Tetrahedral complexes Substitution

Strychnine, the most celebrated member of the Strychnos alkaloids, possesses a complex polycyclic structure which is assembled from only twenty-four skeletal atoms. In addition to its obvious architectural complexity, strychnine s structure contains a contiguous array of six unsymmetrically substituted tetrahedral (asymmetric) carbon atoms of which five are included within one saturated six-membered ring. The intimidating structure of the strychnine molecule elicited the following remark by Sir Robert Robinson in 1952 For its molecular size it is the most complex substance known. 5... 

Beryllium(II) is the smallest metal ion, r = 27 pm (2), and as a consequence forms predominantly tetrahedral complexes. Solution NMR (nuclear magnetic resonance) (59-61) and x-ray diffraction studies (62) show [Be(H20)4]2+ to be the solvated species in water. In the solid state, x-ray diffraction studies show [Be(H20)4]2+ to be tetrahedral (63), as do neutron diffraction (64), infrared, and Raman scattering spectroscopic studies (65). Beryllium(II) is the only tetrahedral metal ion for which a significant quantity of both solvent-exchange and ligand-substitution data are available, and accordingly it occupies a... 

The substituted tris(2-thioimidazolyl)borate ligands with l-R-2-thioimidazole groups where R = tBu and CgH4-/>-CH(CH3)2 yielded tetrahedral complexes on reaction with zinc salts. The... 

Explain why most substitution reactions involving tetrahedral complexes of transition metal ions take place rapidly. 

Explain why substitution reactions of tetrahedral complexes ofBe2+ are slow. 

Although substitution in many tetrahedral complexes is rapid, when the metal ion is Be2+ this is not the case. Explain why this is so. 

Lehn has also reported the hydrogen-bonding templated assembly of receptors based on bipyridine copper and palladium complexes [102]. A mixture of substituted bipyridines (76, 77) (see Scheme 39) with copper(I) triflate generates a mixture of tetrahedral complexes and uncoordinated ligands. 

The situation is quite different with tetrahedral complexes of Ni(0), Pd(0) and Pt(0). We might anticipate that an associative mechanism would be deterred, because of strong mutual repulsion of the entering nucleophile and the filled d orbitals of the d system. Thus a first-order rate law for substitution in Ni(0) carbonyls, and M (P(OC2H5)3)4M = Ni, (Sec. 1.4.1) Pd and Pt, as well as a positive volume of activation ( + 8 cm mol ) for the reaction of Ni(CO)4 with P(OEt)3 in heptane support an associative mechanism. 

The tetrahedral complexes of the d ° Ni(0) system undergo dissociative substitution (Table 4.15). Kinetic data are shown in Table 8.12.243 Infrared monitoring methods feature prominently in these studies (Sec. 3.9.2). 

There have been few studies of substitution in complexes of nickel(II) of stereochemistries other than octahedral. Substitution in 5-coordinated and tetrahedral complexes is discussed in Secs. 4.9 and 4.8 respectively. The enhanced lability of the nickel(II) compared with the cobalt(II) tetrahedral complex is expected from consideration of crystal field activation energies. The reverse holds with octahedral complexes (Sec. 4.8). 

The compounds of zero valent platinum Pt(Ph3P)4 and Pt(Ph3P)3 were first discovered in 1958. Tetrahedral complexes Pt(Pp3)4 and Pt(P(OEt)3)4 undergo nucleophilic substitution by a dissociative mechanism (Sec. 4.8). 

Evidence for the tetrahedral intermediate includes a Hammett p constant of+2.1 for the deacylation reaction of substituted benzoyl-chymotrypsins and the formation of tetrahedral complexes with many inhibitors, such as boronates, sulfonyl fluorides, peptide aldehydes, and peptidyl trifluoromethyl ketones. In these last the chemical shift of the imidazole proton is 18.9 ppm, indicating a good low-barrier H-bond, and the pJQ of the imidazolium is 12.1, indicating that it is stabilized by 7.3 kcal mol 1 compared to substrate-free chymotrypsin. The imidazole in effect is a much stronger base, facilitating proton removal from the serine. 

The coordination of stable silylenes to metal complexes was also reported to produce transition metal silylenoids.40,41 Exposure of unsaturated silylene 16 to Ni(CO)4 resulted in the formation of the disilylene-substituted tetrahedral nickel complex 17 (Scheme 7.3).42,43 Similarly, mixing Ph PAuCI with decamethylsilico-cene 18 produced the silylgold complex 19.44 The 29 Si H NMR spectrum of 19 (8 78 ppm) revealed its silylenoid character. In addition to nickel and gold, other metals, including tungsten,41 platinum,45 iron,40 and ruthenium,46 have been utilized to form silylmetal complexes of stable silylenes. 

In contrast to other tetrahedral complexes, those of Be2+ undergo slow substitution, usually by an SN1 process. What is the basis for this behavior ... 

The solubility of AgCl is therefore sufficient to give [Ag(NH3)2]Cl on treatment with aqueous ammonium carbonate, while AgBr dissolves only in aqueous ammonia, in which Agl is poorly soluble. Aqueous pyridine and substituted pyridines form [Ag(py)]+ and [Ag(py)2]+ ions, but in non-aqueous conditions tetrahedral complexes, such as [Ag(py)4]C104, may be obtained. The tetrahedral acetonitrile adduct [Ag(NCMe)4]+ is also known and is quite stable. There are a number of argen-tate(I) complexes, such as [Ag(NCO)2] and [Ag(ON02)2]- they are linear, with 2-coordinate Ag+.4 Linear coordination is also found in the tetrameric silver amide [Ag N(SiMe3)2 ]4.5... 

Substitution of both cyclopentadienyl ligands in nickelocene proceeds easily with mono- or bidentate ligands. With monodentate ligands tetrahedral complexes Ni(0) are formed, while with bidentate ligands (such as a,a -dipyridyl, 1,10-phenanthroline, etc.) square-planar complexes are produced (225). 

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