Synthesis of a new P2N2 ligand N,N′-bis[2-(diphenylphosphino)phenyl]propane-1,3-diamine, H2L2, and some of its complexes with elements of the nickel triad and rhodium: X-ray structure analyses of the neutral complex [NiL2] and the trans-spanned [Rh(CO)Cl(H2L2)]