The isopropyl chloro complex Tp(Me2)NbCl(i-Pr)(PhC=CMe) (2) [Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate] exhibits a beta -agostic structure in the crystal. The conformation of the alkyl group is such that the agostic methyl group lies in the C alpha -Nb-Cl plane and the nonagostic one, in a wedge formed by two pyrazole rings. As observed by solution NMR spectroscopy, restricted rotation about the Nb-C bond allows the observation of an equilibrium between this species, 2 beta, and a minor alpha -agostic rotamer 2 alpha. A putative third rotamer which would have the secondary hydrogen in the wedge is not observed. Similar behavior is observed for related Tp'NbCl(i-Pr)(R2C=CMe) [Tp' = Tp(Me2), R-2 = Me (3); Tp' = Tp(Me2,4Cl), R-2 = Ph (4)]. The two diastereomers of the sec-butyl complex Tp(Me2)NbCl(sec-Bu)(MeC=CMe) (5) have been separated. In the crystal, 5CR-AS has a beta -agostic methyl group with the ethyl group located in the wedge formed by two pyrazole rings. The same single beta -agostic species is observed in solution. The other diastereomer, 5AR-CS has a beta -agostic methylene group in the solid state, and the methyl group sits in the wedge. In solution, an equilibrium between this beta -agostic methylene complex 5AR-CS beta and a minor alpha -agostic species 5AR-CS alpha, where the ethyl substituent of the sec-Bu group is located in the wedge between two pyrazole rings, is observed. NMR techniques have provided thermodynamic parameters for these equilibria (K = 2 alpha /2 alpha = 4.0 +/- 0.1 at 193 K, DeltaG(193)(o) = -2.2 +/- 0.1, DeltaH(o) = -7.4 +/- 0.1 kJ mol(-1), and DeltaS(o) = -27 +/- 1 J K-1 mol(-1)), as well as kinetic parameters for the rotation about the Nb-C bond (at 193 K, DeltaG double dagger (2) = 47.5 +/- 2.5, DeltaH double dagger = 58.8 +/- 2.5 kJ mol(-1), and DeltaS double dagger = 59.0 +/- 10 J K-1 mol(-1)). Upon selective deuteration of the beta -methyl protons in Tp(Me2)NbCl[CH(CD3)(2)]- (PhC=CMe) (2-d(6)), an expected isotope effect that displaces the equilibrium toward the a-agostic rotamer is observed (K = 2-d(6)beta /2-d(6)alpha = 3.1 +/- 0.1 at 193 K, DeltaG(193)(o) = -1.8 +/- 0.1, DeltaH(o) = -8.3 +/- 0.4 kJ mol(-1) and DeltaS(o) = -34 +/- 2 J K-1 mol(-1)). The anomalous values for DeltaH(o) and DeltaS(o) are discussed. Hybrid quantum mechanics/molecular mechanics calculations (IMOMM (B3LYP:MM3)) on the realistic model Tp(Me2)NbCl- (i-Pr)(HC=CMe) have reproduced the energy differences between the alpha- and beta -agostic species with remarkable accuracy. Similar calculations show that Tp(Me2)NbCl(CH2Me)(HC=CMe) is alpha -agostic only and that Tp(5-Me)NbCl(CH2Me)(HC=CMe), which has no methyl groups at the 3-positions of the pyrazole rings, is beta -agostic only. Analysis and discussion of the computational and experimental data indicate that the unique behavior observed for the secondary alkyl complexes stems from competition between electronic effects favoring a P-agostic structure and steric effects directing a bulky substituent in the wedge between two pyrazole rings of Tp(Me2). All Of the secondary alkyl complexes thermally rearrange to the corresponding linear alkyl complexes via a first-order reaction.