Coinage metals coordination numbers

Most commonly, metal ions M2+ and M3+ (M = a first transition series metal), Li+, Na+, Mg2+, Al3+, Ga3+, In3+, Tl3+, and Sn2+ form octahedral six-coordinate complexes. Linear two coordination is associated with univalent ions of the coinage metal (Cu, Ag, Au), as in Ag(NH3)2+ or AuCL Three and five coordination are not frequently encountered, since close-packing considerations tell us that tetrahedral or octahedral complex formation will normally be favored over five coordination, while three coordination requires an extraordinarily small radius ratio (Section 4.5). Coordination numbers higher than six are found among the larger transition metal ions [i.e., those at the left of the second and third transition series, as exemplified by TaFy2- and Mo(CN)g4 ] and in the lanthanides and actinides [e.g., Nd(H20)93+ as well as UC Fs3- which contains the linear uranyl unit 0=U=02+ and five fluoride ligands coordinated around the uranium(VI) in an equatorial plane]. For most of the metal complexes discussed in this book a coordination number of six may be assumed. 

Aside from the CuF 3 complex, the maximum coordination number for ions of the coinage metals is 4 the complexes of Cu(II) and Au(III) nearly always are tetracoordinated. Coordination numbers of 4 and 3 are known for the univalent ions, but coordination number 2 is much the more usual. 

The interaction of bare monovalent (alkah metals and coinage metals) and divalent (alkaline earth and zinc group metals) cations with the N7 site of piuine NA bases was examined in gas phase using MP2/6-31G(d,p)//HF/6-31G(d,p) level of theory (Burda et al. 1996). A graphical illustration of the dependence of the coordination distance on the atomic number of the cations is displayed in O Fig- 36-1. The intermolecular M-N7 distance monotonicaUy increase with the atomic numbers for the alkali metals and metals of alkaline earths (cf. O Fig. 36-1). This increase of the distances is more pronounced for the alkali metals where it exceeds 1 A. The calculated M-N7 distances for both types of metals correlate well with the known ionic radii, e.g.. Tide (1977). The influence of the relativity for the 5s and 5p electrons (Cs, Ba ) is not as pronounced as for the 5d electrons (Au, Hg ) (Pyykko 1988). 

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