To Bend or Not to Bend! The Dilemma of Allenes

Abstract

Allenes, though expected to be linear, are found to be bent in many examples, especially in the case of cyclic allenes. The bending of allene is attributed to the weakening of π-bond strength across the allene. However, tetrakis(dimethylamino)allene {((CH3)N)2C═C═C(N(CH3)2)}, which is characterized by push−push interactions, has been shown to be linear, thus leading to doubts of the current understanding of the bent allenes. In this article, we report the ab initio MO/DFT, NBO based electronic structure analysis of R2C═C═CR2 (R = H, NH2) with a gradual increase in the number of amino substituents. The results indicate that the allenic π-bond strength and bending potential decrease, with an increase in the amino substitution. Molecular orbital analysis provides necessary clues regarding the delicate balance between orthogonality of the π orbitals and the p orbitals on the central carbon, which dictates the bending potential of the allenes. The dilemma of to bend or not to bend is a unique feature of tetraaminoallenes (NH2)2C═C═C(NH2)2 in comparison to isoelectronic heteroallenes (NH2)2C═N═C(NH2)2+ and (NH2)2C═B═C(NH2)2−.