Planar tetracoordinate carbons (ptC's) can be stabilized by four-membered ring perimeters composed of four bare transition metal atoms. DFT analyses of the MOs, electronic structures, energies, and magnetic properties of these CM4 species (where M represents isoelectronic combinations of Cu, Ni, Ag, and Pd) reveal striking similarities with main group metal ptC analogs (e.g., CAl2Si2, CAl4Na-, and C5Li2). While the CCu42+, CAg42+, and CNiCu3+ ions have the largest HOMO-LUMO sepns., CCu42+ is the best candidate for detection by gas-phase photoelectron spectroscopy. [on SciFinder (R)]