A reaction of [Pt-4(CH3COO)(8)], with an excess of diethyldithiophosphate (Et(2)dtpH) in CH3CN affords a tetrasubstituted derivative, [Pt-4(CH3COO)(4)(Et(2)dtp)(4)], which is a mixture of two isomers. X-ray analyses of the isolated two isomers revealed that Et(2)dtp ligands in both isomers are coordinated in the plane of the Pt-4 cluster core, and that coordination mode of Et(2)dtp is the chelate type in one isomer (2) whereas it is the bridge type in the other (3)H-1 NMR studies showed that both 2 and 3 isomerize slowly in solution to give the same equilibrium mixture. The rate constants for the isomerization in CDCl3 at 40 degrees C were k(1) = 1.2 x 10(-4) s(-1) (for the reaction from 2 to 3) and k(2) = 1.5 x 10(-4) s(-1) (for the reaction from 3 to 2), respectively. Both 2 and 3 undergo the isomerization also in acetonitrile with similar reaction rate, but the mechanism is different from that in chloroform. Activation entropy for the reaction starting from 2 to 3 was -9 +/- 9 J mol(-1) K-1 in chloroform and -47 +/- 11 J mol(-1) K-1 in acetonitrile. The small \Delta S-double dagger\ value for the reaction in chloroform suggests that the isomerization proceeds without bond cleavage through an intermediate when three sulfur atoms from two Et(2)dtp's are coordinated to each platinum. That is, the isomerization in chloroform is caused by rotation of the Pt cluster core within an array of eight S atoms arranged circularly in a single plane. On the other hand, the isomerization in acetonitrile proceeds via a Pt-S bond cleaved intermediate possibly by the solvent assisted mechanism. All the kinetic parameters for the reactions in CDCl3 and CD3CN are reported.