Second sphere ligands, crown ethers

The complexes discussed so far all involve direct interaction between the metal ion and the crown ether. Another important class of crown ether derivatives is that involving second sphere coordination. These compounds involve interaction between the ligands in the primary coordination sphere (such as H2O in M(H20)6"" cations), and the second sphere ligand, the crown ether, often by means of hydrogen bonds. Such a sttuctme is foimd in the [Mn(0H2)6](C104)2 adduct of 18-crown-6 (94). It is not surprising that transition metal/crown ether chemistry often involves adducts, since... 

Fig. 1 Crown ether hosts used as second-sphere ligands for transition metal complex guests...

An important property of crown ethers (also cyclodextrins) is that they can act as second coordination sphere ligands. Thus [Pt(bipy)(NH3)2]2+ gives crystalline (Pt(bipy)(NH3)2[18-C-6] 2+, where there is N—H—O bonding ammonium ions can be similarly coordinated. A cheap ether for catalysis of solid-liquid phase reactions is N(CH2CH2OCH2CH2OMe)3 known as TDA1. 

Second sphere complexes are sometimes obtained from organometallics and crown ethers. In these cases the crown ethers do not interact directly with the metal center of the organometallic compound, but rather with the ligands coordinated to the metal. The interaction usually involves hydrogen-bonding. 

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