The structure of W(CO)₅L (L = pyridine, piperidine) in the lowest ligand field excited state determined by fast time-resolved IR spectroscopy; unexpected C–O bond length changes

Abstract

The frequencies of the v(C0) bands of W(CO)5L (L = pyridine and piperidine) in the lowest ligand field (LF) excited states have been obtained by fast time-resolved IR spectroscopy of the species dissolved in low-temperature glasses, and the shifts from the ground state indicate that the C-0 bonds lengthen on excitation, contrary to the interpretation of the pre-resonance Raman spectroscopy on the ground state.There has been extensive study of the excited state and photochemical properties of the complexes of general formula M(CO)SL, where M = Cr, Mo or W, and L is a nitrogen donor ligand.l-6 It is generally agreed that for W(CO)SL, if L is pyridine 1 or piperidine 2, then the lowest excited state is ligand field (LF) in character ('Al --+ 3E; weak absorption ca. 442 nm for both 1 and 2), too short-lived to be measured at room temperature but with a lifetime, based on emission studies, quoted as: 2.5 1 and 3.1 2 ps in MCH (methyl cyclohexane) at 77 K;1 0.86 1 and 0.82 2 us in EPA at 77 K;2 3.4 1 ps in argon 9 This is because in the LUMO there is considerable interaction between the pz metal orbital and the four equatorial n* orbitals on the CO groups.

Theoretical Investigation on the Absorption and Emission Properties of the Three Isomers of Bis(thiocyanato)(2,2′‐bipyridyl)platinum(II)

“…This type of π* SCN back-bonding has been successfully used by a number of researchers as a spectroscopic tag for TRIR (time-resolved infrared) experiments involving electronic excitation of transition metal complexes. [19][20][21][22][23][24][25][26][27] Similarly, we can analyze the frequency changes in the ground and excited states to investigate the electron transition during the excitation. The frequencies were calculated based on the optimized geometries in the ground and the lowest-lying excited states, respectively.…”

Using picosecond and nanosecond time-resolved infrared spectroscopy for the investigation of excited states and reaction intermediates of inorganic systemsBased on the presentation given at Dalton Discussion No. 6, 9?11th September 2003, University of York, UK.

“…As the halogen was changed from Cl I, the positive ν(CO) shift upon formation of the excited state decreased in the order Cl > Br > I, thus indicating the increase of XLCT character in the lowest excited state. 23 A rare example of a ligand field (LF) excited state is presented in the TRIR study of [W(CO) 5 (py)] in a glass matrix at 77 K. 28 Brozik and co-workers have recently exploited the use of 77 K glasses to obtain the IR spectrum of the LMCT excited state of [Ti(η 5 -C 5 H 5 ) 2 (NCS) 2 ]. 29 Peripheral IR reporter groups such as carboxylic acids and esters on substituted diimine ligands have also been utilised to indirectly probe charge transfer processes using both ns-and ps-TRIR.…”

Ultrafast transient IR spectroscopy and DFT calculations of ruthenium(ii) polypyridyl complexes

“…Time-resolved infrared spectroscopy (TRIR) has been used as a tool to characterise electronically excited states for some time, for instance in ruthenium( ii ), 19 , 20 osmium( ii ) 21 and rhenium( i ) 21 , 22 complexes, and it has also helped to tentatively assign a transient dd state in a tungsten(0) 23 complex. But only recently have ultrafast TRIR methods become available, which in addition allow the evolution of the nuclear structure toward metastable states 21 , 24 and in photochemical transformations 25 upon absorption of a photon to be followed in real time.…”