A first structural and theoretical comparison of pyridinylidene-type rNHC (remote N-heterocyclic carbene) and NHC complexes of Ni(II) obtained by oxidative substitution

A series of cationic pyridinylidene and quinolinylidene complexes of chlorobis(triphenylphosphine)nickel(ii) were prepared by oxidative substitution of Ni(PPh₃)₄ with methylated chloropyridines or chloroquinolines. NMR as well as X-ray crystallographic studies confirmed the trans arrangement of the two phosphines in the products. Calculations, using suitable model compounds at the BP86/TZVP level, clearly differentiate between a standard imidazolylidene complex and new complexes of the NHC-type on the one hand, and new complexes classified as rNHC-types - with the heteroatom distant from the carbene carbon - on the other. The latter form significantly stronger bonds - mainly of an electrostatic nature - with the metal.