Ab initio calculations at 6-31++G(d,p) level have been done on H2CY ... LiF (Y=O,S) complexes choosing ten possible orientations in each complex. The effect of correlation on complex binding energies has been studied via single point MP2 (full) calculations done on 6-31++G(d,p) geometry. Binding energies have been corrected for basis set superposition error. Frequency calculations confirm that H2CO ... LiF and H2CS ... LiF complexes have three and two stable forms, respectively. The most stable form in each complex has been found to have a strong lithium bonding interaction and a secondary hydrogen bonding interaction. NBO analysis has revealed that in this form oxygen donates n(sigma) lone pair while sulfur donates its n(pi) lone pair. In yet another stable form of these complexes, mixed donation of pi and n(sigma) electrons have been observed. (C) 1997 American Institute of Physics.