Electron DIMET

Treatment of TpMo( = CR)(CO)2 (R = C6H4Me-4) initially gives a mixture of TpMoFe( U-CR)(CO) (k = 5, 6), but the saturated molybdenum complex readily loses CO under a nitrogen atmosphere." Parallel reaction between bis(pyrazolyl) borate alkylidyne complexes and Fe2(CO)g similarly provides the 32-valence electron dimetal species BpWFe( -CR)(CO)6." These heteronuclear bimetallic complexes have a rich further chemistry, as summarised in Scheme 78. 

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

Many distibines and dibismuthines have lighter colors in solutions or melts than in the solid state. Crystals of these thermochromic distibines or dibismuthines consist of linear chains of the dimetal compounds with short intermolecular metal-metal contacts. Delocalization of electrons along the chains is possibly responsible for the bathochromic shift between fluid and solid phases. Usually, the /raor-conformation is adopted by the tetraorganodimetal compounds in the solid state. (CF3)4As2 shows the /ra r-conformation also in the gas phase. Photoelectrospectroscopic measurements on Me4Sb2 revealed the presence of gauche- (12%) and trans- (88%) conformed in the gas phase.52... 

DIMET Dynamics Induced by Multiple Electronic Transitions... 

Initial femtochemistry theoretical models were simply generalizations of those for the single-photon DIET processes, i.e., as dynamics induced by multiple electronic transitions (DIMET) [100]. The idea is simply that even if the excited state residence is too short to cause excitation to a ground state continuum after resonant scattering (tR < tc), it can still cause some vibrational excitation in the ground state. If resonant... 

Using fs laser excitation at 620 nm, a 2PC in Y of 0.5ps [399] implicates hot electrons, probably thermalized at Te, as the mechanism for desorption induced by the fs laser (Section 2.6.2). Rotational state distributions are nearly Boltzmann characterized by Tf. The 2PC of internal state distributions was also obtained. Rather surprisingly, significant differences in these 2PC were obtained for T and the state-resolved yield for the two spin-orbit states and this was qualitatively rationalized by a DIMET picture [399]. Where overlap in experiments exist, the qualitative results are similar to those for fs laser induced desorption of NO/Pd(lll) [400,401]. For this latter system, the absolute yield Y is large at typical fluencies used in the experiments and a very hot vibrational distribution was observed (Tv = 2900 K). 

The theoretical model developed to explain these experiments is based on inelastic tunneling of electrons from the tip into the 2ir adsorbate resonance that induces vibrational excitation in a manner similar to that of the DIMET model (Figure 3.44(b)). Of course, in this case, the chemistry is induced by specific and variable energy hot electrons rather than a thermal distribution at Te. Another significant difference is that STM induced currents are low so that vibrational excitation rates are smaller than vibrational de-excitation rates via e-h pair damping. Therefore, coherent vibrational ladder climbing dominates over incoherent ladder climbing,... 

The structure of 11a has been established by X-ray diffraction. The compounds 11 are 30-cluster valence-electron (CVE) dimetal species, and are therefore electronically unsaturated. The electron distribution within the W(fi-C)fth rings may be represented by various canonical forms. However, that shown, which implies that the W=C bond in the ( 5-C2B9H9R 2)(OC)2-W=CC6H4Me-4 moiety formally contributes three electrons to the rhodium center (16-electron valence shell), accords with other results from our laboratory involving electronically unsaturated dimetal compounds with bridging alkylidyne ligands 18). 

Treatment of [Mo(NCMe)(CO)2(ij7-C7H7)][BF4] with 2c in THF affords the dimetal compound [MoW(/i-CC6H4Me-4)(CO)2(f/7-C7H7)(f/5-C2B9H9-Me2)] (17). The latter species, like complex 15a, contains a B—H —Mo three-center two-electron bond (22). 

Thus, in this review we present the desorption phenomena focused on the rotational and translational motions of desorbed molecules. That is, we describe the DIET process stimulated by ultraviolet (UV) and visible nanosecond pulsed lasers for adsorbed diatomic molecules of NO and CO from surfaces. Non-thermal laser-induced desorption of NO and CO from metal surfaces occurs via two schemes of DIET and DIMET (desorption induced by multiple electronic transitions). DIET is induced by nanosecond-pulsed lasers and has been observed in the following systems NO from Pt(0 0 1) [4, 5],... 

Pt(l 11) [6-8], Cu(l 1 1) [9] and Ag(l 1 1) [9], and CO fromPt(00 1) [10] andPt(l 1 1) [11,12]. On the other hand, these molecules are not desorbed from Ni and Pd metal surfaces in spite of the isoelectronic character of the metals Ni, Pd and Pt [13,14]. Desorption induced by subpicosecond-pulsed laser takes place via multiple correlated (and partially coherent) electronic transitions DIMET. DIMET is a very different mechanism from DIET [15-17] and in DIMET the vibrational excitation during the multiple electronic transitions leads to the desorption. Desorption via multiple vibrational transitions has also been observed using an infrared laser [18]. However, these topics are not described in this review. 

A number of small unsaturated molecules or functional groups have been found to add across the M-M triple bond in Cp2Mo2(C0) to give adducts Cp2Mo2(C0) (un), where un = acetylenes ( [), allenes (42), cyanamides (43) and thioketones (44). In all of these adducts, the unsaturated fragments act as 4-electron donors to the dimetal center, thus reducing the M-M bond order from three to one. 

When acting as a ligand toward two or more metal atoms, alkynes have a large repertoire of roles. With respect to a dimetal unit, there are two major types of structures, both describable as rf, they are shown in Fig. 16-28(c) and (d). In the planar structure of Fig. 16-28(c), the alkyne, is a two-electron donor. In structure... 

The ligand-bridged complexes [Rh2(CO)4 (PPh3) (/x-RNXNR)2] (n = 1 or 2, R = aryl, X = N or CMe) undergo reversible one-electron oxidation to isolable monocations (275), which appear to be fully delocalized mixed valence complexes (276). Oxidation also leads to enhanced susceptibility to carbonyl substitution ( = 1, X = CMe), and a drastic shortening in the metal-metal bond distance implies electron loss from an anti-bonding dimetal orbital (275) (Section III,F). 

Most of the electron attacks eventually lead to anti addition of hydrogen to the triple bond. Therefore, it has often been assumed that a irons dianion is a necessary intermediate. Because both free dianions and dimetal adducts may be isolated or detected, either or both may be important in different systems. 

FIGURE 15-10 Bond Order and Electron Count in Dimetal Clusters. (From A. Bino and F. A. Cotton, Inorg. Chem., 1979, 18, 3562 andF. A. Cotton, Chem. Soc. Rev., 1983, 12, 35.)... 

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