The Effect of One Valence Electron: Contrasting (PNP)Ni(CO) with (PNP)Ni(NO) to Understand the Half-Bent NiNO Unit

Reaction of a (PNP)Ni radical with NO finishes in the time of mixing to form a 1:1 adduct with a NO stretching frequency of 1654 cm−1. NMR data of this diamagnetic product indicate C2v symmetry, which is contradicted by the X-ray structure, which shows it to be nonplanar at Ni, with a geometry intermediate between planar and tetrahedral; the planar geometry is thus the transition state for fluxionality giving time-averaged C2v symmetry. The X-ray structure, together with DFT calculations, reveals that the “half-bent” NiNO unit and the intermediate coordination geometry result from a Ni → NO charge transfer, which has a nonintegral value, resulting in a continuum between NO+ (hence Ni0) and NO− (hence NiII). This is related to the nonaxially symmetric character of the Ni → NO back-donation caused by the (PNP) environment on Ni. Steric effects of tBu and even chelate constraints are ruled out as the cause of the unusual electronic and structural features.