Thermodynamic studies of [HPt(EtXantphos)2]+ and [(H)2Pt(EtXantphos)2]2+

[HPt(EtXantphos)(2)](PF6) (where EtXantphos is 9,9-dimethyl-4,5-bis(diethylphosphino)-xanthene) can be prepared by the reduction of Pt(COD)Cl-2 (where COD is 1,4-cyclooctadiene) with hydrazine in the presence of 2 equiv of the diphosphine ligand followed by exchange of Cl- with PF6-. Deprotonation of [HPt(EtXantphos)(2)](PF6) (pK(a) = 27.3 in acetonitrile) leads to the formation of Pt(EtXantphos)(2), which has been characterized by an X-ray diffraction study. Pt(EtXantphos)(2) has a distorted tetrahedral geometry. The average chelate bite angle is 108.2degrees, and the dihedral angle between the two planes formed by the phophorus atoms of each diphophine ligand and platinum is 80.4degrees. Protonation of [HPt(EtXantphos)(2)](+) results in the formation of [(H)(2)Pt(EtXantphos)(2)](2+), which has a pK(a) of 6.8 in acetonitrile. Oxidation of Pt(EtXantphos)(2) with ferrocenium tetrafluoroborate produces [Pt(EtXantphos)(2)](2+). [Pt(EtXantphos)(2)](2+) undergoes two reversible one-electron reductions (E-1/2(II/I) = -0.81 V versus ferrocene and E-1/2(I/O) = -0.97 V), and [HPt(EtXantphos)(2)](+) undergoes a reversible one-electron oxidation (E-1/2(II/III) = +0.23 V). These half-wave potentials and the pK(a) values of [HPt(EtXantphos)(2)](+) and [(H)(2)Pt(EtXantphos)(2)](2+) have been used to calculate five additional homolytic and heterolytic bond-dissociation free energies for these two hydride species and for [HPt(EtXantphos)(2)](2+). The extensive thermodynamic characterization of this hydride system provides useful insights into the factors controlling the reactivity of these complexes.