Square block metal compounds

It is well at this stage to recall the basic premise of the VSEPR approach the electron pairs are similar in energy and repel by either simple electrostatic forces or by the Pauli exclusion principle (75). Within the d-block transition metals, this implies that the 4s, 4p, and 3d electrons should be of similar energy if the model is to work. We know that this is not true, and certainly the energy differences will be greater for metals like Sc, Ti, Zr, Zn, and Hg (27). Worse and different problems exist for the elements Rh, Ir, Pd, Pt, and Au, which exhibit the robust square planar 16-electron structure and which participate in oxidative addition and r uctive elimination reactions. Similarly, the geometry of the common low-spin square planar d compounds of Ni(II) (like Ni(DMG)2 and [Ni(CN)4] "), which do not obey the EAN rule, cannot be deduced from the VSEPR approach. 

Fig. 13 Variation in M M separation for

Pauling further extended the sp"dm hybridization approach to the d-block compounds.3 By varying the relative importance of p and d orbitals, Pauling was able to construct hybrid orbitals that rationalized the geometries and magnetic properties of many transition-metal coordination complexes. For example, the square-planar... 

The group of M. Fujita synthesised coordination cages of this shape while reacting a triazin ligand (L16-L18) with different Pd2+ or Pt2+ compounds. Two of the coordination sites in the square planar environment of these metal atoms are blocked by a coligand, like ethylene diamine, A,A,/V, /V -tetramethylethylene diamine or 2,2 -bipyridine, while the other two sites are used for coordination bonds to the C3 symmetric ligand. In the resulting tetrahedra, compounds could be... 

The pure Sr-doped 123-compound cannot be synthesized at ambient pressure without replacement of some of the other cations. The presence of new structural blocks when substituting square-planar-coordinated Cu atoms of the CuO chain plane, by octahedrally or tetrahedrally coordinated metal atoms opens the possibility to synthesize new superconductors. In the compounds with substitutions that prefer tetrahedral coordinations, M = Co (YSr2CoCu20) [7.57, 7.58], M = Al ((Yo.75Ceo.25)2(Sro.85Yo.i5)2AlCu209) and M = Ga ((Y, Ce) Sr2GaCu207+2n) oxygen-ordered superstructures in the MO plane have been encountered. 

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