Four-coordinate complexes planar

Strontium, aquatetrakis(diacetamide)-structure, 98 Strontium, heptaaqua-dodecaiodide structure, 72 Strontium, octaaqua-stnicture, 84 Structure nomenclature, 124 Substitution reactions, 281-329 base catalyzed, 300 four-coordinate complexes planar d , 311... 

Four-coordinate complexes provide good examples of the early use of preparative methods for establishing stereochemistry. For complexes of the type [Ma2b2], where a and b are unidentate ligands, a tetrahedral structure cannot produce isomerism whereas a planar structure leads to cis and trans isomers (see below). The preparation of 2 isomers of [PtCl2(NH3)2], for instance, was taken as good evidence for their planarity. ... 

Geometry of four-coordinate complexes. Complexes in which the central metal has a coordination number of 4 may be tetrahedral or square planar. 

FIGURE 16.15 (a) Almost all six-coordinate complexes are octahedral. Four-coordinate complexes are either (b) tetrahedral or (c) square planar. 

There is one striking group of exceptions to the otherwise almost unbroken success of Kepert s approach. No model predicated solely upon the repulsions between monodentate ligands (or between bonds) can account for the planarity of some four-coordinate complexes. Yet hundreds of planar (f complexes like [Ni(CN)4] or [PtCl4] are known. Clearly, Kepert s model is to be augmented and we discuss this matter further in Chapter 7. 

The divalent Co(salen) complex (69a) is one of the most versatile and well-studied Co coordination compounds. It has a long and well-documented history and we shall not restate this here. Recent applications of (69a) as both a synthetic oxygen carrier and as a catalyst for organic transformations are described in Sections 6.1.3.1.2 and 6.1.4.1 respectively. Isotropic shifts in the HNMR spectrum of low-spin Co(salphn) (69b) were investigated in deuterated chloroform, DMF, DMSO, and pyridine.319 Solvent-dependent isotropic shifts indicate that the single unpaired electron, delocalized over the tetradentate 7r-electron system in CHCI3, is an intrinsic property of the planar four-coordinate complex. The high-spin/low-spin equilibrium of the... 

In thiosalen-type complexes the Ni11 ion usually has a four-coordinate square planar geometry.1360,1361 Complex [Ni(510)] shows a quasi-reversible reduction at —1.36 V and an irreversible... 

For the tin(II) complexes of the fluorinated Schiff bases derived from amino acids, [47] the 5119Sn chemical shifts in the range from -575 to 582 ppm suggested the four-coordinate square-planar geometry.111... 

Cquare planar complexes are generally of the low-spin d8 type. This includes the four-coordinated complexes of Ni (II), Pd(II), Pt(II), Au(III), Rh(I) and Ir(I). The best known and most extensively studied are the compounds of Pt(II). The kinetics and mechanisms of substitution reactions of these systems have been investigated in considerable detail. Studies on complexes of the other metal ions are rather limited, but the results obtained suggest that their reaction mechanism is similar to that of the Pt(II) systems. This paper briefly surveys some of the available information, and presents the current view on the mechanism of substitution reactions of square planar complexes. 

Platinum(II) complexes containing the tridentate ligands diethylenetriamine (dien) and 2,2 6, 2"-terpyridine (terpy) have provided useful substrates for mechanistic studies in substitution reactions at planar four-coordinated complexes. Moreover, there has been considerable interest in terpy complexes... 

In the same way, the existence of two isomers, yellow and orange, of Pt(NH3)2Cl2 showed that these complexes are square planar, rather than tetrahedral (Fig. 13.2) the yellow one has no electric dipole moment and therefore is the trans isomer, whereas the orange does and is cis. A tetrahedral Pt(NH3)2Cl2 would have just one isomer (with a dipole moment). Four-coordinate complexes of platinum(II), palladium(II), and gold(III) are virtually always square planar, but tetrahedral complexes such as the purple... 

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