Five coordinate silver

More recently, the crystal structures of several silver sulfonates have been determined those containing methanesulfonate,332 bromomethanesulfonate333 and pyridinesulfonate.334 In the first of these, no distinct molecule of the silver salt was found in the unit cell. The silver atom was five-coordinate with a very distorted trigonal bipyramidal arrangement. Ag—O bond distances were in the range 234-263 pm. Each silver was bound to five different methanesulfonates.332... 

Reaction of the Schiff base ligand A,Ar -bis(o-diphenylphosphinobenzylidene)(ethylene-diamine (49 en=P2) with AgBF4 produced a pale yellow salt. The IR spectrum of this complex showed strong bands due to the imino and BF4 group (v(C=N) 1653 cm-1, v(BFj) 1080 cm-1). The crystal structure of the Cu1 analogue was reported and the copper ion was found to adopt a severely distorted tetrahedral geometry. This strain was manifested in its reactivity since both the copper and silver complex reacted with f-butyl isocyanide. In the case of silver(I) a five-coordinate adduct was obtained, [Ag(en=P2)(Bu NC)]BF4.396... 

Figure 11.1. Schematic representation of typical silver (central atom) coordination environments with different coordination numbers and geometries (a) linear two-coordinate (b) triangular three-coordinate (c) T-shaped three-coordinate (d) square planar four-coordinate (e) tetrahedral four-coordinate (f) trigonal bipyramidal five-coordinate (g) tetragonal pyramidal five-coordinate (h) octahedral six-coordinate (i) trigonal prism six-coordinate (j) seven-coordinate (k) tetragonal prism eight-coordinate.

In terms of its coordination chemistry, the silver(I) ion is typically characterized as soft . Although originally believed to only bind ligands in a linear arrangement, it was soon shown that it can adopt a variety of coordination environments, the most common one being a four-coordinate tetrahedral geometry. Square-planar complexes are not rare, and various silver(I) cluster complexes also contain three-and five-coordinate sUver(I) ions. 

On addition of AgN03 to a solution of 16, 17 or 18 in acetonitrile, the d-d transition at ca. 440 nm showed a hypsochromic shift and a decrease in absorbance indicative of 1 1 complex formation [38]. An X-ray crystal structure of the 1 1 silver perchlorate complex of 17 revealed a five-coordinate Ag" ion bound within a distorted square pyramid to four sulfur atoms and a perchlorate ligand [39]. 

An alternative mechanism has been proposed for insertion of ethylene into PtHCl(PEt3)2 involving the intermediacy of a four-coordinate ionic species rather than a five-coordinate covalent one (97). The insertion reaction, which is represented by the forward reaction in Eq. (48), can be accelerated by addition of 1 mole %, of tin(II) chloride, since under these conditions equilibrium is established within 30 minutes at 25° and 1 atm (122). The basis of the suggestion of a four-coordinate cationic intermediate arose from the observation of the similar reaction shown in Eq. (56). In this study it was found that the insertion reaction proceeded rapidly when PtHBr(PPh2Me)2 is treated with ethylene and silver fluorophosphate in acetone as solvent, provided an added base is present. 

Interestingly most of the catalytic systems based on Tp M units contain a coinage metal such as copper or silver. This is also observed for NHC-based catalytic systems, with the addition of gold to those metals. This similarity can be explained in terms of a common feature of both Tp M and (NHC)M moieties with those metals they leave just one coordination site for the catalytic reaction to occur. A second coordination site may also accessible in some cases, for example with Cu(ll)-based systems where five-coordinate geometries are available. In the case of (NHC)M systems (M = Cu, Ag, Au), the linear complexes can accept an incoming ligand (reactant) through the transient formation of three-coordinate intermediates (Scheme 2). 

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