Displacement approximations

Five-coordinate Ni111 complexes (89) have been prepared by oxidation of the square planar Ni11 precursor complexes [Ni(L)X] with either X2 or CuX2, and the crystal structure of the iodo derivative has been determined. The geometry at Ni is best described as square pyramidal, with the Ni atom displaced approximately 0.34 A out of the basal plane towards the apical I atom. EPR confirms the Ni111 oxidation state, in which the unpaired electron of the low-spin d1 system is situated in the dz2 orbital.308,309 In aqueous solution full dissociation of both X anions occurs, while in acetone solution dissociation is not significant. The redox couple [Nin NCN (H20)]+/ [Ni111 NCN (H20)ra]2+ in water is +0.14V (vs. SCE). 

Once again, each haem is also bound to an imidazole of a histidine residue (called the proximal imidazole) from the surrounding globin (Figure 9.5). The iron is displaced approximately 0.6 A from the Fe-N4... 

The chemistry of cluster complexes, e.g. of the sort [FeitSi, (SR) i,] 2, is of particular interest since such complexes are known to be close representations or synthetic analogues of the redox centres present in various iron-sulphur proteins. It is important to know whether the valence electrons are localized or delocalized in such complexes - in fact several studies by e.s.r., n.m.r., and, more recently, resonance Raman spectroscopy have shown that such clusters are delocalized rather than trapped-valence species. This result is linked with the most important biophysical property of iron-sulphur proteins, viz. that of electron transfer. Rapid electron transfer is possible if any consequential geometric rearrangements around the metal atom sites are small, as implied by many resonance Raman results on such cluster complexes (cf. the small-displacement approximation, which provides a basis for enhancement to fundamental but not to overtone bands) (22). Initial studies of [MSi,]2- ions (M = Mo or W) (23,24) have since been supplemented by studies of dinuclear species e.g. [(PhS)2FeS2MS2]2 (25) and cluster species... 

If one thermocouple was displaced approximately 2 mm toward the side, top, or bottom of the sample chamber, curve (2) was obtained. It can be seen that the displaced thermocouple heats up more quickly than the symmetrically located thermocouple, thus causing the deviation of the curve up to A. After the temperature difference has been established, the curve remains essentially constant for the remainder of the run. In curve ( ), it is seen that loose packing leads not only to a displacement of the curve, but also to maxima and minima, presumably due to the shift of support particles as they expand during the heating process. 

In such a way, we shall express the full displacement approximation in the rectangular region by two-dimensional spline as follows... 

A close analogue of the hydrocarbons are the fluorocarbons. The fluorine atoms are able to induce quadrupolar interactions leading to an isoenergetic double potential minimum displaced approximately 10° from the equatorial configuration. These minima are of considerable significance as they force the molecule to adopt a helical conformation and this structure is retained in the solid phase. Ultrasonic measurements combined with NMR studies of short chain fluorinated hydrocarbons confirm these theoretical predictions and indicate that facile exchange between trans minimum energy states occurs. 

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