Theoretical Predictions and Single-Crystal Neutron Diffraction and Inelastic Neutron Scattering Studies on the Reaction of Dihydrogen with the Dinuclear Dinitrogen Complex of Zirconium [P2N2]Zr(μ-η2-N2)Zr[P2N2], P2N2 = PhP(CH2SiMe2NSiMe2CH2)2PPh