Unexpected Structural Preference for Aggregates with Metallophilic Ag-Au Contacts in (Trimethylphosphine)silver(I) and -gold(I) phenylethynyl complexes. An Experimental and Theoretical Study.

Me3P)AuCCPh is a molecular compound associated into chains through aurophilic contacts, while its silver analogue exists as the ionic isomer [(Me3P)2Ag]+[Ag(CCPh)2]-. Equivalent mixtures of the two compounds (Au:Ag = 1:1) in dichloromethane yield the ionic mixed-metal compound [(Me3P)2Ag]+[Au(CCPh)2]-, which is stable in solution only in the presence of an excess of the Me3P ligand. Under these conditions it can be crystallized, and in the crystals the cations and the anions (with idealized D3d and D2h symmetry, respectively) are aggregated via heterometallophilic bonding, generating linear −Ag−Au−Ag−Au− chains. The solid was found to be also unstable at room temperature, owing to a rapid loss of four out of six tertiary phosphines, which leads to a product of the composition [(Me3P)2Ag]+[Ag2Au3(CCPh)6]-. By crystal structure analysis, the anion was shown to have three [PhCCAuCCPh]- anions associated via two Ag+ cations to give a Ag2Au3 core unit of quasi-D3h symmetry. Structure and bonding in this anion have been analyzed through density functional calculations of the [Ag2Au3(CCH)6]- model and shown to be largely ionic in nature. Deviations of the experimental and calculated geometrical details could be traced to the electrostatic field in the crystal. In the unit cell, the cluster anions are associated with the [(Me3P)2Ag]+ cations (of idealized D3h symmetry) via heterometallophilic contacts in which two of the three gold atoms are involved.