Infrared and micro-Raman spectra of crystalline Cr(CO)(5)(CNBu-t) and cis-Cr(CO)(4)(CNBu-t)(2) have been recorded in the 3000-390 cm(-1) region at ambient temperature and selected pressures up to 45 kbar with the aid of diamond-anvil cells. The vibrational data indicate that both the mono- and the disubstituted complex undergo a phase transition (possibly second order) at similar to 11 and similar to 6 kbar, respectively. The nu(CN) modes are significantly more pressure sensitive in the low-pressure phases than are the nu(CO) modes. The pressure dependences (d nu/dP) in the high-pressure phases are similar. The Cr-CO and Cr-CNBu-t pi-back-bonding interactions are apparently increased in the low-pressure phases by the application of pressure. This effect is greatest for the CO ligands, affording additional evidence for the weaker pi-acceptor capacity of the CNBu-t ligand. This interpretation is supported by the fact that the nu(CO) modes involving vibrations of CO groups trans to CNBu-t have significantly higher pressure dependences than do the other nu(CO) vibrations. High-pressure vibrational spectroscopy is another useful way of probing the relative pi-back-bonding capabilities of different pi-acid ligands in organometallic complexes.