Magnetic Circular Dichroism Spectrum of the Molybdenum(V) Complex [Mo(O)Cl₃dppe]: <i>C</i>-Term Signs and Intensities for Multideterminant Excited Doublet States

The molybdenum(V) complex [Mo(O)Cl3dppe] [dppe = 1,2-bis(diphenylphosphino)ethane] is considered as a model system for a combined study of the electronic structure using UV/vis absorption and magnetic circular dichroism (MCD) spectroscopy. In order to determine the signs and MCD C-term intensities of the chlorido → molybdenum charge-transfer transitions, it is necessary to take the splitting of the excited doublet states into sing-doublet and trip-doublet states into account. While transitions to the sing-doublet states are electric-dipole-allowed, those to the trip-doublet states are electric-dipole-forbidden. As spin–orbit coupling within the manifold of sing-doublet states vanishes, configuration interaction between the sing-doublet and trip-doublet states is required to generate the MCD C-term intensity. The most prominent feature in the MCD spectrum of [Mo(O)Cl3dppe] is a "double pseudo-A term", which consists of two corresponding pseudo-A terms centered at 27000 and 32500 cm–1. These are assigned to the ligand-to-metal charge-transfer transitions from the pπ orbitals of the equatorial chlorido ligands to the Mo dyz and dxz orbitals. On the basis of the theoretical expressions developed by Neese and Solomon (Inorg. Chem.1999, 38, 1847−1865), a general treatment of the MCD C-term intensity of these transitions is presented that explicitly considers the multideterminant character of the excited states. The individual MCD signs are determined from the corresponding transition densities derived from the calculated molecular orbitals of the title complex (BP86/LANL2DZ).