Tetravalent main-group-element hydrides of form AH (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 AH systems in D symmetry, the ground state is a. The exact electronic configuration, however, is dependent on the electronegativity of the main group element. The distortion to tetrahedral geometry has b symmetry. For these AH systems, the a→b and e→e one-electron charge-transfer transitions are most active in the b mode.