Structure and coordinate bonding nature in hydrideboryl complexes of tungsten(IV) and tantalum(V)

Abstract

<i>Ab initio </i>calculations at the SCF, MP2, CASSCF and CASPT2 levels of theory with basis sets using relativistic pseudopotentials have been carried out for the model complexes [Cl₂ WH {B(OH)₂}], <i>endo- </i>[Cl₂TaH₂{B(OH)₂}]and <i>exo- </i>[Cl₂TaH₂{B(OH)₂}] in order to investigate the nature and energetics of the interaction between transition metal and boryl ligand. The geometry of the [Cl₂ WH {B(OH)₂}] has been optimized at SCF and CASSCF level and the geometry of the tantalum complexes optimized at SCF level. The results are consistent with formation of metal -boryl complexes rather than metal-borane a-complexes. The calculated binding energies for complexes ICI2 [Cl₂ WH {B(OH)₂}] (-197. 13 kcal/mol at SCF,- 192.60 kcal/mol at MP2, -57.99 kcal/mol at CASSCF and -94.1 6 kcal/mol at CASPT2), <i>endo- </i>[Cl₂TaH₂{B(OH)₂}] (-189.08 kcal/mol at SCF, -168.61 kcal/mol at MP2, -49.78 kcal/mol at CASSCF and -82.12 kcal/mol at CASPT2) and <i>exo- </i>[Cl₂TaH₂{B(OH)₂}] (-184.52 kcal/mol at SCF, -165.64 kcal/mol at MP2, -47.03 kcal/mol at CASSCF and -79.16 kcal/mol at CASPT2) indicate that the bonding of boryl ligand with tungsten and tantalum is substantial. The electronic structures of [Cl₂WH{B(OH)₂}], <i>endo </i>and <i>exo </i>isomers of [Cl₂TaH₂{B(OH)₂ }] have been analyzed in detail on the basis of charge distributions and orbital populations.