© 2020 by the author. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited. Open Access Advances in Chemical Research Original Research Theoretical investigation of the single and double ionization spectra of ()  , = and  Behnam Nikoobakht 1, * , Gulzari L. Malli 2 , Martin Siegert 3 1. School of Chemistry, The University of Sydney, Sydney, Australia; E-Mail: bnikoobakht@gmail.com. 2. Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada; E-Mail: malli@sfu.ca. 3. IT services,Simon Fraser University, Burnaby, BC, Canada V5A 1S6, Canada; E-Mail: siegert@sfu.ca. * Correspondence: Behnam Nikoobakht; E-Mail: bnikoobakht@gmail.com Academic Editor: Maxim L. Kuznetsov Special Issue: Coordination Chemistry and Metal Complexes Advances in Chemical Research 2020, volume 2, issue 4 doi:10.21926/acr.2004010 Received: April 26, 2020 Accepted: November 10, 2020 Published: December 14, 2020 Abstract In this work, we study the single and double ionization spectra of the M(CO) 6 , with M= (W and Cr) complexes by applying the four-component algebraic diagrammatic construction and Fock-space coupled cluster methods to extend earlier studies based on less demanding approaches. The computed single and double ionization potentials are in good agreement comparing with the available experimental results. The electronic structures of the cationic molecular systems are carefully investigated by computing accurately single and double ionization potentials. The final state characterization is relied on group theoretical considerations of the contributing orbitals and allowed for a clear assignment. Energy level diagrams show the effect of spin-orbit (SO) coupling starting from scalar relativistic results and for the heavy representative M(CO) 6 with M = (W and Cr) nonadditivity effects of the SO and electron correlation can be observed requiring a consistent treatment of both contributions.