Inverse Coordination-The Janus Face of Coordination Chemistry

I would like to draw attention to the novel chemical concept of inverse coordination, which discloses another face of coordination chemistry (practically ignored so far as a distinct field) and deals with the formation of metal complexes in which the donor and acceptor positions are reversed. This concept emerged after R.C. Mulvey used the “inverse crown” paradigm [1], to describe macrocyclic structures in which “the arrangement of Lewis acidic [acceptor] and Lewis basic [donor] sites is opposite to that encountered in conventional crown ether complexes”. The extension of this paradigm resulted in the “inverse coordination” concept [2]. The inverse coordination can be defined as the formation of metal complexes in which the arrangement of the acceptor and donor sites is opposite to that occurring in conventional coordination complexes [3]. The relationship between traditional coordination complexes and inverse coordination complexes could be regarded as a Janus face image, as suggested by Scheme 1.