Sequential bonding of CO molecules to a titanium dimer: A photoelectron velocity-map imaging spectroscopic and theoretical study of Ti2(CO)n- (n = 1-9).

Abstract

Binuclear titanium carbonyl cluster anions, Ti2(CO)n- (n = 4-6), are produced via a laser vaporization supersonic cluster source and characterized by mass-selected photoelectron velocity-map imaging spectroscopy. Quantum chemical calculations are carried out for Ti2(CO)n- (n = 1-9) to explore the trend of sequential bonding of CO molecules to a titanium dimer. It has been found that the CO molecules bind to Ti2 in a side-on fashion and form a stable Ti2[η2(μ2-C, O)]3 structure at n = 3, the motif of which retains up to n = 5. Starting at n = 6, a new building block of two CO groups side-on-bonded to Ti2 is favored, the structure of which persists up to n = 9. In the larger clusters (n = 6-9), the side-on-bonded CO molecule can be stabilized via the removal of two electrons from an anionic titanium carbonyl, which is different from the effect of charge on CO binding in rhodium carbonyls where bridge-bonded CO molecules are selectively destabilized by the removal of an electron from a neutral rhodium carbonyl. The present study provides a stepwise picture for molecular-level understanding of CO bonding on transition-metal clusters, which is directly relevant to the elementary processes of CO at metal catalysts.