Variable-temperature X-ray crystallographic studies: a complementary tool for charge-density investigation of soft (organometallic) bonds.

Comparative X-ray diffraction studies at different temperatures are used to complement the information usually extracted from 'single-point' charge-density studies. The response of soft bonds against (temperature driven) changes of the crystalline environment might be indicative of some intramolecular mode associated with an incipient bonding. This is in fact confirmed for the anion [FeCo(CO)(8)](-) in the [(Ph(3)P)(2)N](+) salt. In contrast, the [Cr(2)H(CO)(10)](-) anion, whose conformation is known to strongly depend on the counterion, is actually insensitive to T variations in the [K(C(18)H(36)N(2)O(6))](+) salt.