The assembly of the essential diferric cluster/tyrosyl radical cofactor of the R2 subunit of Escherichia coli ribonucleotide reductase from apoR2 with Fe2+ and O-2 or diferrous R2 with O-2 has been studied by a variety of rapid kinetic methods. An intermediate X, formally an Fe3+/Fe4+ diiron cluster, oxidizes tyrosine 122 to the tyrosyl radical concomitant with its own reduction to the diferric cluster generating the R2 cofactor. To characterize the properties of X, rapid freeze quench methods have been used in conjunction with Mossbauer, ENDOR, and EPR spectroscopies. These studies are extended here to include rapid freeze quench EXAFS. A short, 2.5 Angstrom Fe-Fe vector and a 1.8 Angstrom Fe-O interaction have been identified in nine independent samples of X. These samples have been generated using both wild-type and a mutant protein in which the essential tyrosine has been replaced by phenylalanine (Y122F). The short Fe-Fe interaction is neither present in diferrous or diferric R2 nor in samples of X that have aged to decay the intermediate. Several structural models which are consistent with the data are presented.