The new Pt-Au hydride cluster compound [(H)Pt(AuPPh(3))(9)](NO3)(2) (3) has been synthesized and characterized by NMR, FABMS, and single-crystal X-ray diffraction [triclinic, , a = 17.0452(1) Angstrom, b = 17.4045(2) Angstrom, c = 55.2353(1) Angstrom, alpha = 89.891(1)degrees, beta = 85.287(1)degrees, gamma = 75.173(1)degrees, V = 15784.0(2) Angstrom(3), Z = 4 (two molecules in asymmetric unit), residual R = 0.089 for 45 929 observed reflections and 3367 variables, Mo K alpha radiation]. The Pt(AuP)(9) core geometry is a distorted icosahedron with three vertices vacant. The Pt-Au, Au-Au, and Au-P distances are within the normal ranges observed in other Pt-Au clusters. This cluster is a catalyst for H-2-D-2 equilibration in homogeneous solution phase and has been used in a general mechanistic study of this reaction catalyzed by Pt-Au clusters. We previously proposed that a key step in the mechanism for catalytic H-2-D-2 equilibration is the dissociation of a PPh(3) ligand to give a cluster with an open Au site for bonding of H-2 or D-2. This was based on qualitative observations that PPh(3) inhibited the rate of HD production with [Pt(AuPPh(3))(8)]-(NO3)(2) (1) as catalyst. In order to test this hypothesis, phosphine inhibition (on the rate of HD production) and phosphine ligand exchange kinetic experiments were carried out with [(H)(PPh(3))Pt(AuPPh(3))(7)](NO3)(2) (2) and 3. In this paper we show that the rate constant for phosphine dissociation determined from the PPh(3) inhibition rate study of H-2-D-2 equilibration with cluster 2 is nearly identical to the rate constant for dissociative phosphine ligand exchange. The slower rate for H-2-D-2 equilibration observed with 3 compared with 2 (5.5 x 10(-3) vs 7.7 x 10(-2) turnover s(-1)) is explained by its smaller rate constant for phosphine dissociation (2.8 x 10(-5) vs 2.9 X 10(-5) s(-1)). The fact that clusters 2 and 3 show similar kinetic behaviors suggests that the PPh(3) dissociation step in the catalytic H-2-D-2 equilibration is general for 18-electron hydride Pt-AuPPh(3) clusters.