THEORETICAL COMPARISON BETWEEN NUCLEOPHILIC AND ELECTROPHILIC TRANSITION-METAL CARBENES USING GENERALIZED MOLECULAR-ORBITAL AND CONFIGURATION-INTERACTION METHODS
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Ab initio calculations are reported on several transition metal carbenes and their dissociated fragments. In a better than minimal basis set, the orbitals involving the metal-carbene double bond (, , *, and *) are optimized by the generalized molecular orbital (GMO) method and used in a full configuration interaction (CI) calculation for the four electrons in the MC bond. In this manner we maintain the physical significance inherent only in small CI calculations while obtaining the major portion of near-degenerate correlation energy for these four electrons. Our results suggest electrophilic and nucleophilic metal carbenes arise from two different bonding schemes. Electrophilic, 18-electron, metal carbenes can be considered as bonding between singlet metal and singlet carbene fragments, whereas nucleophilic, often electron-deficient, metal carbenes can be considered as bonding between triplet metal and triplet carbene fragments. This conclusion is illustrated using fragment and atomic deformation densities, molecular orbital maps, molecular orbital diagrams, and theoretical thermodynamics. Interchanging singlet and triplet carbene fragments with the metal fragments indicates the metal fragment is slightly more important than the carbene fragment in determining the stability and the electronic properties of metal carbenes. The MC dissociation energy for electrophilic (CO)5MoCH(OH) is calculated to be 60 kcal/mol. The calculated MC dissociation energy for nucleophilic CpCl2NbCH2 is 74 kcal/mol. The latter compound appears to have a stronger bond. The calculated rotational barrier of the methylene in CpCl2NbCH2 is 14.6 kcal/mol, in good agreement with NMR experiments on similar compounds. 1984, American Chemical Society. All rights reserved.