Iron K-edge X-ray absorption data for the iron-molybdenum cofactor ('FeMoco') from Klebsiella pneumoniae reported here provide the first evidence for long-range structural order in the cofactor [Fe...Fe(Mo) = 0.368 nm in addition to Fe...S = 0.22 nm and Fe...Fe(Mo) = 0.27 nm] and, in contrast with previously published data [Antonio, Teo, Orme-Johnson, Nelson, Groh, Lindahl, Kauzlarich & Averill (1982) J. Am. Chem. Soc. 104, 4703-4705], indicate that most of the iron centres are not co-ordinated to light (oxygen, nitrogen) atoms. This demonstrates that presently available chemical models for FeMoco are inadequate.

ing to trapped silylene (22, 24) or silanone (23) was found. An even smaller amount of 7 (<2%) was identified by GCMS. When compared with the results of copyrolysis of 7 with excess dimethyldimethoxysilane (vide supra) under the same conditions, many of the numerous unidentified compounds formed can be attributed to redistribution reactions. No attempt was made to isolate the presumed trapping product or to provide positive identification of the redistribution products.Copyrolysis of 21 with 1,3-Butadiene. Trisilane 21 (0.3448 g, 1.2 X mol) was pyrolyzed in a vertical flow system with 1,3-butadiene as the carrier gas. At 510 "C (35 mL/min butadiene flow rate), a mass recovery (after evaporation of volatiles) of 46% was obtained. No compound of m / e 212 (corresponding to silylene trapping product) was found by GCMS.Acknowledgment. The support of this work by the National Registry No.

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Systems containing either iron or copper coordinated to sulphur and molybdenum are well known to be of biological importance, the former in the iron-molybdenum protein and cofactor of nitrogenase and the latter in molybdenum-induced copper deficiency in ruminants. X-ray absorption spectroscopy has been used to study the metal K-edge EXAFS of such systems and to provide new informatioin about their structure