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

The bond is an important concept in chemistry, and the metal-carbon (M-C) bond in particular is a central feature in organometallic chemistry. Synthesis of stable complexes with five coplanar M-C bonds is challenging. Here, we describe the synthesis of two different types of stable complexes with five coplanar M-C 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 show that the M-C bonds in these complexes have primarily a covalent character. Besides the nature, there are also a conjugation component among the metal center and carbons, which causes delocalization. This work expanded the coplanar M-C bonds to five.