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 R3, Z = 6) and contain cations with a tetrahedral Ag/Au-As₄ 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 Å; r(Au^I) = 1.37 Å is found to be 6% smaller than r(Ag^I) = 1.46 Å. 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 Å.