Bond order is an important concept and a quantitative index for characterizing chemical bonds, which essentially reflects the number of electron pairs shared by two bonding atoms. Here, we calculated various bond orders of C 18 -(CO) n (n = 6, 4, and 2) via different methods, and the bond lengths and bond orders of each precursor are collectively plotted in Figs. S3-S5. It is found that there is a good correlation between bond length and various bond orders of each species, that is, the longer the C-C bond length is, the smaller the C-C bond order is, and vice versa. Similarly to the case of C 18 , the bond orders of the long and short bonds in C 18 -(CO) n (n = 6, 4, and 2) calculated by all methods are larger than 1.0 and smaller than 3.0, respectively, suggesting that some of the electrons from the short C-C bonds, which are formally triplet bond, are delocalized to the long C-C bonds. The existence of electron delocalization in C 18 -(CO) n (n = 6, 4, and 2) implies that it is possible for them to generate induced ring current in external magnetic field, which is the topic we will focus on below.