Ab initio calculations (DMOL method) including the estimate of the total energy and the full optimization of the geometrical parameters have been used to study the electronic structures and the coordination geometries of the model systems [{P(CH2CH2PH2)(3)}M(H)(L)](+) (M = Fe, L = H-2, C2H4, CO, N-2; M = Ru, L = H-2) Single crystal X-ray analyses have been carried out on the complexes [(PP3)Fe(H)(eta(2)-H-2)]BP4 . 0.5THF (1 . 0.5THF), [(PP3)Fe(H)(CO)]BPh(4) . THF (3 . THF), and [(PP3)Ru(H)(eta(2)-H-2)]BPh(4) . 0.5THF (5 . 0.5THF) [PP3 = P(CH(2)CH(2)PPh(2))(3)]. Crystal data : for 1 . 0.5THF, triclinic P1 (No. 2), a = 17.626(3) Angstrom, b = 14.605(3) Angstrom, c = 12.824(4) Angstrom, alpha = 90.09(2)degrees, beta = 103.87(2)degrees, gamma = 107.46(2)degrees, Z = 2, R = 0.082; for 3 . THF, triclinic P1 (No. 2), a = 12.717(2) Angstrom, b = 14.553(1) Angstrom, c = 17.816(2) Angstrom, alpha = 72.90(1)degrees, beta = 76.82(2)degrees, gamma = 89.71(1)degrees, Z = 2, R = 0.067; for 5 . 0.5THF, monoclinic P2/1a (No. 14), a = 19.490(5) Angstrom, b = 19.438(2) Angstrom, c = 16.873(5) Angstrom, beta = 110.96(2)degrees, Z = 4, R = = 0.074. On the basis of theoretical calculations, X-ray analyses, and multinuclear NMR studies, all of the complexes of the formula [(PP3)M(H)(L)]BPh(4) [M = Fe, L = H-2 (1), C2H4 (2), CO (3), N-2 (4); M = Ru, L = H-2 (5), C2H4 (6)] are assigned a distorted octahedral structure where the hydride (trans to a terminal phosphorus donor) and the L ligand occupy mutually cis positions. The theoretical calculations indicate that the H-2 ligand in the eta(2)-dihydrogen-hydride derivatives 1 and 5 is placed in the P-M-H plane (parallel orientation) and that there is an attractive interaction between the H and H-2 ligands. XPS measurements, performed on the iron complexes, show that the Fe --> L back-bonding interaction plays a leading role in 3. It is concluded that the stronger metal-H-2 bond in the dihydrogen-hydride complex 1, as compared to the Ru analog 5, is due to the greater d(metal) --> sigma*(H-H) back-donation as well as a more efficient interaction between the terminal hydride and an H of the dihydrogen ligand. This cis effect is suggested to contribute to the relative stability of the iron complexes, which increases in the order C2H4 < N-2 < H-2 < CO.