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The problem of intramolecular hydrogen atom exchange in the OsH3(BH4)(PR3)2 system is examined from both theoretical and experimental points of view, through ab initio MO calculations on the OsH3(BH4)(PH3)2 system at the MP2, MP4, and CCSD(T) computational levels and variable-temperature 1H NMR studies on the OsH3(BH4)(PiPr3)2 complex. Three different exchange processes are fully characterized from a theoretical point of view through location of intermediates and transition states. Experimental results supporting the existence of these three different exchange processes and providing definitely its intramolecular nature are also presented.

The structure of the d 1 -Ti(BH 4 ) 3 complex is studied by means of ab initio UHF calculations including correlation energy at the MP2 level. Twenty structures which differ by the coordination mode of the BH 4 - ligands are optimized through an analytical gradient method, using the double-ξ basis set I. In agreement with the experimental data, the C 3h (η 3 , η 3 , η 3 ) structure is found to be the most stable, a result which is confirmed by calculations with the extended basis set II. The geometry of the absolute minimum and the energy ordering of the other structures are rationalized through electron counting around the metal atom

The structure of the d 1 complex Ti(BH 4 ) 3 (PH 3 ) 2 is studied by means of ab initio UHF calculations including correlation energy at the MP2 level. This complex is used as a model for the Ti(BH 4 ) 3 (PMe 3 ) 2 complex characterized by Girolami et al. Sixteen structures which differ by the borohydride coordination modes are optimized through an analytical gradient method. The energetical ordering of these structures is analyzed through an electron count. We find that the optimal structure corresponds to a η 2 ,η 2 ,η 3 coordination mode, a result in disagreement with the experimental data, which have been interpreted in terms of η 1 ,η 1 ,η 2 coordination

The »overlap graph«, where vertices represent atomic orbitals and edges, their in-phase or out-of-phase overlaps, suggests that, . in the thermal electrocyclic opening of oxirane, the second oxygen lone pair has little effect on the allowed conrotatory mode but lowers the activation energy of the forbidden disrotatory mode. This effect does not exist in the electrocyclic opening of aziridine, which is one reason why the energy gap between the allowed and forbidden processes should remain substantial. In the course of an aziridine ring opening, the nitrogen's hydrogen may remain in the plane of the heavy atoms or may move out into either of the two half-spaces defined by this plane. Three different disrotatory pathways are thus conceivable. They are compared by a perturbational treatment using simple Hiickel theory. Localization of the transition states on these three reaction paths - i. e. determination of their »early« or »late« character, as measured by the magnitude of the rotation of the two methylene groups, for example - can be made with the help of »natural correlation diagrams«. This provides a simple criterion for predicting the breakdown of the perturbational treatment, when pathways of similar energies are compared: if one transition state is much earlier (later) than the other(s), no safe conclusion can be reached. These qualitative arguments are fully supported by ab initio calculations (ST0-3G + (3 X 3) CI and 4-31G + extensive CI). Substituent effects and possible cases of violation of orbital symmetry requirements are discussed.