The use of parameter ratios to characterize the formal order of chemical bonds

While bonding parameters such as electron localization function (ELF) bond basin populations and atoms-in-molecules (AIM) delocalization indices may n ot in an absolute sense reflect our simple ideas of bond orders, ratios of these parameters referenced to a suitable standard may be a more viable mea sure of the bonding situation. In the present paper we have calculated both these quantities for the bonds between heavy atoms for all the simple two- heavy atom hydrides and diatomics capable of multiple bond formation for el ements in the first two long rows of the periodic table and have compared t he ratios of these parameters relative to the arbitrarily chosen double bon d case. We find that a division into clearly defined formal bond-order clas ses occurs for the AIM delocalization index and is apparent thought less cl ear for ELF bond basin populations. The division agrees with our intuitive chemical ideas regarding formal bond order and should be a useful tool in c haracterizing general bonding situations. (C) 2001 Elsevier Science B.V. Al l rights reserved.