Covalent radii of four-co-ordinate copper(I), silver(I) and gold(I): Crystal structures of [Ag(AsPh3)(4)]BF4 and [Au(AsPh3)(4)]BF4

The complexes [Ag(AsPh3)4]BF4 and [Au(AsPh3)(4)]BF4 were prepared from AgBF4 and 4 equivalents of AsPh3, and from equimolar quantities of [Au(AsPh3)Cl] and AgBF4 and 3 equivalents of AsPh3, respectively, in dichloromethane solution. Single crystals of the two compounds are isomorphous (trigonal, space group R (3) over bar, Z=6) and contain cations with a tetrahedral Ag/Au-As-4, core. From the Ag/Au-As distances measured at 199 K the covalent radii of four-co-ordinate silver(I) and gold(I) have been calculated using an accepted standard covalent radius for four-co-ordinate arsenic(III), r(As-III) = 1.20;Angstrom; r(Au-I) = 1.37 Angstrom is found to be 6%;, smaller than r(Ag-I)=1.46 Angstrom. This contraction reflects the strong influence of relativistic effects on atomic radii. From other data, r(Cu-I) for four-co-ordinate copper is estimated to be 1.29 Angstrom.