Four-coordinate complexes

Iron hahdes react with haHde salts to afford anionic haHde complexes. Because kon(III) is a hard acid, the complexes that it forms are most stable with F and decrease ki both coordination number and stabiHty with heavier haHdes. No stable F complexes are known. [FeF (H20)] is the predominant kon fluoride species ki aqueous solution. The [FeF ] ion can be prepared ki fused salts. Whereas six-coordinate [FeCy is known, four-coordinate complexes are favored for chloride. Salts of tetrahedral [FeCfy] can be isolated if large cations such as tetraphenfyarsonium or tetra alkylammonium are used. [FeBrJ is known but is thermally unstable and disproportionates to kon(II) and bromine. Complex anions of kon(II) hahdes are less common. [FeCfy] has been obtained from FeCfy by reaction with alkaH metal chlorides ki the melt or with tetraethyl ammonium chloride ki deoxygenated ethanol. 

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. 

Four-coordinate complexes exhibit lower isomer shifts than six-coordinate compounds. Metal-ligand bonds are shorter and more covalent if the coordination number is smaller because of less steric hindrance and less overlap with antibonding 2g orbitals in the case of four as compared to six bonds. 

In the four-coordinate complexes (12) (X = Cl, Br) the phenol phosphine is coordinated as a monodentate.151 In the presence of base, the phenolate O atom deprotonates and coordinates to give (13). The five-membered chelate ring in this complex is resistant to protonation, and ringopening is not observed even in the presence of CO or C2H4. 

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... 

Tri-ra-butylphosphite, tri-ra-butylarsine, and tri-ra-butylstibine complexes of platinum(O) have been prepared by the reaction of the ligand with pt(COD)2] or, in the case of the phosphite, by reaction with [Pt(PPh3)4].38 All form four-coordinate complexes when 4 mole-equivalents of ligand are used, but when 2 or 3 mole-equivalents of the phosphite ligand are used, mixtures of three- and four-coordinate complexes are observed.38... 

Four-coordinate complexes with monodentate ligands have also been reported. The X-ray crystal structures of three modifications of the compound [Au(PPh3)4]BPh4 have been determined, none of which shows the expected simple tetrahedral geometry.2540 Thus, the chloroform solvate... 

The 9Be NMR spectroscopy has been used in determination of the chiral binaphthyldiimine Be(II) complex [49].118 The measured 8Be value of 4.84 ppm and the line width of 40 Hz indicated the presence of a four-coordinate complex. The calculated 9Be NMR parameters were in good agreement with experimental results. 

Mononuclear, four-coordinate complexes [(Ls)Cun(NEt3)], [(Ls)Cun(NHEt2)] and [(Ls)Cun(H2N-CH2-Ph)] have been isolated and crystallographically characterized (216, 217). 

The NMR spectra of the two-coordinate stannylenes in solution show values of Sn ranging from about 1150 (e.g., in ArSnl) to 3750 (in (Ar3Sn)Sn ), with a large anisotropy. The stannylenes behave as Lewis acids, for example, in the three- or four-coordinate complexes (e.g., 78, 79, and 80), which are formed when the molecule carries an intramolecular ligand, and as Lewis bases, particularly in complexing to transition metals (e.g., 81, 82, and 83). The dimerization of stannylenes to give distannenes can be regarded as a result of this amphoteric character (Equation (179)). 

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