Using quantum chemical computations and analysis of electron distribution (MO, DOS, ELF) we showed that in some carbon-doped silicon and germanium clusters, it is possible to achieve a planar tetracoordinate carbon with enhanced stability. While the driving force for C-planarization in the square dications CX42+ (with X = Si, Ge) is electron delocalization on X-4 frame together with single bonds along C-X bonds, the larger neutral CSi9 and CGe9 clusters enjoy combined stabilization from both electronic effect and geometrical constraint of the X-9 cages. In CX9, an additional electrostatic interaction reinforces stabilization within the CX4 moiety in maintaining the ptC configuration. (C) 2014 Elsevier B.V. All rights reserved.