Synthesis and Bonding in the Diamagnetic Dinuclear Tantalum(IV) Hydride Species ([P2N2]Ta)2(μ-H)4 and the Paramagnetic Cationic Dinuclear Hydride Species {([P2N2]Ta)2(μ-H)4}+I- ([P2N2] = PhP(CH2SiMe2NSiMe2CH2)2PPh):  The Reducing Ability of a Metal−Metal Bond

The controlled reaction of the Ta(V) trimethyl species [P2N2]TaMe3, where [P2N2] = PhP(CH2SiMe2NSiMe2CH2)2PPh, under 0.5 atm of hydrogen gas produces a partially hydrogenated Ta(V) species, ([P2N2]TaMe2(H), of unknown structure. Under 4 atm of hydrogen gas, further hydrogenation does not produce the complex [P2N2]TaH3; instead, reduction of the tantalum center occurs to yield the dinuclear Ta(IV) hydride ([P2N2]Ta)2(μ-H)4. This diamagnetic tetrahydride fails to react with many reagents, including ethylene and carbon monoxide; however, upon addition of iodomethane, {([P2N2]Ta)2(μ-H)4}+I- is produced as a paramagnetic green crystalline solid. The number of hydrides in this reaction product was confirmed by a deuterium labeling study. The results of a variable-temperature magnetic susceptibility study of this tetrahydride cation can be partially modeled with the Curie−Weiss law and a large correction for temperature-independent magnetism. Ab initio calculations using density functional theory were performed in an attempt to further understand the influence of the macrocyclic ligand in the bonding in these complexes.