Ligands square planar

See also Chapter 4. The molybdenum (II) alkoxides isolated as alcohol or amine solvates of [Mo(OR)2L2] composition are the typical examples of binu-clear clusters with a quadruple metal-metal bond not supported by any bridging ligands. Square planar coordinated (not taking into account the M-M bond) metal atoms are situated exactly in front of each other, which leads to the molecules with eclipsed conformation. 

Mixed-ligand square-planar platinum(II) complexes of the type [Pt(L) (L Xmnt)] (15), where L = L = P(OPh)3, P(OEt)3, PPh3 L + L =l,5-cyclooctadiene (cod), l,2-bis(diphenylphosphino)methane (dppm) have also been investigated for their photoluminescent properties (59, 90, 91). These complexes luminesce in the solid state at 298 K and at 77 K in frozen solvent... 

With rich luminescent properties, long-lived CT excited states and a variety of bimolecular photochemical reaction pathways, the mixed-ligand square-planar diimine dithiolene complexes of d8 metal ions show great promise for solar-energy conversion, as luminescent probes or in photocatalytic applications. Complexes of this type have also received attention for the nonlinear optical properties, such as second harmonic generation, related to the MMLL CT excited state (14, 129). 

Figure 10-27(b) compares square-planar and tetrahedral structures. For strong-field ligands, square planar is preferred in all cases exeept d, d, d, and d. In those eases, the angular overlap approach predicts that the two structures are equally probable. For weak-field ligands, tetrahedral and square-planar structures also have equal energies in the d, d, and d cases. 

Design some preparation processes to make mixed-ligand square planar complexes using the trans effect. 

A trimethylplatinum(iv) complex with tris(3,5-bistrifluoromethylpyrazolyl)borate as a ligand has been prepared and studied by H-F two-dimensional NMR spectroscopy the spectra exhibit long-range G-H-F-C interaction which is evidenced also by IR spectra.1,4,7-Triazacyclononane coordinates to the Pt(ii) center as a bidentate ligand and to the Pt(iv) center as a facial tridentate ligand. Square-planar dimethylplatinum(ii) complex 1006 is in equilibrium with the octahedral hydrido (dimethyl)platinum(iv) complex 1007 in MeOH via reversible protonation of the Pt(ii)... 

The previous examples demonstrate optical isomerism in octahedral complexes. Tetrahedral complexes can also exhibit optical isomerism, but only if aU four coordination sites are occupied by different ligands. Square planar complexes do not normally exhibit optical isomerism as they are superimposable on their mirror images. 

Evans and co-workers investigated the effect of a number of -symmetric bis(oxazoline) ligands on the copper(II)-catalysed Diels-Alder reaction of an N-acyloxazolidinone with cyclopentadiene. Enantiomeric excesses of up to 99% have been reported (Scheme 3.4). Evans et al." suggested transition state assembly 3.7, with a square planar coordination environment around the central copper ion. In this scheme the dienophile should be coordinated predominantly in an cisoid fashion in... 

The effects of these ligands on the second-order rate constants for the Cu (ligand) catalysed reaction of Ic with 2 are modest In contrast, the effects on IC2 are more pronounced. The aliphatic Oramino acids induce an approximately two-fold reduction of Iv relative to for the Cu" aquo ion. For the square planar coordinated copper ions this effect is expected on the basis of statistics. The bidentate ligands block half the sites on the copper centre. 

Gold Compounds. The chemistry of nonmetallic gold is predominandy that of Au(I) and Au(III) compounds and complexes. In the former, coordination number two and linear stereochemistry are most common. The majority of known Au(III) compounds are four coordinate and have square planar configurations. In both of these common oxidation states, gold preferably bonds to large polarizable ligands and, therefore, is termed a class b metal or soft acid. 

This is of most importance in square-planar and octahedral compounds where ligands, or more specifically donor atoms, can occupy positions next to one another (cw) or opposite each other (trans) (Fig. 19.11). 

Green-yellow salts of the tetrahedral [MX4] (X = Cl, Br, I) ions can be obtained from ethanolic solutions and are well characterized. Furthermore, a whole series of adducts [MnX2L2] (X = Cl, Br, I) are known where L is an N-, P- or A -donor ligand, and both octahedral and tetrahedral stereochemistries are found. Of interest because of the possible role of manganese porphyrins in photosynthesis is [Mn (phthalocyanine)] which is square planar. The reaction of aqueous edta with MnC03 yields... 

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