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
P. Kircher et al., 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, Journal of organometallic chemistry, 562(2), 1998, pp. 217-227
[{(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.