The available experimental and theoretical data [1] indicate that tetraatomic molecules of the type X2CE (X=H, F, Cl; E=O, S) are planar in their ground electronic states, while in the lowest excited singlet 1A2 and triplet 3A2 states they are unstable with respect to the out-ofplane distortion of B1 symmetry leading to the bent structures. Moreover, the η2 -coordinated X2CE molecules are also nonplanar [2]. In this work the pseudo Jahn-Teller effect (PJTE) is employed to explain peculiarities of geometry of molecules from the above series in their excited singlet and triplet electronic states. The wavefunctions of the ground state, the excited 1A2 and 3A2 states, and several higher excited 1B2 and 3B2 electronic states of the X2CE molecules have been calculated by ab initio RHF CI method using 6-311G* basis set. Parameters of the PJTE (primary force constants K0, vibronic coupling constants V, and resulting force constants K= K0-V 2 /Δ) were estimated by fitting the ab initio data for the adiabatic potential energy surfaces (APES) of the systems to the general formulas. Calculations show firstly that for all the systems the vibronic coupling in the triplet 3A2 state is stronger than in the singlet 1A2 one; this explains the larger out-of-plane distortion of molecules in their triplet states. The vibronic contribution Kv= V 2 /Δ to the curvature of APES with respect to the bending coordinate for the H2CS molecule is two times less than that for the H2CO resulting in Kb1 (H2CS)= -0.14 eV/Å 2 , Kb1 (H2CO)= -0.54 eV/Å 2 . It explains a weaker distortion of the first system compared with the second one. For the F2CO and Cl2CO molecules the values of both K0 and V are greater than these parameters for the H2CO. As a result we have Kb1(F2CO)= -7.75 eV/Å 2 and Kb1 (Cl2CO)= -2.86 eV/Å 2 . The distortion of these two systems (in their excited states) is stronger than that of the H2CO molecule. Using two model complexes, Fe(CO)2(PH3)2(η2 -CH2O) I and Fe(PH3)4(η2 -CH2O) II as examples, we show that the bending of the CH2O structure by coordination is also due to the PJTE induced by the orbital charge transfers (OCT). Earlier we have shown that the curvature of the adiabatic potential (AP) of the coordinated CH2O molecule in the b1 direction (bending mode) can be estimated as follows [3]: 2 1,6 1 2 ( )/ b a coord free K  K  q f  , where Δq is the OCT from Fe to the empty 2b1 MO of CH2O, f is the PJT vibronic coupling constant and 2b1,6a1 is the energy gap between the interacting 2b1 and 6a1 states. From ab initio calculations the following values of parameters are obtained: Kb1 free(CH2O)=0.56eV/Å2 , f = 3.05eV/Å, 2b1,6a1=3.26eV. Calculated OCTs in the complexes under consideration are q=0.41e for I, and q=0.53e for II. With these numerical data we get the following resulting values of the curvature: K(I)= -0.61 eV/Å 2 and K(II)= -0.95 eV/Å 2 . It is seen that in the coordinated state the curvature of AP of the CH2O molecule in the b1 direction becomes negative in both complexes. This explains the origin of the geometry of coordination of this molecule and provides for numerical estimates of the strength of the distortion.