THF solution the dimanganese complex Mn-2(mu-H)(mu-PCyH)(CO)(8) (1) is deprotonated by excess DBU (1,8-diazabicyclo[5.4.0]undecen-7-en) to an anionic species which on addition of 2 equiv of ClAuPPh(3) at 20 degrees C in 1 h gives the isomers Mn-2(AuPPh(3))(2)(mu(4)-PCy)(CO)(8) (2) and Mn-2(mu-AuPPh(3))(mu(3)-PCy(AuPPh(3)))(CO)(8) (3). From the dissolved mixture of isomers the thermodynamically more preferred isomer 2 was crystallized and identified by a single crystal X-ray analysis, whereas 3 was characterized by P-31 NMR data. A solution of 2 gives rise to an equilibrium mixture of 2 and 3 in the chosen solvents. The factors influencing the equilibrium position for the enantiotropically convertible mu(3)/mu(4)-bridged isomers were determined by P-31 NMR measurements. The variation of the reagent ClAuPR(3) with R = Ph, p-C(6)H(4)OMe, p-C6H4F, Cy, Et, and (CH2)(2)CN and the solvent in the aforementioned reaction system showed that the equilibrium between the isomers Mn-2(AuPR(3))(2)(mu(4)-PCy)(CO)(8)/ Mn-2(mu-AUPR(3))(mu 3-PCY(AuPR(3))) (CO)(8) depends on electronic and/or steric factors and on the polarity of the solvent. The mu(3)-bridged isomer 3 is favored py a +I effect of the R groups, and the mu(4)-bridged isomer 2 by a +M effect of such groups. In spite of the +I effect of Cy in PCY3 it was possible to separate a mu(3)-bridged isomer, Mn-2-(mu-AuPCy(3))(mu(3)-PCy(AuPCy(3)))(CO)(8) (5), due to a bigger steric strain in the related (mu(4)-P) bridged isomer, The molecular structure of 5 was ascertained by an X-ray analysis. The structure of 5 can be derived from 1 by the exchange of both hydrogen atoms for AuPCy(3) groups as substituents. The molecular structure shows a typical edge-sharing coordination bioctahedron with a Mn-Mn bond length of 3.041(2) Angstrom. From 5 to the other isomer 2 one of the Au-Mn bonds converts to an Au-Au bond and the additional generation of an Au-P bond leads to a mu(4)-bridged P atom, Combined with these alterations the strongly distorted Mn(2)AU(2)(mu(4)-P) core is formed which has trans-positioned an opened and closed Mn-Au edge, The inequivalence of the edges of 2 in the solid disappears in solution. On the time scale of the P-31 NMR method there are two fluxional Mn-Au bonds in the course of a topomerization process. To our knowledge, this is the first example of a permanent valence isomerization of heterometallic bonds in a cluster compound. This and other features of the complex reaction system are discussed.