Tetravalent main-group-element hydrides of form A H 4 (A = B, C, N, O, Al, Si, P, and S, where A is a tetravalent atom or ion) are known to distort from the square planar to tetrahedral geometry. For all A H 4 systems in D 4h symmetry, the ground state is a 1g. The exact electronic configuration, however, is dependent on the electronegativity of the main group element. The distortion to tetrahedral geometry has b 2u symmetry. For these A H 4 systems, the a 2u →b 1g and e u →e g one-electron charge-transfer transitions are most active in the b 2u mode.