In a more preferred embodiment, L1 and L2, which may be the same or different, are phosphines of the formula PR1R2R3, wherein R1 is a C3-C12 secondary alkyl or a C5-C12 cycloalkyl group, and R2 and R3 are independently selected from the group consisting of a C6-C12 aryl, a C1-C10 primary alkyl, a C3-C12 secondary alkyl and a C5-C12 cycloalkyl.