Pseudoaromaticity in Metal Chelates : Biguanides and N' -amidinoisourea Complexes

Department of Chemistry, Indian Institute of Technology, Kharagpur-721 302, India,

Evidences for the sustenance of ring current In metal biguanides and metal N -amidinoisoureas have been reviewed. Electrophilic substitution reactions i.e. halogenation, sulphonation, nitration, have been successfully performed on Cu(Il), Ni(II), Cr(lll) and Co(III) complexes Diazotization reaction on acid stable complexes of Pt(II), Pd(II) and Au(l) has been reported. Preparation of robust water stable boron, carbon and silicon compounds with these ligands indicates electron delocalization in the ring system. The band (^\(\approx\)200 nm) assigned to aromatic metal chelate ring systems has been red·shifted in their halogen derivatives. X-ray investigation on copper(II) and nickel(II) biguanides indicate shortening of C - N bond. Stabilities of halogeno-substituted compounds are less than the unsubstituted ones. For substituted and unsubstituted Cu(II) complexes,

lengthening of M - N bond nearer to halogen atom and shortening of -- C- NH₂ bond has been attributed to drifting of delocalised electron cloud towards halogen. Nmr data on nickel(II) biguanide suggest sustenance of ring current. XPS data indicate absence of any quaternary nitrogen atom in metal biguanides. Pseudoaromatic behaviour as evidenced has been attributed to strong \(\pi\) electron delocalizatlon over the entire complex molecule.