Coordination chemistry square-planar geometries

Various mixed tridentate ligands with P,N,0/S/N donor sets have been explored in Ni11 chemistry. For example, condensation of 2-(diphenylphosphino)aniline with substituted (5-chloro-, 5-nitro, 5-bromo-, 5-methoxy-, and 3-methoxy-) salicylaldehydes yields (253).697 The deprotonated ligand coordinates through its P,N,0 donor set in a square planar geometry with some distortion, which is probably due to the bulk of the phosphine group and to the bite angle of the P,N chelate. 

The most common oxidation state of palladium is H-2 which corresponds toa electronic configuration. Compounds have square planar geometry. Other important oxidation states and electronic configurations include 0 ( °), which can have coordination numbers ranging from two to four and is important in catalytic chemistry, and +4 (eft), which is octahedral and much more strongly oxidizing than platinum (IV). The chemistry of palladium is similar to that of platinum, but palladium is between 103 to 5 x 10s more labile (192). A primary industrial application is palladium-catalyzed oxidation of ethylene (see Olefin polymers) to acetaldehyde (qv). Palladium-catalyzed carbon—carbon bond formation is an important organic reaction. 

The coordination geometry around copper(II) peptide complexes is generally tetragonally distorted octahedral, although there are some cases where square planar and square pyramidal geometries can also be found. X-ray crystal stmcture determinations have shown that copper(III) peptide complexes have square-planar geometry (see Copper Inorganic Coordination Chemistry). In this section, we discuss copper(II) peptide complexes copper(III) peptide complexes are reviewed elsewhere. ... 

The compound oo[K2PdSeio] [126] provides an intriguing extra facet of this supramolecular chemistry. There are two interpenetrating but different diamondoid lattices, one with [Pd(Se4)4/2] units and the other with [Pd(Se6)4/2p units. The local coordination at Pd is square planar, even though the Pd atoms are located at the tetrahedral connection sites of the diamondoid lattice. The conversion between tetrahedral and square planar geometries is achieved by means of the flexible Se, chains. Note the discussion in Section 8 about the cation influences on the crystallization of palladium polyselenide compounds. 

In the chemistry of group 8 and 9 metal complexes, consideration of octahedral complexes is required, whereas with group 10 metal complexes, particularly in Pd(II) complexes with tertiary phosphine ligands, the reactions proceed mostly under the constraint of square planar geometry with possible involvement of unstable five coordinate species. Thus mechanistic considerations of the reaction courses are somewhat simpler. 

We have extended this chemistry to the later transition metals in the preparation of the "homoleptic" Ni(0) complex of ligand 8. Based on its diamagnetic behavior, we propose a square planar geometry for 12. Thus our ligand complement also promotes C-F activation with the metal undergoing a dio to d transformation. The coordinatively unsaturated nature of 12 may allow for further reaction chemistry. 

The metal complexes discussed thus far bear little resemblance to the vast majority of common transition-metal complexes. Transition-metal chemistry is dominated by octahedral, square-planar, and tetrahedral coordination geometries, mixed ligand sets, and adherence to the 18-electron rule. The following three sections introduce donor-acceptor interactions that, although not unique to bonding in the d block, make the chemistry of the transition metals so distinctive. 

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