Reported herein are the first thermodynamic hydricities (
GM+) for a collection of niobocene
and tantalocene compounds (
1-
13) of general formulas (C
5R
4R')
2M(H)(L) (R = H, Me; R' =H, Me, SiMe
3; L = CO, PEt
3, (H)
2, PhCCPh, CH
2=O, =O)
and (C
5H
4SiMe
3)
2Nb(Cl)(L') (L' =O=C=CEtPh, O=C=CEt
2). The studies
were carried out in acetonitrile solution,
and thecompounds sho
w hydride transfer free energies ranging from 74 to 95 kcal/mol, as determinedby bracketing studies involving a kno
wn series of hydride acceptors. The data indicate thatthe free energies of hydride transfer are heavily influenced by colig
ands, but less so by 4dvs 5d effects in isostructural niobium or tantalum compounds.