The new isomeric ruthenium/zirconium dihydrides of the formula (PPh(3))HRuH(mu-PMe(2)Cp*)(2)ClZrCl (1, 2) (Cp* = C(5)Me(4)) have been characterized by elemental analysis and NMR (H-1, P-31 and H-1 relaxation data). Complex 1, stabilized by Cl and H bridges, has been isolated from the room temperature reaction between RuH2(H-2)(PPh(3))(3) and (PMe(2)Cp*)(2)ZrCl2. The X-ray crystallographic study of 1 revealed a bimetallic complex. The six-coordinate Ru atom and the five-coordinate Zr atom are held together by two bifunctional phosphinocyclopentadienyl ligands and by H and Cl bridges. Crystal data for 1 : monoclinic space group P2(1)/c, a = 13.901(2) Angstrom, b = 18.205(6) Angstrom, c = 16.633(3) Angstrom, beta = 92.43(1)degrees, V = 4206 Angstrom(3), Z = 4, d(calc) = 1.472 g cm(-3), R(F) = 0.056, R(w)(F) = 0.058. Complex 2 with two H bridges and terminal Cl ligands at Ru and Zr has been obtained by an irreversible isomerization of 1 in the presence of HNEt(3)BPh(4). This transformation has been proposed to occur through slow protonation of one of the phosphorus ligands with the five-coordinate Ru center formed by undergoing rapid pseudorotation. Complexes 1 and 2 do not react with H-2, N-2, or 3,3-dimethyl-but-1-ene. Treatment of 1 with 1 equiv of NaHBEt(3) in C6D6 gives a mixture of new trihydrides (PPh(3))HRu(mu-Cl)(mu-H)(mu-PMe(2)Cp*)(2)ZrH (3) and (PPh(3))HRu(mu-H)(2)(mu-PMe(2)Cp*)(2)ZrCl (4). Complex 3 transforms to 4 upon standing in solution for a period of several days. Under the same conditions, complex 2 leads smoothly to trihydride 4. Both trihydrides are new and have been characterized by H-1, P-31 NMR, and H-1 NMR relaxation data. Complexes 1 and 4 are fluxional in solution at room temperature. showing hydride exchange between the terminal and bridging positions. The variable-temperature H-1 NMR spectra allowed determinations of the Delta G(double dagger) values of 16.4 (313 K, THF-d(8)) and 13.5 kcal/mol (295 K, toluene-d(8)) for the exchange in complexes 1 and 4, respectively. Possible exchange mechanisms have been discussed. Complex 2 is rigid on the NMR time scale.