With regard to the myth, taught to entire generations of chemistry students and perpetuated online in chemistry portals and by outdated tutorials, textbooks, and in classroom lectures, that elements beyond the second period can expand their octet by utilizing available d orbitals, consider this:
Since the 90’s, quantum chemists have shown this idea to be experimentally incorrect as it is energetically unfeasible to use d-orbitals for extra bonds (Kalemos & Mavridis, 2011; Durrant, 2015; Cowley, 2015; Northumbria, 2015). Indeed, the possibility of extensive d-orbital participation has been discredited more than a quarter century ago (Reed & Schleyer, 1990; Magnusson, 1990). As back then Cooper, Cunnigham, Gerratt, Karadakov, & Raimondi stated in a JACS article published by the ACS (Cooper et. al, 1994):
“Indeed, models based on d2sp3, dsp2, and dsp3 hybrid orbitals are still in widespread use among professional chemists and are described in many of the most widely used textbooks. It is tempting to speculate as to why such models continue to survive when there is so much theoretical evidence which does not support them.”
See references below. Additional references, links, and topic discussion are given at http://www.minerazzi.com/tools/bond-order/calculator.php
References
Cooper, D. L., Cunnigham, T. P., Gerratt, J., Karadakov, P. B., & Raimondi, M. (1994). Chemical Bonding to Hypercoordinate Second-Row Atoms: d Orbital Participation versus Democracy. J. Am. Chem. Soc. Vol. 116, No. 10, pp 4414-4426. doi: 10.1021/ja00089a033.
Kalemos A. & Mavridis, A. (2011). Myths and Reality of Hypervalent Molecules. The Electronic Structure of FClOx, x = 1-3, Cl3PO, Cl3PCH2, Cl3CClO, and C(ClO)4. J. Phys. Chem., 115, (11), pp 2378-2384.
Magnusson, E. (1990). Hypercoordinate molecules of second-row elements: d functions or d orbitals?. J. Am. Chem. Soc., Vol. 112, No. 22, pp. 7940-7951.
Reed, A. E. & Schleyer, P. V. R. (1990). Chemical bonding in hypervalent molecules. The dominance of ionic bonding and negative hyperconjugation over d-orbital participation. J. Am. Chem. Soc., 112, pp. 1434-1445.
