Nickel complexes four-coordinate structures

Copper and nickel complexes of the tridentate l-(2-carboxyphenyl)-3,5-diphenyl- (169 X = C02 R = R = Ph) and 1-(2-hydroxyphenyl)-3,5-diphenyl-(169 X = 0 R = R = Ph) formazans were prepared118 by the interaction of the formazan and the appropriate metal acetate in alcohol and were assigned the three-coordinate structures (170 X = O, C02 R = R = Ph M = Ni, Cu) since the diamagnetic nickel complexes were found to be unimolecular in benzene solution. Treatment of the nickel complex (170 X = O, R = R = Ph M = Ni) with pyridine gave a violet crystalline adduct which was assigned the four-coordinate structure (171 X = O R = R = Ph M = Ni). A product similar to the latter could not be obtained from the nickel complex of l-(2-carboxyphenyl)-3,5-diphenylformazan but nickel complexes of this type were obtained from both l-(2-hydroxyphenyl)- (169 X = O, R = CN R = Ph) and l-(2-carboxyphenyl)- (169 X = C02 R = CN R = Ph) 3-cyano-5-phenylformazans. In all three cases a considerable shade change occurred on going from the three-coordinate complex to the pyridine adduct. 

ESMS has been used to characterize the intermediate Nin-complexes formed in the coupling reaction of 2-bromo-6-methylpyridine in the presence of Raney nickel (Scheme 1) [45]. The composition of the intermediate had already been determined previously by elemental analysis, but the ES mass spectra, showing a strong peak for the ion [Ni2(dmbp)2Br3]+, pointed to a dimeric structure. It was concluded that this ion was formed by the loss of Br from the dimeric structure 1. An alternative explanation is that the intermediate has the more common four-coordinate structure 2, and that the observed peak was due to the ion-paired species [Ni2(dmbp)2Br2]2++Br. The dimeric nature of the intermediate was confirmed by a cross experiment when mixtures of differently substituted pyridines were reacted, mixed ligand dinickel species were observed in the ES mass spectra. 

Thus the Berry coordinate represents a viable option for intramolecular exchange in rhodium and iridium complexes, in contrast to platinum and palladium complexes. Nickel complexes, on the other hand, can adopt either tetrahedral or square-planar conformations in the four-coordinate structures, and therefore the fact that these complexes can take on any of the three conformations is not surprising. This analysis is described in detail in Reference 67. 

In nickel and palladium dithiobenzoato complexes, four-membered chelate rings are formed (366), whereas, in the corresponding phenyl-dithio acetates [M2(S2CCH2Ph)4], the dithio ligands act as bridging groups between the two metal atoms, with the formation of binuclear units (367). The molecular structure of the latter compounds shows that each metal atom is coordinated to four sulfur atoms and to the other metal atoms in a square-pyramidal geometry. Other evidence for... 

In contrast, Ni[pz(iV-Me2)8] C60 (lOOd) has a totally different supramolecular structure featuring a noncentrosymmetric 1 1 complex with a strongly warped pz unit. The plane of diametrically opposite pyrrole rings is being rotated by -30° with respect to each other about their N—N vectors with the nickel and the four coordinated nitrogen atoms are coplanar to within 0.006 A. Each C60 unit lies within a cleft formed by two steeply inclined (by -12°) Ni-pz molecules (Fig. 33), the shortest Ni—C (C60) distances are 3.29 and 3.64 A. 

Most of the nickel compounds in the solid state and almost all in aqueous solution contain the metal in the oxidation state +2, which, by consequence, can be considered the ordinary oxidation state for nickel in its compounds. The electronic structure and stereochemistry of nickel(II) were reviewed in 1968.6 The most stable electronic configuration of the free Ni ion is [Ar]3d8 which is also the ground state configuration in its complexes. The overwhelming majority of nickel(II) complexes have coordination numbers of four, five and six. Complexes with coordination numbers of three, seven and eight are still quite rare. 

Relevant structural data for selected mixed phosphine carbonyl complexes are shown in Table 5. In all these complexes the nickel(O) atom is four-coordinate in a pseudotetrahedral geometry with the Ni—CO linkage essentially linear, the Ni—C—O angles being in the range 173-178°. In complex (14) the np3 ligand bonds through the three phosphorus atoms and the r. kel is in a pseudotetrahedral environment.103... 

In contrast to the cyanides, other complexes of nickel with a coordination number of four, e,g, [Ni(NH3 J2+S04 , show paramagnetism corresponding to one or two unpaired electrons, which has been regarded as evidence for a tetrahedral structure of the complex ion. This has not been confirmed experimentally. 

Many (although not all) spectroscopic data on metal-substituted derivatives and their binary and ternary complexes have also been interpreted as indicative of a four-coordinate metal.Even nickel(II) and copper(II), which have little tendency to adapt to a pseudotetrahedral ligand environment, do so in LADH, the electronic structure of the latter resembling that of blue proteins (Figure 2.36). ... 

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