Cadmium complexes, five coordinated

Biirgi studied also a series of five coordinated cadmium complexes, 38, that contain three equatorial sulfur ligands, but in which the fourth and fifth, axial ligands, X and Y, are sometimes iodine, sometimes sulfur, and sometimes oxygen (84). The structural correlations have a clear interpretation in terms of the ligand exchange reaction and are reminiscent of the kind of process that is believed to occur in S 2-type nucleophilic substitution reactions ... 

N3CI plane (207). The other halids and the cadmium complexes are isomor-phous with the appropriate forms, and undoubtedly possess related five-coordinate structures. A number of studies of the vibrational spectra of the [M(terpy)X2] species have been described, all of which support the formulation as isomorphous five-coordinate complexes (124, 171, 225, 371). The copper(II) complexes [Cu(terpy)Cl2] are isostructural, and a number of studies of the paramagnetic species doped into a host matrix of [Zn(terpy)Cl2] have been reported (23, 213). The zinc complex [Zn(terpy)Cl2] exhibits absorption maxima at 22,650 and 18,000 cm (23, 348). The mercury(II) halide adducts are not so well characterized, but may be prepared by the direct reaction or HgX2 with terpy (171) or by trans-metallation of [Ph2Sn(terpy)Cl2] with HgCl2 (471). They are thought to possess similar, five-coordinate structures. The structures of the 1 1 adducts of the nitrates M(N03)2 terpy are not known with any certainty (171,328). A Cd NMR study of Cd(N03)2 terpy has been reported (430). 

Support for the above conclusions was also obtained from an analysis of the Cd NMR and solid state MAS spectra of the five, six, and seven coordinate cadmium texaphyrin complexes 116, 156, and 157, respectively [70]. From the solid-state MAS results, a single tensor was observed for the five-coordinate complex 116 with a corresponding isotropic chemical shift of 194 ppm. The MAS spectrum of a complex prepared in the presence of pyridine, however, revealed two tensors, presumed to be due to a mixture of six and seven coordinate species. Based upon the isotropic chemical shifts and the symmetry of the tensors the six coordinate species was assigned to the isotropic shift at 188 ppm and the seven coordinate species assigned to the isotropic shift at 221 ppm. The MAS spectrum of the benzimidazole complex of cadmium texaphyrin consists of only a single... 

No compound with five-coordinate carbon as the central atom has yet been crystallized. Burgi, however, has studied a series of trigonal bipyramidal complexes of divalent cadmium with three equatorial sulfur ligands and with axial ligands iodide, hydroxyl, or sulfur groups ( Y and X). 

The sodium salt of Bm has been prepared and employed to obtain homoleptic complexes [M(Bm)2] (M = Zn, Cd, Hg). [Cd(Bm)2] shows a distorted tetrahedral [MS4] central core with two weak vicinal M H-B bonds.61 [Cd(SR)(Bm)] (R = Ph, p-Tol, C6F5) have been synthesized by an analogous procedure. [Cd(S-p-C TUMehBm)] exhibits a dimeric structure in the solid state, in which each five-coordinate cadmium atom displays a trigonal-bipyramidal geometry.62... 

Fig. 5.1. Pathways of the 8, 2 reaction H + CH4. 1 Calculated by the gradient method [19] 2 calculated in the reaction coordinate regime by varying V2 with the rest of parameters optimized 3 according to the data of an X-ray structural mapping of the five-coordinate complexes of cadmium XI as described in Refs. [27,32]. The PRDDO and STO-3G methods yield consistent results...

In the second example, O Brien et al. (1849) prepared [M S2CN(Me) CH2CH2CH2NMe2 2] (M = Zn, Cd) from trimethylpropane-1,3-diamine. The zinc complex 483 is polymeric. Each five-coordinate metal center carries two chelating dithiocarbamates and are linked via coordination of a pendant dimethylamino group. Interestingly, the analogous cadmium complex adopts a dimeric centrosymmetric structure in the solid state. Although these examples... 

The cadmium(ll) chelate of the tripodal tetradentate ligand tren also yielded the mixed-ligand complex cation of the salt [Cd(tren)(xan)]C104 H20 [29], where the metal ion is only five-coordinated and the anionic xan ligand forms the Cd-N7 bond (2.224 A) reinforced by the intramolecular interligand H-bond (tren)N-H" 06 (2.889 A, 117.6°). In the compotmd traui-[Cd(theo)2(H20)4]-2DMF [30], each theophilinate(l-) anion binds the cadmium(ll) center by the Cd-N7 bond (2.300 A) in cooperation with an intramolecular H-bond (aqua)0-H - 06 (2.689 A, 158.5°). 

The ligand 6,13-dimethyl-l,4,8,ll-tetra-azacyclotetradecane-6,13-diamine coordinates as a hexadentate ligand to zinc in neutral aqueous solution. Potentiometric titrations were used to determine the stability constant for formation. The pXa values were determined for five of the six possible protonation steps of the hexamine (2.9, 5.5, 6.3, 9.9 and 11.0).697 Studies of the syn and anti isomers of 6,13-dimethyl-1,4,8, ll-tetraazacyclotetradecane-6,13-diamine reveal that they offer different shapes for metal binding, which is reflected in the stability constants for 1 1 zinc ligand ratio complexes. The selectivity of binding to the zinc ion compared to the cadmium(II) ion by both isomers is significant.698... 

A series of complexes of the type ML(SCN) (C104)2-, (M - Zn, Cd, or Hg x = 1 or 2) has been prepared where L is en and its tetramethyl derivative, diethylenetriamine and its pentamethyl derivative, triethylenetetramine and its hexamethyl derivative, and bis(ethylenediamine). The complexes are either monomeric with four-, five- or six-coordinate metal, or polymeric containing bridging thiocyanate the perchlorate is always ionic. The thiocyanate is generally bonded through nitrogen to zinc and cadmium (and through sulfur to mercury).181... 

Using a different metal salt, Cd(NOj)2, instead of CdClj oxidation reaction results in a slightly higher yield of the cadmium texaphyrin complex [60, 65]. However, upon purification, the product is obtained as a mixture of crystalline and non-crystalline solids. A single crystal X-ray diffraction study of the crystalline portion of the sample gave an unexpected result. The structure obtained (Fig. 20) revealed a six-coordinate pentagonal pyramidal cadmium (II) complex 156 (c.f. Scheme 20) where one of the two possible axial ligation sites is occupied by a benzimidazole [65]. The five donor atoms of the pentadentate texaphyrin macrocycle complete the coordination sphere about the cadmium with the cadmium... 

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