Experimental and theoretical evidences for the formation of transition metal complexes with five coplanar metal–carbon σ bonds

The σ bond is an important concept in chemistry, and the metal–carbon (M–C) σ bond in particular is a central feature in organometallic chemistry. Synthesis of stable complexes with five coplanar M–C σ bonds is challenging. Here, we describe our synthesis of two different stable types of complexes with five coplanar M–C σ bonds, and examine the stability of such complexes which use rigid conjugated carbon chains to chelate with the metal center. Density functional theory (DFT) calculations showed that the M–C σ bonds in these complexes have primarily a covalent character. Besides the σ nature, there are also π conjugation component among the metal center and carbons, which causes delocalization. This work expanded the coplanar M–C σ bonds to five.