Transition Met. Chem., 17, 455-457 (1992) Nitrato(6,6'-dimethyl-2,2'-bipyridine)silver(I) crystal structure 455 Crystal and molecular structure of bipyridine)silver(I). A dimer held by interactions nitrato(6,6'-dimethyl-2,2'- stacking and silver-silver Nandanavanam Venkatalakshmi, Melath V. Rajasekharan* School of Chemistry, University of Hyderabad, Hyderabad-500134, India lrimpan I. Mathews Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India Summary The crystal and molecular structure of Ag(dmbp)NO3, where dmbp=6,6'-dimethyl-2,2'-bipyridine, has been determined. The crystal consists of dimers formed by two symmetry-related molecules. Each dimerie unit is held together by (weak) Ag--Ag and heteroaromatic ring stacking interactions. Stacking interactions lead to a slip- ped stack arrangement of the dimers. The coordination around silver is very nearly planar (tetrahedral distortion angle = 15~ with the nitrate ion acting as an unsym- metrical chelating ligand. Introduction Silver(I) Complexes of 2,2'-bipyridine and related ligands have long been known ~1'2~but, with one exception (3~,have not been structurally characterised, presumably due to the difficulty in obtaining suitable single crystals. While bis- chelate coordination is more commonly found with these ligands, mono-chelate complexes are known for 2-methyl- phenanthroline ~4),2,9-dimethylphenanthroline(5~,and 6,6'- dimethylbipyridine~6~. Some of these ligands also stabi- lise the copper(I) oxidation state which has an interesting photochemistry involving charge-transfer excited states 17). The origin of the flattening distortion in the copper(I) complexes has been a subject of controversy, being vari- ously attributed to an admixture of the Jahn-Teller active charge transfer excited state with the ground state ~8~, and to crystal packing forces (9~ as well as to stacking inter- actions between heteroaromatic ligands (5~. In this paper we describe the crystal structure of Ag(dmbp)NO3 which has a near-planar geometry, with the nitrate ion acting as an unsymmetrical chelating ligand. The stacking interaction between the aromatic rings is further reinforced by silver-silver interactions leading to the formation of dimeric units in the lattice. Experimental The preparation of Ag(dmbp)NO3 has already been de- scribed ~6~. Colourless fibre-type crystals were obtained by dissolving the sample in 0.1 M HNO 3 and concentrating the solution in a desiccator over H2SO4. Crystal data AgClzH12N303, M = 354.1, monoclinic, a = 7.674(4), b = 20.022(9), c = 16.775(6)/~, fl = 96.00(4) ~ U = 2563/~3, by least squares refinement of 25 centred reflections (2 = * Author to whom all correspondence should be directed. 0.7107/~), space group C2/c, Z=8, De= 1.835gcm -3, D m= 1.80 g cm- 3. Crystal dimensions, 0.2 x 0.2 x 0.05 mm, #(Mo-K~) = t5.6cm- 1 Data collection and processin9 CAD4 diffractometer, ~o - 20 mode, graphite-monochro- mated Mo-K, radiation; 3166 reflections measured (2 < 0 < 23) giving 1055 with F o > 3a(Fo); absorption correction based on ~ scans, with transmission coefficients0.81 1.67. Structure analysis and refinement The structure was solved by Patterson and Fourier methods and refined by a full matrix least squares method. The ring hydrogen atoms were allowed to ride on the respective carbon atoms with a common isotropic thermal parameter which was refined. The methyl groups were refined as rigid groups with fixed isotropic thermal para- meters for the hydrogen atoms. The function minimised was Yw([Fo[ - I Fc 1)2;weighing scheme w- 1 = o.2(Fo), with a(Fo) from counting statistics; final R 0.122, R' 0.104; shift/e.s.d.<0.1; maximum residual electron density 3 e/~ - 3 near Ag; < 0.7 e/~ - 3 elsewhere. The high R-factor was due to the poor quality of the crystal, resulting in an overall low scattering power. The computer programs used include SHELX ~1~ and ORTEP ~Jl~. Atomic scatter- ing factors were taken from ref. 12 and corrections for anomalous scattering for Ag from ref. 13. Final atomic coordinates for non-hydrogen atoms are listed in Table 1. Selected bond distances and angles are given in Table 2. Additional material deposited with the Editor comprises thermal parameters and a complete list of bond distances and angles, observed and calculated structure factors and a packing diagram. Results and discussion" The structure consists of [Ag(dmbp)NO3] 2 units made up of two symmetry related molecules connected by a weak Ag--Ag bond. The coordination of each Ag § ion (Figure 1) is with the two nitrogen atoms of the dmbp and the two oxygen atoms of the nitrate ion. The Ag--N distances are approximately equivalent (average = 2.35 ~) and com- parable to that found in Ag(tmbp)2BF4 (2.320 fit) 13), where tmbp = 4,4',6,6'-tetramethyl-2,2'-bipyridine. The nitrate coordination is highly unsymmetrical, similar to that found in Ag2(dpm)2(NO3) 2 where dpm = bis(diphenyl- phosphino)methane 1141. The coordination polyhedron is nearly planar with a slight tetrahedral distortion (di- hedral angle between Ag, N(1), N(2) and Ag, O(1), 0(2) planes = 15~ and the two trans-bonds are nearly linear 0340-4285 9 1992 Chapman & Hall