Square Planar and Octahedral Species

Macrocyclic complexes (continued) nickel(II), 44 93-94 eatalysis, 44 119-125 configurational isomerization, 44 126 electrochemical properties, 44 112-113 electronic absorption spectra, 44 108-112 reactions, 44 118-119 square-planar and octahedral species, 44 116-118... 

E. Equilibrium Between Square-Planar and Octahedral Species in Coordinating Solvent... 

Whereas Ni(acac)2(H20)2 has a mononuclear (raws octahedral structure,1536 the complex Ni(acac)2 has been found to possess a trinuclear structure (195) in which Ni06 octahedra share faces via bridging acac groups.1534 A reddish brown compound having the same stoichiometry has been extracted from a solution of Ni(acac)2 in CS2. It is assumed that the former compound contains both square planar and octahedral species in a 1 3 ratio.1550... 

Resonance Raman spectral results on square planar and octahedral species are given in Table 7. They are similar in most aspects to those obtained for tetrahedral species. 

Table 7. Resonance Raman spectral data on square planar and octahedral species... 

Two or more species with different physical and chemical properties but the same formula are said to be isomers of one another. Complex ions can show many different kinds of isomerism, only one of which we will consider. Geometric isomers are ones that differ only in the spatial orientation of ligands around the central metal atom. Geometric isomerism is found in square planar and octahedral complexes. It cannot occur in tetrahedral complexes where all four positions are equivalent... 

Six-coordinate complexes are expected to be distorted from pure octahedral symmetry by the Jahn-Teller eEFect and this distortion is generally observed (Chapter Ml. A number of five-coordinate complexes are known, both square pyramidal and trigonal bipyramidal. Four-coordination is exemplified by square planar and tetrahedral species as well as intermediate configurations. 

T. Waters, X.-B. Wang and L.-S. Wang, Coord. Chem. Rev., 2007, 251, 474. Review describing the use of electrospray to transfer negatively charged transition metal complexes to the gas phase for photoelectron spectroscopy. Examples discussed include square planar and octahedral halide complexes, metal-metal bonded species, transition metal bis(diihiolene) complexes, and mononuclear and polynuclear iron-sulfur clusters. 

Much information has been gained on the mechanism of C-H bond-forming reductive elimination (see Equation 8.9). In addition to creating an understanding of C-H bond formation, this information has been used to understand the mechanism of the opposite reaction, the oxidative addition of C-H bonds. Because reductive eliminations of alkanes are faster from three- and five-coordinate species than from four- and six-coordinate species, square planar and octahedral complexes often dissociate or associate a dative ligand prior to reductive elimination. However, elimination to form a C-H bond from a four- or six-coordinate complex can also be fast enough that it occurs directly from the alkylmetal-hydride complexes prior to ligand dissociation. 

Another interesting isomerization process can take place between square planar and octahedral systems in coordination compounds. 16.45 shows how the z lx -y separation changes as two trans ligands are brought closer to the square plane. When AE is small enough then a high spin d species is formed, as... 

Se3Bri3- < > SeCls , TeClj-, TeCle ", etc.< > The anion structures are much as expected with the Se species featuring square planar (pseudo-octahedral) units, and the trinuclear Se " anions as in the tellurium analogue above. See also p. 776. There are, in addition, a fascinating series of bromoselenate(II) dianions based on fused planar SeBr4 units, e.g. Se3Brg ", Se4Bri4 ,... 

Organometallic compounds of rhodium have the metal center in oxidation states ranging from +4 to -3. but the most common oxidation states are +1 and +3. The Rh(I) species have a d electron configuration and both four coordinated square planar and five coordinated trigonal bipyramidal species exist. Oxidative addition reactions to Rh(I) form Rh(III) species with octahedral geometry. The oxidative addition is reversible in many cases, and this makes catalytic transformations of organic compounds possible. Presented here are important reactions of rhodium complexes in catalytic and stoichiometric transformations of organic compounds. 

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