Metal ions divalent square-planar

J. Solvent Exchange and Ligand Substitution on Divalent Square-Planar Transition Metal Ions... 

Divalent Square-Planar Transition Metal Ions... 

The second category is the transition metal ions, all of which in Fig. 1 are six-coordinate with the exception of Pt2+ and Pd2+, which are square-planar four-coordinate (6-9). Their labilities are strongly influenced by the electronic occupancy of their d orbitals. This is illustrated by the divalent first-row transition metal ions, which should exhibit similar labilities to Zn2+ on the basis of their rM instead, however, their labilities encompass seven orders of magnitude. On a similar basis, the trivalent first-row transition metal ions might be expected to be of similar lability to Ga3+, but instead they exhibit a lability variation of 11 orders of magnitude, with Cr3 being at the... 

A statistical report of the oxygen coordination environments about divalent metal ions in minerals and other inorganic (oxidic) compounds was published (105) in 1984. The coordinations about the Cu2+ ion (VIrCu 0.73 A) shows a pronounced square planar tendency (IV sq. pi.). Quite often, however, one or two additional oxygen atoms are located perpendicular to the square plane, yielding a distorted "V"-coordination (IV + I square pyramid) or a highly distorted octahedron, "VI"-coordination (IV + II), see Figure 20. 

Metal ions in aqueous solution generally hydrolyze to form a series of mononuclear ind polynuclear hydroxy complexes. Iron(III) for example forms [FeOH] ", [Fe(OH)2], Fe(OH)3(aq), [Fe(OH)4] , [Fe2(OH)2] and probably other polynuclear species. The formation af a monohydroxy complex (equation 1) can be used as a guide to the degree of electron withdrawal by the cation. Typical pJCa values for various cations are listed in Table 2. (The pJCa values of and HjO are -1.7 and 15.7 respectively.) The tetrahedrally coordinated [Be(H20)4] ion is more acidic than expected when compared with octahedrally coordinated aqua ions of divalent metals. Each hydrogen must carry a somewhat greater portion of the positive charge than it would in an octahedral complex, and this makes removal of a proton easier. Similar considerations apply to square planar Pd (aq). Mercury(II) and tin(II) are also more acidic than would be expected. The high acidity of mercury(II) is due to the unusual stability of linear H2O—Hg—OH, while Sn (aq) has between two and three coordinated water molecules in the inner coordination sphere. ... 

Structure I requires a tetrahedral coordination around the metal and a divalent metal ion (Zn2+ or Be2+, for example, as these ions have been found before in similar systems such as [Zn(CN)2]n and [CBe2]J. Structure II may use Li+ ions, meanwhile Pt2+ may be the right choice for structures III and IV, as it often exists in a square planar arrangement. In fact, the theoretical analysis showed that compounds I-Zn2+ and II-Li+ were stable, having a tetragonal and orthorhombic unit cells, respectively (Figure 13),... 

From the reaction of divalent, first-row hexaaqua transition metal ions and MS4 (M = Mo, W), [Co(MS4)2] and [Ni(MS4)2] can be isolated, typically as quaternary phosphonium salts. The [MS4] ions act as bidentate ligands to the divalent metal ion, but the coordination geometries are different - tetrahedral for Co and square planar for Ni. These compounds model, in a structurally well-defined way, sulfide-bridged Co-Mo and Ni-Mo interactions and may serve as references for such interactions in CoMoS and NiMoS phases. 

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