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)AuC≡CPh is a molecular compound associated into chains through aurophilic contacts, while ≡ its silver analogue exists as the ionic isomer [(Me₃P)₂Ag]⁺[Ag(C≡CPh) ₂]^-. Equivalent mixtures of the two compounds (Au:Ag = 1:1) in dichloromethane yield the ionic mixed-metal compound [(Me ₃P)₂Ag]⁺[Au(C≡CPh)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 D_3d and D_2h 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(C≡CPh) ₆]^-. By crystal structure analysis, the anion was shown to have three [PhC≡CAuC≡CPh]^- anions associated via two Ag⁺ cations to give a Ag₂Au₃ core unit of quasi-D_3h symmetry. Structure and bonding in this anion have been analyzed through density functional calculations of the [Ag₂Au ₃(C≡CH)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 D_3h symmetry) via heterometallophilic contacts in which two of the three gold atoms are involved.