M = CH2 systems, where M is a metal from 4th up to 7th period, are studied at DFT level using B3LYP functional and small-core quasirelativistic pseudopotential or fully relativistic four-component methodology. We obtained structural data for 44 elements, M, and our results can be used to infer double-bond lengths for these elements. Our results also suggest that the bonding of these M = CH2 systems can be understood by a simple pictorial approach, even when spin-orbit effects are present. (c) 2008 Elsevier B. V. All rights reserved.