the 
 formation of halogen bonds from iodopentafluorobenzene and 
 1-iodoperfluorohexane to a series of bis(5-cyclopentadienyl)metal 
 hydrides ( cp2tah3 , 1 ; cp2mh2 , m = mo , 2 , m = w , 3 ; cp2reh , 4 ; cp2ta(h)co , 5 ; cp = 5-cyclopentadienyl ) is demonstrated by 1h nmr spectroscopy . 
 interaction enthalpies and entropies for 
 complex 1 with c6f5i and c6f13i are reported ( h 
 = 10.9  0.4 and 11.8  0.3 kj / mol ; s = 38  2 and 34  2 j/(molk ) , 
 respectively ) and found to be stronger than those for 1 with the hydrogen - bond donor indole ( h 
 = 7.3  0.1 kj / mol , s = 
 24  1 j/(molk ) ) . for the more reactive complexes 25 , 
 measurements are limited to determination 
 of their low - temperature ( 212 k ) association constants with c6f5i as 2.9  0.2 , 2.5  0.1 , < 1.5 , 
 and 12.5  0.3 m1 , respectively .