pears at ca. -65 "C; then, below -80°C, a singlet appears at 6 = 8.2, which we assign to the q2-form. A singlet for the aldehyde proton of the q'-form could not be observed, either because of too low a concentration of the ql-form-<8% at -80°C (extrapolated)-or because the signal is excessively broadened due to the E1.Z isomerization. The 13C-NMR spectrum of 3a is also temperature-dependent : At -80°C (CD2Clz), the resonance of the C(Se)-atom appears at 6=74.0 (for comparison, C(Se) in (CO)5W(q1-Se=CPh2): 240.0"], [((q5-C5H5)Mn(C0)212(~2q2-Se=CH2)]: 56.7l6]); at -15 "C on the other hand, a C(Se)-signal can no longer be observed.q'-3a and q2-3a also differ in the UV/VIS spectrum: '1'-3a is deep blue (&,,,=597 nm), like the ql-selenoketone complexes, whereas qz-3a is yellow-green (Amax =425 nm). Hence, on cooling from room temperature to -78"C, a solution of 3a in hexane changes its color reversibly from blue to yellow-green.The X-ray structure analysis of a crystal of 3a obtained from hexane at -78°C shows the q2-coordination of the Se=C(H)Ph ligand (Fig. 1). The C-Se distance is only slightly shorter than the sum of the carbon and selenium single-bond radii. The selenobenzaldehyde ligand is almost coincident with 04-C4-W-C5-05, CO(4) and CO(5) are bent away from it, while CO(2) and CO(3) are bent towards it.

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Der Austausch des H‐Atoms in CCH durch Br oder I gelingt mit N‐Brom‐bzw. N‐Iodsuccinimid unter Silbersalz‐Katalyse. Mit dieser Methode ließen sich unter anderem zahlreiche 17α‐Brom‐ 2 und 17α‐Iodethinyl‐Steroide 3 herstellen. Anders als bei älteren Methoden müssen Hydroxy‐ (und Carbonyl‐)gruppen in 1 nicht geschützt werden.

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