Tuning of the transition metal hydrogen bond: how do trans ligands influence bond strength and hydridicity?

Density functional calculations using the BP86, BLYP and PW91 functionals have been performed for the complexes W(dmpe)2H(X) (dmpe = (CH3)2PCH2CH2P(CH3)2); with X = CH, I; NO, II; N, III; CMes (Mes = 2,4,6-(CH3)3C6H2), Ia; CPh, Ib; CMe, Ic; and CtBu, Id. The W–H bond strength increases in the order III < I < II, with an approximate difference of 20 kJ mol−1 between each pair. A perturbational analysis relates this effect to a variation in energy of the metal fragment orbital involved in bonding. The polarization of the W–H bond increases in the order III < I < II. The different functionals produce bond energy terms which differ in the range of about 8 kJ mol−1, BP86 predicting the stronger, and PW91 the weaker bonds. The optimized geometric parameters are similar for all three functionals, with the exception of the W–P bonds, which are calculated to be about 4 pm longer with the BLYP approach. Bulkier carbyne ligands influence the coordination geometry of the dmpe ligands due to steric effects, which in turn influences the unit energy of the metal fragment orbital and thus the orbital interaction energy of the W–H (secondary trans influence).