The structure of the 1:1 adduct of [Rh(cod)(NH3)2][PF6] (cod = 1,5‐cyclooctadiene) and α‐cyclodextrin (α‐CD) has been elucidated by an X‐ray structure analysis. The tub‐shaped cod ligand lies almost exactly above the center of the α‐CD cavity, into which it partilly protudes. The two NH3 ligands interact with four secondary OH groups.

In the second sphere co-ordination of a cyclobutane-I ,1-dicarboxylatodiammineplatinum(ii) by a-cyclodextrin, stabilisation of the crystalline adduct is achieved by the cyclobutane ring of the guest penetrating into the cavity of the host and the ammine ligands on the guest participating in the formation of two ( N -H a -0 ) hydrogen bonds with secondary hydroxy groups a t C(3) on neighbouring a-D-glucopyranosidic units in the host.

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The second sphere coordination ['] of transition metal complexes bearing hydrophobic ligands by cyclodextrins is a relatively unexplored field.[21 Only two such supramolecular adducts have been ~haracterized'~] structurally in the solid state by X-ray crystallography. In both cases, the hydrophobic ligand of the complex penetrates the cavity of the cyclodextrin via the wider aperture of the receptor associated with the face bearing the secondary hydroxy groups. 'H-NMR spectroscopy reveals that the hydrophobic ligand also enters the cavity in solution, though unequivocal structural information regarding the preferred face[4J at which adduct formation occurs has not been obtained. A large number of transition metal complexes bearing hydrophobic trialkylphosphane ligands find applications in catalysis.[51 In addition, some are employed in the treatment of rheumatoid arthritis.[61 Our interest"' in the second sphere coordination of transition metal complexes has led us to investigate adduct formation between trialkylphosphaneplatinum complexes and cyclodextrins and their methylated derivatives in aqueous solution. We also describe the first example of a transition metal complex adduct with a cyclodextrin in which a crystalline adduct is formed as a result of occupation of the narrower primary hydroxy-group-bearing face of the receptor by the substrate. I+] Present address:metal complex, an analogous shift to higher frequency (0.104 ppm) was observed for the protons of the PMe3 ligand in the complex 1 in the presence of 1 mol equivalent of PCD. These results indicate that, in solution, the PMe3 HO A OH Fig. 1. Top: Structure of the adduct formed between @CD and 1 in the crystal viewed looking into the PCD face carrying the secondary hydroxy groups. Only the PMe3 ligand lies in the cavity of the CD above the lower rim of primary hydroxy groups. The unresolved component of the adduct which lies within a plane parallel to the secondary hydroxy-group-bearing face, and also the peripheral water molecules, have been omitted for clarity. Bottom: In the side-on view of the adduct formed between PCD and 1 in the crystal, the degree of the penetration of the PMe, ligand of 1 into the cavity of PCD is easily discernible.-Crystal data: orthorhombic, a = 19.431 (7) ligand, in common with other hydrophobic first sphere ligands, enters the cavity of the cyclodextrin. Furthermore, the associated chemical shift differences vary with the concentration of the 1 : 1 adduct in a manner that verifies the 1 : 1 stoichiometry. Quantitative treatmentl7I of the concentration dependence in D 2 0 at 22°C afforded an average value[*] for the association constant (K,) for adduct formation of 2950 mol-' kg, corresponding to a value for the free energy of adduct formation (-AGO) of 19.6 kJ mol -I. The K , value is ten times higher than those so far found13' for adduct formation between a C D and other transition metal complexes.[91 Interestingly, however, the absence of 'H-NMR chemical shift changes in the spectrum of a 1 : 1 molar mixture of 1 an...