Crystal structures of AuCN and AgCN and vibrational spectroscopic studies of AuCN, AgCN, and CuCN+

The crystal structures of AuCN and AgCN have been determined by powder neutron diffraction measurements. The structure of AuCN consists of rows of linear AuCN chains parallel to [001] with alternating long Au-C = 2.06(2) and short Au-N = 1.82(2) Angstrom. The An atoms form sheets and are bonded to 6 other An atoms at a distance of 3.396(2) Angstrom, so that the local environment of each Au atom can be described as a compressed scalehedron. The Au---Au distance is within the range expected for an "aurophilic attraction" between these atoms. The AgCN structure is very similar to that for AuCN, in that it consists of rows of AgCN chains with alternating long Ag-C = 2.15(6) and short Ag-N = 1.86(5) Angstrom. The major difference is that the Ag---Ag separation within the AE sheets is noticeably longer, 3.881(5) Angstrom, so that there are no significant Ag---Ag interactions. The IR spectra of MCN show nu(CN) at 2170, 2164, 2236; nu(MC,MN) at 591, 480, 598; delta(MCN) at 326, 272, 358; and delta(NMC) = 168, 112, 224 cm(-1) for M = Cu, Ag, and An, respectively. This pattern of band positions strongly suggests that CuCN has the same infinite linear chain structure as AgCN and AuCN. Anomalies in the previously reported IR spectrum of CuCN are shown to be due to the formation of an unusual CuCN/KBr/H2O product in KBr disks, which is possibly an intercalation compound involving incorporation of KBr and H2O between the chains in the CuCN structure.