Key words: Lewis basicity
Springer Online Journal Archives 1860-2000
Chemistry and Pharmacology
Abstract. Quantum chemical calculations using gradient-corrected density functional theory (B3LYP) and ab initio methods at the MP2 level are reported for the geometries and bond energies of the nitrido complexes Cl2 (PH3)3ReN–X (X = BH3, BCl3, BBr3, AlH3, AlCl3, AlBr3, GaH3, GaCl3, GaBr3, O, S, Se, Te). The theoretical geometries are in excellent agreement with experimental values of related complexes which have larger phosphine ligands. The parent nitrido complex Cl2(PH3)3ReN is a very strong Lewis base. The calculated bond dissociation energy of Cl2(PH3)3ReN–AlCl3 is D e = 43.7 kcal/mol, which is nearly as high as the bond energy of Me3N–AlCl3. The donor-acceptor bonds of the other Cl2(PH3)3ReN–AY3 complexes are also very strong. Even stronger N–X bonds are predicted for most of the nitrido-chalcogen complexes, which exhibit the trend X = O ≫ S 〉 Se 〉 Te. Analysis of the electronic structure shows that the parent compound Cl2(PH3)3ReN has a Re–N triple bond. The Re–N σ bond is clearly polarized towards nitrogen, while the two π bonds are nearly nonpolar. The Re–N σ and π bonds become more polarized toward nitrogen when a Lewis acid or a chalcogen atom is attached. Bonding in AY3 complexes should be described as Cl2(PH3)3ReE≡N→AY3, while the chalcogen complexes should be written with double bonds Cl2(PH3)3Re=N=X. The charge-decomposition analysis indicates that the nitrogen-chalcogen bonds of the heavier chalcogen complexes with X = S, Se, Te can also be interpreted as donor-acceptor bonds between the nitrido complex acting as a Lewis base and the chalcogen atom with an empty p(σ) orbital acting as a Lewis acid. The nitrido oxo complex Cl2(PH3)3 Re=N=O has a covalent N–O double bond.
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