Theoretical versus experimental charge and spin-density distributions in trans-[Ni(NH3)4(NO2)2]

The experimental charge and spin-density distributions for [Ni(NH3)4(NO2)2] have been compared with theory at various levels. Ab initio unrestricted Hartree 13Fock (UHF) and discrete variational X;1(DVX;1) Hartree 13Fock 13Slater molecular orbital (m.o.) calculations are reported together with cellular ligand field (c.l.f.) results. The UHF and DVX;1 approaches yield closely similar descriptions of the charge and spin densities, and qualitatively reproduce the main features of both types of experimental data, namely the Ni 13N covalence is strong, the NO2 13 ion is a better <3 donor than the NH3 molecule, and the Ni 13N <0-bonding is small. Both theories indicate quite appreciable O(NO2) participation in the bonding and antibonding m.o.s involving nickel. C.l.f. calculations which include only the Ni 13N interactions reproduce the experimental d 13d spectra and the signs of the single-crystal paramagnetic anisotropies quite well, but assign a weaker <3-donor role to the nitrite ligand relative to NH3. An extension of the model to include explicit Ni 13O interactions is more satisfactory and places the NO2 13 ion as the stronger <3 donor consistent with the other theoretical and experimental data

Mechanism of formation of [(PMe3)(3)Rh(-C equivalent to C-R)(2)(H)] via C-H oxidative addition: Isomerization, alkyne exchange, and hydride replacement

The mechanism of formation of mer,trans-[(PMe3)3Rh(−C⋮C−R)2H] from [(PMe3)4Rh(Me)] and terminal alkyne has been studied. The initial step of the reaction is the elimination of methane and the formation of the trigonal bipyramidal complex [(PMe3)4Rh(−C⋮C−R)], a reaction that is complete in time of mixing at −78 °C. This intermediate undergoes an oxidative addition reaction with a second equivalent of alkyne to give fac-[(PMe3)3Rh(−C⋮C−R)2H] as the kinetic product. This fac isomer is not stable above −20 °C and isomerizes to the thermodynamic product mer,trans-[(PMe3)3Rh(−C⋮C−R)2H]. fac-[(PMe3)3Rh(−C⋮C−R)2H] will exchange alkynyl groups with free alkyne, a reaction that has a lower energetic barrier than the isomerization to mer,trans-[(PMe3)3Rh(−C⋮C−R)2H]. Density functional theory studies on all stages of the formation of mer,trans-[(PMe3)3Rh(−C⋮C−R)2H] have been carried out and give ground state energies in line with those experimentally observed. Once formed, mer,trans-[(PMe3)3Rh(−C⋮C−R)2H] is configurationally stable and not prone to scrambling, although it will react with chloroform, whereupon the hydride is replaced by chloride. The initial product of this reaction is mer,trans-[(PMe3)3Rh(−C⋮C−R)2Cl], and this compound has been studied by single-crystal X-ray diffraction. In solution mer,trans-[(PMe3)3Rh(−C⋮C−R)2Cl] isomerizes slowly to mer,cis-[(PMe3)3Rh(−C⋮C−R)2Cl]