 Free Re2(CO)10 (point symmetry D4d ) has a CO stretch representation of 2A1+E2 + E3+ 2B2 +E1, where 2B2 + E1 are IR active.For an axially perturbed (C4v) Re2(CO)10 molecule, the CO stretch representation was found to be 2E+B1+B2+3A1, where the IR active modes are 2E+3A1.Its identity can also be confirmed by mass spectrometry, using the isotopic pattern of rhenium (185Re and 187Re).