The synthesis and
structures of nitrile complexes of V(N[tBu]Ar)3,2(Ar = 3,5-Me2C6H3), are described. Thermochemical
and kinetic data for their formation were determined by variable temperature
Fourier transform infrared (FTIR), calorimetry, and stopped-flow techniques.
The extent of back-bonding from metal to coordinated nitrile indicates
that electron donation from the metal to the nitrile plays a less
prominent role for2than for the related complex Mo(N[tBu]Ar)3,1. Kinetic
studies reveal similar rate constants for nitrile binding to2, but the activation parameters depend critically on the
nature of R in RCN. Activation enthalpies range from 2.9 to 7.2 kcal·mol–1, and activation entropies from −9 to −28
cal·mol–1·K–1in an
opposing manner. Density functional theory (DFT) calculations provide
a plausible explanation supporting the formation of a π-stacking
interaction between a pendant arene of the metal anilide of2and the arene substituent on the incoming nitrile in favorable
cases. Data for ligand binding to1do not exhibit this
range of activation parameters and are clustered in a small area centered
at ΔH‡= 5.0 kcal·mol–1and ΔS‡=
−26 cal·mol–1·K–1. Computational studies are in agreement with the experimental data
and indicate a stronger dependence on electronic factors associated
with the change in spin state upon ligand binding to1.