Synthesis and characterization of mixed-phosphine osmium polyhydrides: Hydrogen delocalization in [OsH₅P₃]⁺ systems

The hexahydrido complex OsH6(PiPr3)2 (1) reacts with PHPh2 to give molecular hydrogen and the tetrahydride OsH4(PHPh2)(PiPr3)2 (2). However, the formation of OsH2(PHPh2)2-(PiPr3) 2 (3) as a consequence of the substitution of a second hydrogen molecule from 2 by PHPh2 does not occur. The treatment of 2 with 1.0 equiv of PHPh2 in toluene at 80 °C leads after 3 days to OsH2(PHPh2)3(PiPr3) (4). The preparation of 3 requires the previous acidolysis of 2 with HBF4, which gives [OsH5(PHPh2)(PiPr3)2] BF4 (5). In contrast 2, the addition of PHPh2 to 5 affords [OsH3(PHPh2)2(PiPr3) 2]BF4 (6) and molecular hydrogen. Deprotonation of 6 with Et3N yields 3. The skeleton of the cation of 5 has been determined by X-ray diffraction. The configuration is consistent with a Y-shaped OsP3 disposition with the osmium atom in the common vertex. Complex 5 also reacts with methanol and water to give [OsH5-{P(OMe)Ph2}(PiPr3)2 ]BF4 (7) and [OsH5{P(OH)Ph2}(PiPr3)2]B F4 (8), respectively. The addition of Et3N to 7 affords OsH4{P(OMe)Ph2}(PiPr3)2 (9). A theoretical study on the OsH5(PH3)3+ model complex reveals that although a static description is fully consistent with a classical pentahydride assignment, the formation of a dihydrogen is a very low energy costing process, on both thermodynamic and kinetic grounds. Thus, these polyhydride systems might be better described as possessing delocalized hydrogen atoms. A further QM/MM IMOMM study on the actual OsH5(PR3)+ systems indicates that the inclusion of bulky phosphine substituents plays a role against the stability of dihydrogen forms, because of the higher steric congestion of lower coordination number complexes arising from repulsions between bulky phosphines. Although IMOMM calculations improve significantly the agreement with experimental structures, they do not change the validity of the aforementioned statement concerning delocalization.