The Lewis acid/base stabilized phosphanylboranes and -ar-sanes [(CO)(5)W((H2EBH2NMe3)-N-.)] (1a: E = P; 1b: E = As) have been shown to react with the platinum(0) complex [(Ph3P)(2)-Pt(C2H4)] under oxidative addition of the E-H bond to the platinum center. The complexes cis-[(Ph3P)(2)Pt(H)(mu-(EHBH2NMe3)-N-.)W(CO)(5)] (2a: E = P; 2b: E = As) are formed. Complex 2a is unstable in solution at room temperature and slowly reacts with loss of carbon monoxide to form [(Ph3P)(2)- Pt(mu-H)(mu-(EHBH2NMe3)-N-.)W(CO)(4)] (3a: E = P). An analogous complex 3b (E = As) is formed from 2b only by refluxing in CH2Cl2. The reaction of 2a to 3a can be reversed by addition of CO, whereas the arsenic compound 3b does not show this reactivity pattern. All new compounds have been comprehensively characterized by spectroscopy and X-ray crystallography. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).