The metal carbonyl displays two kinds of bindings in the form of the terminal and the bridging modes. The infrared spectroscopy can easily distinguish between these two binding modes of the metal carbonyl moiety as the terminal ones show ν(CO) stretching band at ca. 2100-2000 cm⁻¹ while the bridging ones appear in the range 1720−1850 cm⁻¹. The carbonyl moiety can bridge between more than two metal centers (Figure 2).