AB-INITIO AND EXPERIMENTAL STUDIES ON THE STRUCTURE AND RELATIVE STABILITY OF THE CIS-HYDRIDE-ETA-2-DIHYDROGEN COMPLEXES [(P(CH(2)CH(2)PPH(2))(3))M(H)(ETA(2)-H-2)](+) (M=FE, RU)

Ab initio calculations (DMOL method) including the estimate of the tot al energy and the full optimization of the geometrical parameters have been used to study the electronic structures and the coordination geo metries 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 h ave 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)R u(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) Angst rom, b = 14.553(1) Angstrom, c = 17.816(2) Angstrom, alpha = 72.90(1)d egrees, 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)degr ees, Z = 4, R = = 0.074. On the basis of theoretical calculations, X-r ay 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 octah edral structure where the hydride (trans to a terminal phosphorus dono r) and the L ligand occupy mutually cis positions. The theoretical cal culations indicate that the H-2 ligand in the eta(2)-dihydrogen-hydrid e derivatives 1 and 5 is placed in the P-M-H plane (parallel orientati on) 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(met al) --> sigma(H-H) back-donation as well as a more efficient interact ion between the terminal hydride and an H of the dihydrogen ligand. Th is 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 .