Nickel four-coordinate planar

Nickel four-coordinated by three pyrrole nitrogens and by one 2894 extra nitrogen atom from the nitrene fragment macrocycle not planar... 

As expected for four-coordinate planar d metal centers, the nickel complexes listed in Table II are practically diamagnetic. The magnetic moment at ambient temperature ranges from 0 BM (Ni(salen)) to 0.4 BM (Ni [H4](Me)L ). ... 

The M(dioxime-BR2)2 class of complexes 120-122 with four-coordinate metal ions in a square-planar environment has attracted attention in view of possible columnar M M interactions that may result in interesting semiconducting properties in the solid state [182]. Therefore, a series of nickel(II) complexes... 

For the known nickel sites in biological systems, four-coordinate square planar, five-coordinate, and six-coordinate octahedral geometries are found.1840-1846 In general, the flexible coordination geometry of nickel causes its coordination properties in metallo-biomolecules to be critically influenced by the protein structure. 

Most four-coordinate nickel(ll) complexes are square planar. They are of red, brown and yellow color and practically aU are diamagnetic. Some examples are red bis(dimethylglyoximato)nickel(II) and the yellow tetracyanonick-elate(ll) ion, [Ni(CN)4]2-... 

In all the complexes shown in Table 9 (with one exception) the nickel atom is four-coordinated by two phosphorus atoms and by two carbon atoms in a distorted planar arrangement (24a). The plane containing the nickel and phosphorus atoms and the plane containing the nickel and the coordinated carbon atoms form a dihedral angle which varies between 4° and 27° (24b), depending on the coordinated alkene. In the [Ni(p3)(C2F4)2] complex (25) the nickel atom is five-coordinated by three phosphorus atoms of the tridentate ligand and two carbon atoms of tetrafluoroethylene. 

Most of the nickel(l) complexes are four-coordinate, either tetrahedral (phosphine and arsine complexes) or square planar (macrocyclic and dithiolene complexes), but five-coordinate complexes are also easily formed in the presence of tetradentate tripodal ligands. 

Four-coordinate d8 complexes can display a closely related electronic and geometric equilibrium between paramagnetic tetrahedral and diamagnetic planar isomers. Numerous examples are known in nickel(II) chemistry (80). In this case, as well as with the octahedral complexes described above, there is no change in the coordination number of the metal ion. 

Nickel(II) complexes display a variety of equilibria which involve spin state changes. Planar four-coordinate complexes are invariably diamagnetic. These can undergo an intramolecular isomerization to paramagnetic tetrahedral four-coordinate species. Alternatively, the planar complexes can coordinate additional ligands to form five- and six-coordinate paramagnetic complexes. The additional ligand molecules can be Lewis bases in solution, or solvent molecules, or, in par-... 

Less expected, perhaps, are results on substitution reactions of four-coordinate nickel(II) chelate complexes which occur in equilibrium between planar and tetrahedral isomers. Despite the longer bond lengths of the tetrahedral isomers, it is the planar isomers which undergo the substitution reactions (140). 

Nickel(II) is a 3d8 ion and has two unpaired electrons (5 = 1) when it is six coordinated or four coordinated pseudotetrahedrally. In the latter configuration, sharp proton NMR signals are obtained. When it is planar four coordinated nickel(II) is always low spin diamagnetic (5 = 0). Five-coordinated nickel(II) complexes can either be high (5 = 1) or low spin (5 = 0) depending on the nature of the donor atoms. 

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. 

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