Kompleksy diwodorowe metali grup przejściowych. Cz. 1 Metody otrzymywania i badania struktury

In the first part of this review preparative methods and structural studies of s-bonded dihydrogen complexes of the transition group metals have been described. The former includes the most common procedures of direct reaction of coordinatively unsaturated complexes with hydrogen gas and protonation of metal hydride species. The M-(ƞ²-H₂) bonding in these complexes is best described as a σ_(HH) electron density donation to an empty metal d orbital of σ symmetry σ_(HH)→d_σ(M) augmented by a synergistic back-donation from filled metal d orbitals (d_π(M)→ σ̽_(HH)). From among methods of determining solid state structure, the most accurate are the neutron diffraction studies that have been successfully applied to 11 complexes. Progress in broader application of this method is slow due the need for preparation of large crystals and the shortage in the world of neutron scattering and diffraction facilities. In solution, the most reliable data are acquired by the NMR methods Large J_(HD) coupling constant values of 12 to 34 Hz and short minimal spin-lattice relaxation time values of ~3 msmin<~40 ms are characteristic of complexes containing M-(ƞ₂-H₂) bonds. The dependence of T^min on speed of rotation ensures that T^min values measured for fast spinning H₂ ligands are 4 times longer than those observed for slow rotational motion regimes and that the H-H distance for these two types of motion is linked by the relation (...). Taking into account the generally accepted view that dihydrogen complexes may be regarded as intermediates along the pathway to H₂ oxidative addition. Morris and Maseras proposed the following classification of hydrogen containing complexes based on the distance between the hydrogen atoms in these complexes: 0.8-1.1 Å ('stretched' dihydrogen complexes), 1.3-1.6 Å (compressed dihydrides), >1.6 Å (classical dihydrides).