Nickel complexes thermochromic

The formation in solution of nickel complexes with TV-substituted ethylenediamines has been studied over a long period by many authors.707 Solutions of nickel(II) complexes with TV-substituted diamines often exhibit equilibria between pseudotetrahedral and pseudo-octahedral species. These equilibria are displaced towards the pseudotetrahedral species when the temperature increases. Complexes Ni(TV,TV-Et2en)2X2691 with poorly coordinating anions are thermochromic. This behaviour has been investigated by means of calorimetric and NMR studies.708,709... 

For the sake of completeness, nickel complexes with bis(thiosquaramide) ligands such as [Ni(sq— S4XI2 (sq = squaric acid) (45) are to be noted. They exhibit thermochromic behavior in pyridine solution. Their dinuclear structure is concluded from elemental analyses and spectroscopic data. Complexes such as [Ni(S20—H)2] serve as stabilizers in color photography (46a). They are assumed to exhibit six-coordinate Ni(II) centers. 

KEYWORDS liquid crystal, thermochromism, nickel complex, structure analysis... 

The complexes exhibit thermochromism as the 3dz2-4pz transition for the nickel(II) and the 4dz2-5pz transition for the palladium(II) ions are sensitive to the intermolecular separation. Therefore, there is a blue shift of these bands as the temperature is increased corresponding to an increase in the internal separation. X-ray studies for a palladium(II) complex, where n = 12, supports this premise as the intermolecular separation is 3.3 A at room temperature compared to 3.5 A at 150°C. 

Several nickel(II) complexes containing bidentate nitrogen-donor ligands are known to exhibit reversible thermochromism. For example, [Ni2Cl4Q2], where Q is shown below. Figure 11.2 ... 

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