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

Ab initio calculations at the SCF, MP2, CASSCF and CASPT2 levels of theory with basis sets using relativistic pseudopotentials have been carried out f or the model complexes [Cl2WH{B(OH)(2))}] endo-[Cl2TaH2{B(OH)(2)}] and exo[ 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 [Cl2WH{B(OH)(2)}] has been optimized at SCF and CASSCF level and the geome try of the tantalum complexes optimized at SCF level. The results are consi stent with formation of metal-boryl complexes rather than metal-borane sigm a -complexes. The calculated binding energies for complexes [Cl2WH{B(OH)(2) }] (-197.13 kcal/mol at SCF, -192.60 kcal/mol at MP2, -57.99 kcal/mol at CA SSCF and -94.16 kcal/mol at CASPT2), endo-[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 exo[Cl2TaH2{B(OH)(2)}] (-184.52 kcal/mol at SCF, -1 65.64 kcal/mol at MP2, -47.03 kcal/mol at CASSCF and -79.16 kcal/mol at CAS PT2) indicate that the bonding of boryl ligand with tungsten and tantalum i s substantial. The electronic structures of [Cl2WH{B(OH)(2)}], endo and exo isomers of [Cl2TaH2{B(OH)(2)}] have been analyzed in detail on the basis o f charge distributions and orbital populations.