (113). The oxidized nickel complexes have absorption maxima around 325 and 240 nm (e = 5240 and 11,000 M I cm-1, respectively for [NiM1H 3G4]-). Reduction potentials (116) (Table II), measured by cyclic voltammetry, show a small dependence on ligand structure which can be correlated... 

Fig. 6. Relationship between Imax and Hammett constants of substituent groups of nickel complex with a correlation coefficient of 0.985.

Fig. 24. Correlation diagram for the I.E. of allyl-nickel complexes adapted from reference 13°)...

The higher reactivity of nickel complexes than the palladium congeners was examined theoretically. Fig. 9.1 illustrates a schematic diagram for the orbital correlation on the ethane elimination from cw-M(CH3)2(PH3)2 complexes (M = Ni, Pd) [8,10]. This scheme presumes the least motion process that maintains the C2v symmetry of precursor complex throughout the reaction. The C-M-C angle becomes narrow while the P-M-P angle is gradually extended as the reductive elimination proceeds. Finally, ethane is eliminated with formation of a linear M(PH3)2 complex. 

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). 

Experimentally, it has been shown that the reductive elimination of R2 is easier for a nickel complex than for the isoelectronic palladium-containing species. This result may be readily understood from the correlation diagram in Figure 4.12. 

Phosphines and related P(III) compounds typically serve as ancillary ligands, but the dissociation of these ligands is crucial to the reactivity described in later chapters. Tolman correlated the ligand cone angle with the equilibrium for dissociation from NiL compounds.The extent of ligand dissociation in these nickel complexes and in related palladium complexes increases in the order PMej < PMe Ph < PMePlq < PEtj < PPhj < PPr j < PCyj < PPhBu, . 

Many of these nickel carbonyl-base compounds have been prepared primarily for use in infrared studies, some of the conclusions of which are summarized briefly in Section II, C 30,41,46,47,48,50,51,127,349). The phosphine-nickel complexes have catalytic activity in the polymerization of acetylenes, and the mechanisms of these polymerizations have been studied 350, 351). Interest in these catalysts has led to an investigation of their phosphorus-31 NMR spectra, which may be qualitatively correlated with the accepted ideas on metal-ligand bonding (72). 

The spectra for a series of nickel complexes of the type [L Ni(CO) ], when L = PR, PPh2Y. PPhY2, P(0R )3 and PCl have revealed a correlation of 6 C O with both IR stretching force constants and Kabachinik a-parameters [237]. The carbonyl resonance is found to be deshielded with increasing electron density at the metal (see Table 32). The order of this increase in these complexes is... 

Fig. 8. One-dimensional and two-dimensional chemical-shift correlated P-NMR spectra of Ni(a-[18]ane P402)-2Bp4 in acetonitrile. X, Designates artifact peaks at the rf carrier position. (see Fig. lb) was a 90° pulse. The nickel complex was kindly provided by Professor Mario Ciampolini.

Here we comment on the shape of certain spin-forbidden bands. Though not strictly part of the intensity story being discussed in this chapter, an understanding of so-called spin-flip transitions depends upon a perusal of correlation diagrams as did our discussion of two-electron jumps. A typical example of a spin-flip transition is shown inFig. 4-7. Unless totally obscured by a spin-allowed band, the spectra of octahedral nickel (ii) complexes display a relatively sharp spike around 13,000 cmThe spike corresponds to a spin-forbidden transition and, on comparing band areas, is not of unusual intensity for such a transition. It is so noticeable because it is so narrow - say 100 cm wide. It is broad compared with the 1-2 cm of free-ion line spectra but very narrow compared with the 2000-3000 cm of spin-allowed crystal-field bands. 

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