Me(L)] + (L ) MeCN (4 + ) or THF (5 + )). For comparison the neutral tantalum derivatives Ta(N t Bu)(N 2 N py )R (R ) Me (6) or η 1 -allyl (7)) were synthesized by reaction of Ta(N t Bu)(N 2 N py )Cl(py) with MeLi or (allyl)MgCl. Compound 6, valence isoelectronic with 2 + , was crystallographically characterized. Although both 2 + and 6 possess trigonal bipyramidal geometries at the metal, the methyl ligand in 2 + lies in the equatorial plane (with NPh trans to pyridyl), whereas in 6 the opposite arrangement of methyl and imido ligands is found. Reaction of 1 with 0.5 equiv of BAr F 3 gave the fluxional Me-bridged cation [{W(NPh)(N 2 N py )Me} 2 (µ-Me)] + (8 + ); 8 + was also formed by direct reaction of 1 with 2 + . The methyl cation 2 + underwent facile methyl group exchange with Cp 2 ZrMe 2 and ZnMe 2 as established by spin saturation transfer and deuterium labeling studies. Although a stable intermediate was not spectroscopically observed for either reaction, for the latter case a likely adduct was identified by DFT calculations on a model system and features coordination of Zn to the imido nitrogen and a Zn-Me‚‚‚W interaction. Reaction of 2 + with AlMe 3 formed [W{MeC(2-C 5 H 4 NAlMe)(CHNSiMe 3 )(CH 2 NSiMe 3 )}(µ-NPh)Me 2 ] + (9 + ) and CH 4 by deprotonation of a CH 2 linkage of N 2 N py . DFT (B3PW91) calculations on model systems of the type M(NR){HC(2-C 5 H 4 N)(CH 2 SiH 3 ) 2 }(X) (X ) Cl, Me) showed that there is an unambiguous electronic preference for the imido ligand to lie trans to the pyridyl nitrogen. This geometry allows optimal π-donation from the imido and the amido nitrogen atoms. Inclusion of the steric bulk of the SiMe 3 groups and the R group (Ph or t Bu) on the imido ligand through ONIOM(B3PW91:UFF) calculations showed that the underlying electronic preference for the imido ligand to be trans to pyridyl can be reversed because of increased steric repulsions between the imido and amido N-substituents in this isomer. These cause a misdirection of the amido lone pair π-donation, which in turn destabilizes the metal-imido ligand π-bonding.

Experimental and DFT computational studies of the reactions of the cationic diamidepyridine-supported methyl tungsten complex [W(NPh)(N 2 N py )Me] + (2 + ) with the heterocumulenes CO 2 , CS 2 , RNCO, and RNCS (R ) tert-butyl or aryl) are described, together with comparative studies of the group 5 compounds M(N t Bu)(N 2 N py )Cl(py) (M ) Nb (13) or Ta ( 14)) and Ta(N t Bu)(N 2 N py )Me (N 2 N py ) MeC(2-C 5 H 4 N)(CH 2 NSiMe 3 ) 2 ). In all of the reactions of 2 + the heterocumulene inserted exclusively into a W-N amide bond to give unstable intermediates (isolated for certain isocyanate substrates), which subsequently underwent rearrangement via a 1,3-migration of a SiMe 3 group. DFT studies of the reaction of 2 + with CO 2 showed that insertion into the W-Me bond is in fact thermodynamically preferred, but that insertion into W-N amide is kinetically more facile. Cycloaddition to the WdNPh bond was neither kinetically nor thermodynamically viable. The reactions between Ta(N t Bu)-(N 2 N py )Me and CO 2 or RNCO in contrast gave complex mixtures, as did those between 13 and 14 and CO 2 . DFT studies showed that in a neutral but otherwise identical tantalum analogue of 2 + the preference for substrate attack at the M-N amide bond is much less pronounced. Reactions of 13 and 14 with the more sterically discriminating RNCO (R ) t Bu or p-tolyl) did, however, exclusively afford M-N amide insertion/1,3-SiMe 3 migration products, and Nb(N t Bu){MeC(2-C 5 H 4 N)(CH 2 NSiMe 3 )(CH 2 NC(OSiMe 3 )N t Bu)}Cl was structurally characterized. The previously reported reaction of Ti(N t Bu)(N 2 N py )(py) with ArNCO (Ar ) 2,6-C 6 H 3 i Pr 2 ) was re-evaluated and shown to involve an analogous Ti-N amide insertion/1,3-SiMe 3 migration reaction, ultimately forming Ti(N t Bu){MeC(2-C 5 H 4 N)(CH 2 NSiMe 3 )(CH 2 NC(N(Ar)-SiMe 3 )O)}(py).