Tripyrrolylphosphine reacts with the cluster Rh-6(CO)(15)(NCMe) to afford the disubstituted Rh-6(CO)(14)(mu(2)-P(NC4H4)(3)) derivative (2) via the Rh-6(CO)(15)P(NC4H4)(3) intermediate (1) with eta(1)-P coordination. In the solid state, 2 has the phosphine occupying a bridging position where it is bonded to two neighboring Rh atoms in the Rh-6 octahedron through the P-atom and an approximately tetrahedral alpha-carbon atom of one of the pyrrolyl rings. This can be described by the interaction of an electron pair, associated with a negative charge on one of the canonical forms of the NC4H4 ring, with the adjacent Rh center. H-1 NMR spectra show that the solid-state structure is retained in solution, but the phosphine is not rigid, and three distinctive dynamic processes are observed. Each of these represents independent hindered rotation of inequivalent pyrrolyl rings about P-N bonds, the ring involved in the interaction with the Rh-6 skeleton displaying the highest activation barrier with DeltaH(not equal) = 15.8 +/- 0.1 kcal mol(-1) and DeltaS(not equal) = 1.4 +/- 0.3 cal K-1 mol(-1). The assignment has been confirmed by H-1 TOCSY and EXSY experiments, and a mechanism is proposed. The formation of 2 from 1 is reversible in the presence of CO, which is highly unusual for bridged clusters. The kinetics of the forward and reverse reactions have been studied, and the values of DeltaHdegrees and DeltaSdegrees for formation of 2 (+1.3 +/-0.5 kcal mol(-1) and -9 +/- 2 cal K-1 mol(-1), respectively) show that the Rh-C bond in the bridge is comparable in strength with the Rh-CO bond it replaces. The intrinsic entropy of 2 is exceptionally unfavorable, overcoming the favorable entropy caused by CO release, and this allows the reversibility of bridge formation. The reactions proceed via a reactive intermediate that may involve agostic bonding of the ring. The reverse reaction has an exceedingly unfavorable activation entropy that emphasizes the unique nature of 2.