Influence of hydrogen bonding on ‘soft’ coordination geometries: further examples Jonathan W. Steed a, *, Karin Johnson a , Christina Legido a , Peter C. Junk b a Department of Chemistry, King’s College London, Strand, London, WC2R 2LS, UK b School of Chemistry, Monash University, Wellington Road, Clayton, Vic. 3800, Australia Received 8 August 2002; accepted 8 October 2002 Abstract The X-ray crystal structures of five new crown ether complexes of Na  and Ag  are reported, namely: [Ag(18-crown- 6)(H 2 O)(NO 3 )] (4), [Ag(18-crown-6)(CF 3 SO 3 )] (5), [Na(18-crown-6)(H 2 O) 2 ](CF 3 SO 3 ) × /18-crown-6 (6), [Na(18-crown- 6)(MeOH) 2 ](BPh 4 )(7), [Na 2 (dibenzo-30-crown-10)(H 2 O)(MeOH)](BPh 4 ) 2 × /MeOH (8). Compounds 4 and 7 exhibit intramolecular OH /ÁÁÁ /O crown hydrogen bonds resulting in marked distortions of the complex in order to simultaneously maximise ion  /dipole interactions with the metal centre and satisfy the hydrogen bonding requirements of the system. Compound 6 exhibits intermolecular hydrogen bonding interactions to the uncomplexed crown ether, while compound 5 does not contain any hydrogen bonding protons but forms a surprising intermolecular pseudo -agostic CH 2 /ÁÁÁ /Ag interaction. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Interaction; Hydrogen bonding; Proton; Intramolecular 1. Introduction There is a growing body of work that highlights the influence that ‘crystal packing forces’ can exert on the conformational and geometric characteristics of crystal structures [1  /5]. Non-covalent interactions such as hydrogen bonds are of particular importance because of their strong and highly directional nature [6 /8]. Cases where intramolecular interactions are observed are of particular interest because intramolecular hydrogen bonds observed using diffraction techniques in the solid state are likely to persist in the solution phase because, unlike intermolecular interactions, they are not subject to solution concentration effects. The ultimate example of the importance of comparability of solution and solid state interactions is in the study of the tertiary structure of proteins and other biological molecules [9  /11]. We have published a number of studies on the influence of strong hydrogen bonding on metal coordi- nation geometry in crown ether complexes [1,3,4]. The crown ethers, with their poor hydrogen bond donor and good acceptor characteristics, coupled with the confor- mational rigidity lent by the macrocyclic ring, represent excellent model compounds for systematic studies on the competition between covalent, coordination and hydro- gen bonding interactions. In particular we have recently reported a series of non-complementary 18-crown-6 complexes of Na  such as [Na(18-crown-6)(H 2 O)X] (X /NO 3 , 1; ClO 4 , 2; ReO 4 , 3) in which the metal ion is ostensibly too small to fit the macrocyclic cavity [3]. As a result, additional intramolecular, stabilising hydrogen bonding interactions occur between the coordinated aqua ligand and the crown ether. We have also demonstrated the general importance of hydrogen bonding in crown ether complexes by an extensive survey of compounds in the Cambridge Structural Database (CSD) [5]. We now report the preparation and structures of several additional examples which strongly highlight the paramount importance of strong hydrogen bonding interactions in determining overall structure in these types of ‘soft’ system. By the term soft we understand a metal complex possessing a non-rigid * Corresponding author. Tel.:  /44-20-7848-2117; fax:  /44-20- 7848-2810. E-mail address: jon.steed@kcl.ac.uk (J.W. Steed). Polyhedron 22 (2003) 769  /774 www.elsevier.com/locate/poly 0277-5387/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0277-5387(02)01408-0