Binding of Nitriles and Isonitriles to V(III) and Mo(III) Complexes: Ligand vs Metal Controlled Mechanism

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 for 2 than for the related complex Mo(N[tBu]Ar)3, 1. Kinetic
studies reveal similar rate constants for nitrile binding to 2, 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–1 in 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 of 2 and the arene substituent on the incoming nitrile in favorable
cases. Data for ligand binding to 1 do not exhibit this
range of activation parameters and are clustered in a small area centered
at ΔH‡ = 5.0 kcal·mol–1 and 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 to 1.