Structural Conversion of Sulfur-Bridged Polynuclear Complexes by Metal Replacement Reaction

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Coordinated thiolato S atoms in mononuclear Co (III) complexes have been recognized to bind with other metal ions to form S-bridged polynuclear complexes. In fact, considerable progress has been made in the chemistry of S-bridged polynuclear complexes containing [Co (aet) (en) _2] ^<2+> or fac (S) - [Co (aet) _3] units (aet=2-aminoethanethiolate, en=ethylenediamine). On the other hand, no S-bridged polynuclear complexes containing [Co (aet) _2 (en)] ^+ units have long been studied, because of the difficulty in preparation of [Co (aet) _2 (en)] ^+ used as a starting mononuclear complex. We have recently found that the facile reaction of [Ni (aet) _2] with [CoCl_2 (en) _2] ^+ gives an S-bridged CO^<III>Ni^<II>Co^<III> trinuclear complex, [Ni {Co (aet) _2 (en)} _2] ^<4+> (1), in which the central Ni^<II> atom is coordinated by four Satoms from two C_2-cis (S) - [Co (aet) _2 (en)] ^+ units. In order to better understand the chemistry of S-bridged polynuclear complexes composed of octahedra … More l thiolato units, it is desirable to investigate other S-bridged polynuclear complexes composed of [Co (aet) _2 (en)] ^+ units.In the present study, it was found that the replacement of the Ni^<II> atom in [Ni {Co (aet) _2 (en)} _2] Cl_4 is achieved by the reaction with excess CdCl_2 in water. However, the reaction product was not a corresponding S-bridged Co^<III>Cd^<II>Co^<III> trinuclear complex [Cd {Co (aet) _2 (en)} _2] ^<4+>, but a neutral dinuclear complex [CdCl_3 {Co (aet) _2 (en)}] (2), in which the Cd atom is situated in a trigonal-bipyramidal environment. When 2 was treated with a large amount of NaNO_3 in water, a cationic Co^<III>_4Cd^<II>_2 hexanuclear complex [Cd_2Cl {Co (aet) _2 (en)} _4] ^<7+> (3) was produced. In 3 each Cd^<II> atom also adopts a trigonal-bipyramidal geometry, coordinated by a bridging Cl atom besides four S atoms from the two C_2-cis (S)- [Co (aet) _2 (en)] ^+ units. Attempts to remove the bridging Cl atom from 3 by treating with a large amount of NaNO_3 in water were unsuccessful. Accordingly, it is reasonable to assume that the Co^<III>Cd^<II> polynuclear structures composed of C_2-cis (S) - [Co (aet) _2 (en)] ^+ units are stabilized by the preference of Cd (II) for the trigonal-bipyramidal geometry. The large covalent radius of the Cd^<II> atom and the small S-Cd-S bite angle restricted by the C_2-cis (S) - [Co (aet) _2 (en)] ^+ metalloligand seem to be responsible for this geometrical preference. Less