PREPARATION AND SPECTROSCOPIC PROPERTIES OF THE ETA-2-DIHYDROGEN COMPLEXES [MH(ETA-2-H2)(PR2CH2CH2PR2)2]+ (M = FE, RU, R = PH, ET) AND TRENDS IN PROPERTIES DOWN THE IRON GROUP TRIAD

Complexes trans-[M(H)(eta-2-H2)L2]BF4 (L = PPh2CH2CH2PPh2 = dppe (1Fe, 1Ru), PEt2CH2CH2PEt2 = depe (2Ru)) are prepared by reaction of cis-M(H)2L2 with 1 equiv of HBF4.Et2O. Deprotonation of 1Ru by BuLi at 200 K, gives thermally unstable trans-Ru(H)2(dppe)2, which isomerizes to cis-Ru(H)2(dppe)2. Tetraphenylborate salts of the complexes 1Fe, 2Fe, and 2Ru are prepared by reaction of trans-M(Cl)(H)L2 with NaBPh4 under 1 atm of H-2. 2Fe, 1Ru, and 2Ru can also be made directly from the complexes cis-MC12L2 by reaction with 1 atm of H-2, excess NaBPh4, and 1 equiv of NaOEt (or NaO(t)Bu) in THF. Some properties of the complexes [Os(H)(H-2)(L)2]+ (L = dppe (10s), depe (20s)) are included to reveal trends down the triad of metals; ReH3(dppe)2 also provides useful comparisons. The terminal hydride stretching mode, nu(M-H), increases in frequency as Fe < Ru < Os, and the P-31 chemical shifts increase in the order Fe < Ru < Os as expected for isostructural complexes. However, indicators of dihydrogen vs dihydride character show that Ru is out of place in the periodic order. H-1 NMR spectra of isotopomers trans-[M(H)(eta-2-HD)L2]+ and trans-[M(D)(eta-2-HD)L2]+ give couplings 1J(H,D) that decrease as Ru > Fe > Os. The chemical shifts of the HD for these two isotopomers are quite different because of the higher trans influence of D than H. The chemical shift difference, delta(dihydrogen) - delta(terminal hydride), for complexes 1 and 2 also decreases as Ru > Fe > Os. The T1 values of the dihydrogen nuclei, T1(H-2), and the hydride ligand, T1(H), were determined over the temperature range of 190-300 K for the complexes 2Fe and 2Ru in acetone-d6. Analysis of these and other data suggests H-H distances for the rapidly spinning H-2 ligand of the Fe and Ru complexes are comparable at 0.87 +/- 0.02 angstrom. An overall ordering of increasing distances is Ru almost-equal-to Fe < Os. The lability of dihydrogen as judged by the qualitative H2/D2 rates of exchange increases as Os < Fc < Ru. The equilibrium constant for H2 binding and hence the strength of the H-2-metal bond likely increases as Ru < Fe < Os. Thus, the Ru complexes have the strongest H-H interaction and weakest metal-dihydrogen interaction. The [RuHL2]+ unit is a poorer pi-back-bonder than the corresponding complexes of either Fe or Os and forms weaker sigma-bonds than Os. Electrochemical and infrared data both indicate that the ease of oxidation of the binding sites for N2 and Cl- decreases as ReH(dppe)2 >> [FeH(depe)2]+ > [FeH(dppe)2]+ > [MH(depe)2]+ > [MH(dppe)2]+ (M = Ru, Os). H atom exchange between H-2 and hydride ligands is monitored by variable-temperature H-1 NMR, and spectra are simulated to give DELTA-G values that decrease as Ru > Fe > Os and dppe > depe. This exchange likely proceeds via the homolytic cleavage of the H-H bond.