Ab initio calculations on electron deficient molecules : boron hydrides and transition metal carbenes

Abstract

The effect of electron correlation on the electron distribution and bonding of diborane is examined in several basis sets. The generalized molecular orbital method is used to define optimized orbitals for the configuration interaction calculations. Electron correlation shifts electron density away from the hydrogens, both terminal and bridging, and into the interior of the cluster and increases the direct B-B contribution to the bonding. We have also calculated the dissociation energy of diborane (B(,2)H(,6) (--->) 2BH(,3)). The experimental value is 35 kcal mol('-1), while without electron correlation the theoretical value is only about 20 kcal mol('-1). Electron correlation increases the stability of the cluster by about 15 kcal mol('-1). Ab initio calculations of B(,5)H(,9), B(,5)H(,11), and 1,2-C(,2)B(,4)H(,6) have been performed in a double-(zeta) basis and with extended configuration interaction. Theoretical deformation densities are reported. Support is given for a postulated B-C-B open 3-center bond in 1,2-C(,2)B(,4)H(,6). Ab initio calculations are reported on several transition metal carbenes and their dissociated fragments. 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. The M=C dissociation energy for electrophilic (CO)(,5)Mo=CH(OH) is calculated to be 60 kcal mol('-1). The calculated M=C dissociation energy for nucleophilic CpCl(,2)Nb=CH(,2) is 72 kcal mol('-1). The latter compound appears to have a stronger (pi) bond. The calculated rotational barrier of the methylene in CpCl(,2)Nb=CH(,2) is 14.6 kcal mol('-1). The potential energy surface of the carbene in CpCl(,2)Nb=CH(,2) has been studied by ab initio techniques. M-C-H angles as small as 78(DEGREES) and M-C-R angles as large as 170(DEGREES) have been measured in metal alkylidenes, L(,n)M=CHR. Our calculations imply that these severe distortions are due to steric factors operating in a very flat potential energy surface. Distortions in 18 electron species are predicted to be smaller than in electron deficient species.