Complexes of group 12 Zn, Cd, Hg

Cadmium is a member of Group 12 (Zn, Cd, Hg) of the Periodic Table, having a filled d shell of electrons 4valence state of +2. In rare instances the +1 oxidation state may be produced in the form of dimeric Cd2+2 species [59458-73-0], eg, as dark red melts of Cd° dissolved in molten cadmium halides or as diamagnetic yellow solids such as (Cd2)2+ (AlCl [79110-87-5] (2). The Cd + species is unstable in water or other donor solvents, immediately disproportionating to Cd2+ and Cd. In general, cadmium compounds exhibit properties similar to the corresponding zinc compounds. Compounds and properties are listed in Table 1. Cadmium(TT) [22537 48-0] tends to favor tetrahedral coordination in its compounds, particularly in solution as complexes, eg, tetraamminecadmium(II) [18373-05-2], Cd(NH3)2+4. However, solid-state cadmium-containing oxide or halide materials frequently exhibit octahedral coordination at the Cd2+ ion, eg, the rock-salt structure found for CdO. 

The complexes show dynamic behavior, i.e. the p-tol groups change positions indicating movement of both the NYN skeleton and the H atom. The activation decreases in the order Ni" > Pd" > Pt". In the light of the structure of (12) it seems likely that in PdCl(l,3-ij-C3H5) (RNYNR)H which has been prepared from [PdCl(l,3- /-C3H5)]2 and (RNYNR)H (Y = CH, N) the triazene is also coordinated via the imine N atom to Pd and not via the amino N atom, as was proposed, Finally complexes of (RNYNR)H and Ag, Zn", Cd", Hg , Co and Rh have been reported. ... 

Several chalcogen-bridged clusters of the group 12 metals (Zn, Cd, Hg) have been synthesized utilizing silylated reagents. Mostly, the structures of these complexes are based on adamantane-like cages, and are similar to II-VI clusters synthesized by other approaches, as described in... 

X = Y = CO M = Mo, X = CO, Y=T] -allyl), the K -N,N,N coordination pattern has been observed. The complexes [M(Tm )Me] (M = Zn, Cd, Hg) have been recently reported and investigated for their ability to exchange alkyl and sulfur donor hgands between the Group 12 metals. A number of cationic Pt(IV) carbene complexes [Pt(=C(OMe)(NHR)) (Tp )(Me)2] (OTf) have been synthesized (R=Et, Pr, Pr, Bn OTf=trifluoromethanesulfonate) by methylation with MeOTf of the Pt-carboxamido precursors [Pt(C(=0)(NHR))(Tp )(Me)2]. A computational study on the mechanism of trimerization of aUcynes in the presence of a hydrotris(pyrazolyl)borate iridium catalyst [lr(Tp)(Ti -HC=CH)2] and the effect of substituent groups in R-C=C-R (R = Me or OCOMe) has been reported, demonstrating that two mechanisms for the formation of the benzene complex, including the intramolecular [4 + 2] cycloaddition and Schore mechanisms, are possible in this reaction. 

Hg and, to a less extent, Cd are soft acids and prefer to coordinate with soft base ligands. The three ions are more polarising than those of group 2. Hence their compounds deviate more from ionic character than those of group 2. Parallel to this, group 12 metals form more numerous and more stable complexes than those of group 2. However, Cd(OH)2 is more basic than the amphoteric Zn(OH>2 whereas Hg(OH>2 is only a very weak base. The chalcogenides, especially those of Zn and Cd are semiconductors and may act as photocatalysts. 

The elements Zn, Cd, and Hg follow Cu, Ag, and Au, respectively. Each has a filled ( -l)d shell plus two ns electrons. While Cu, Ag, and Au all give rise to ions or complexes in which one or even two d electrons are lost, that is to compounds in oxidation states II and III, no such compounds have ever been isolated for the Group 12 metals. Thus, while Cu, Ag, and Au are classified as transition elements, Zn, Cd, and Hg are not. For mercury it has been claimed that at -78°C [Hg cyclam](BF4)2 can be oxidized electrochemically in acetonitrile to give the [Hg cyclam]3+ ion, with a half-life of ca. 5s, and there have also been theoretical arguments that HgIV might exist, for example, in HgF4. ... 

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