Nonrelativistic and quasirelativistic density-functional calculations have been performed aimed at investigating structures and bonding in the cationic methylene complexes MCH(2)(+) of nickel, palladium, platinum, iridium, and gold. Relativistic effects on the bond dissociation energies of these complexes amount to +7, +15, +51, -3, and +66 kcal/mol, respectively. The influence of relativity on bond energies is analyzed in detail and discussed in terms of the s-orbital stabilization and d-orbital destabilization. The relativistic contraction of the M(+)-CH2 bond is found to be small for the group 10 metals and iridium but significant for gold.