text: The simplest analogue that can be formed as X is varied is . This anion was first isolated by Becker et al. by reacting the phosphaethynolate anion with carbon disulphide. Unlike PCO, PCS shows ambidentate nucleophilic tendencies towards the W(0) complex mentioned above.
  This is the result of a reduced difference in electronegativity between E and X thus neither atom offers a substantial advantage over the other in terms of providing ionic contributions to bonding. As a result, the average electron density in PCS is spread over the entire anion  whereas in PCO, most electron density is localised on the phosphorus atom  as this is the atom which bonds to form the thermodynamically favourable product.