PI-BONDING IN THE STAR-TYPE COMPOUND [((CO)(5)CR)(3)SN](2-) - HOW TO RATIONALIZE ITS ELECTRONIC ABSORPTION-SPECTRUM AND ITS OUT-OF-RANGE SN-119-NMR SHIFT OF DELTA = 3924

[{(CO)(5)Cr}(3)Sn](2-), 1, is obtained from Na-2[Cr-2(CO)(10)] or K-2[ Cr(CO)(5)] by reaction with SnCl2 under well specified conditions on t hree different routes. The structure of 1 as determined for its salt [ Ph4P](2).1. The star-type anion 1 shows a D-3h arrangement of the [Cr3 Sn] core. At each chromium center two equatorial carbonyl groups are c lose to the coordination plane of the tin center, with the remaining t wo roughly pointing into the direction of the idealized C-3-axis. Thes e 'out-of-plane' carbonyl groups are bent towards the center with Sn-C r-C-CO angles of ca. 82 degrees. The unconventional C-s symmetry of th e [(CO)(5)Cr] entities thus resulting, is well-rationalized in terms o f tin-pi-CO back-bonding within the EHT model. EHT calculations also substantiate the idea that 1 is to be considered as a 4-center, 6 pi-s ystem, isoelectronic, in a formal sense, to well-known pi-systems of t his type, like [CO3](2-) or [C(CH2)(3)](2-). 1 has a prominent electro nic absorption in the visible range, which is attributed to a pi-pi t ransition from the occupied e-set of the pi-system into the empty pi LUMO of the compound. This assignment is in line with the results of C I calculations on model compounds. The most unconventional property of 1 is its Sn-119-NMR resonance, which is found at delta = 3924, well o utside the low-field limit hitherto documented in Sn-119-NMR spectrosc opy. In terms of the models describing the pi-bonding situation in 1, this extreme low-field shift is not unexpected: the low-lying pi LUMO will greatly increase the paramagnetic contribution to the shift. (C) 1998 Elsevier Science S.A. All rights reserved.