Four-fold planar coordination

Several models for the structure of the twin walls have been proposed (30, and ref. therein). The simplest of these is illustrated in Figure 4. The twin wall is formed by three consecutive diagonals. The copper atoms located on the central one have two-fold coordination with corresponding oxygen stoichiometry of x = 6.0. Those located on the two neighboring rows on each side of the central diagonal have three-fold coordination and oxygen stoichiometry x = 6.5. Within each domain, away from the boundary, the composition is x = 7.0 and copper has the usual four-fold planar coordination of... 

We might well have asked why 3d-4s hybridization on Cu1 did not lead to an egg shaped core instead of a disk shaped core. To a first approximation, these two situations would indeed to be degenerate. With hybridization to an egg shaped core, the occurrence of Cu1 in three-fold of four-fold planar coordination should be no surprise. This, of course, actually happens in the n-type copper oxides such as Nd2 xCexCu04. 

In all cases where the structure has been determined, the Au attains planar four-fold coordination and polymerizes as appropriate to achieve tliis. The halides for instance are dimeric but with the cyanide, which forms linear rather than bent bridges, tetramers are produced ... 

There are three well-established oxides of copper containing Cu and/or Cu" viz. CU2O, CU4O3 and CuO. Their structures are all based on a cubic-close packing (c.c.p.) of copper (somewhat distorted in C114O3 and CuO) with oxygen inserted into tetrahedral sites in such a way as to be a solution to the problem of having Cu in linear two-fold coordination and Cu in planar four-fold coordination. These have been discussed from this point of view elsewhere , and it has been speculated that the observed structures are, in fact, the only solutions to the problem. 

In general, the crystal chemistry of borates is similar to that of silicates differences arise from the fact that boron combines with oxygen not only in four-fold (tetrahedral) but also in three-fold (trigonal planar = triangular) coordination. As a result, silicate chemistry is considerably less complicated than borate chemistry. 

Figure 2 shows the Q band (35 GHz) powder spectrum of Co(acacen) doped into Ni(acacen) as a typical example of the EPR spectra of planar, four-fold coordinated, low-spin Co(II) Schiff base complexes. The structure, due to the three principal values of the g and A tensors is nicely resolved. Both tensors are clearly orthorhombic, with one large and two similar, smaller principal values. Powder spectra yield however no information on the orientation of the tensors with respect to the molecular frame. This information is extremely important in complexes having low molecu-... 

The spectrum in Figure 9 is characteristic for four-fold coordinated Co(II) complexes, with a planar N2O2 coordinatioi sphere independent of the detailed structure of the ligand (57, 70, 37). This shows, in contrast to the EPR spectra, that the ab-... 

The typical distortion is an elongation along one four-fold axis, so that there is a planar array of four short Cu—L bonds and two trans long ones. In the limit, of course, the elongation leads to a situation indistinguishable from square coordination as found in CuO and many discrete complexes of Cu11. Thus the cases of tetragonally distorted octahedral coordination and square coordination cannot be sharply differentiated.— —------------... 

The t/ configuration is represented by a group of very stable oxidation states. Of those that favour four-coordinate planar coordination, platinum(II) stands out because the level of reactivity places its complexes within the range of rates conveniently studied by conventional, classical techniques and because the stability of the oxidation state is high. Oxidation state (II) is even more stable for palladium but the 10 fold greater reactivity tends to restrict the scope of any study. Complexes of gold(III) are generally more labile than those of Pt but can conveniently be studied by classical techniques however, they can be reduced to Au by many of the soft nucleophiles. Closer examination has shown that, as a rule, the reduction follows, rather than accompanies, the act of substitution " and much more should be done in this area. There are cases when the reduction is direct, e.g. ... 

Allan and Mackrodt 8,iS9 gg well as Tokura, Takagi, and Uchida have studied doped lanthanum and neod)tmium copper oxide systems. Both systems, as shown in Figure 9, have planar CUO4 units, and this feature is considered essential for high temperature superconductors. The Cu atoms are six-fold coordinated in La2Cu04, whereas they are four-fold coordinated in Nd2Cu04. 

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