Side-on coordination

H2 could be quantitatively removed at room temperature either by partial evacuation or by sparging the solution with argon. Definitive confirmation that the complexes did indeed contain 7J--H2 came from X-ray and neutron diffraction studies on the bisftri t-propylphosphine) analogue at —100°, which revealed the side-on coordination of H2 as shown in Fig. 3.2. During the past decade many other such compounds have been prepared and studied in great detail, and the field has been well reviewed. ... 

Particularly interesting examples of mechanistic considerations can be found in the work of Graham [124-126], Schubert [127,128] and Crabtree [129]. Cophotolysis of hexamethylbenzenechrom- (or tungsten) tricarbonyl or Cp Mn(CO)3 with diphenylsilane yields, after cleavage of CO, a side-on coordinated silane with a M(H)Si 3c2e bond 30 [130, 131],... 

The complexes are formed with 68% (R = t-Bu) and 73% [R = Me2Si(t-Bu)N] yield in warm (85°C) toluene in the course of 90 min. Surprisingly, the B—N distance is extended only a little ( 0.03 A), compared with the free iminoborane. In contrast to this side-on coordinated iminoborane, the zirconium complex (rj -Cp)2ZrCl(H)... 

In neat low-temperature C02 matrices, C02 is side-on coordinated to Ni atoms and the binding energy is weak (18kcal mol"1).2438,2439 The calculated OCO angle is 145°, which is quite a large value compared to those of other C02 complexes. In dinitrogen matrices, the binding of C02 is enhanced due to formation of [Ni(N2)ra(C02)] (n 1, 2).2437... 

Cations Au+ have been embedded between parallel -C=C-Au-C=C- units connecting two calixarenes (Scheme 82). The symmetrical side-on coordination is slightly distorted to allow for aurophilic bonding.51... 

Figure 21 Side-on coordination modes for dichalcogenide ligands E22 (a) k2E3,E2 (b) g-1 k2E],E2 2k2E1, E2 (c) ii-l E1, E2 2kE3 (d) h3-1k2E , E2 2kE] 3kE2...

Infrared absorption of an unstable hydroperoxo species had been observed at 230 K by Tozzola et al. (63). A peak at 886 cm-1, strongly overlapping the peak at 877 cm-1 attributed to physisorbed H202, was attributed to TiOOH (771 end-on coordination), although a band at 837 cm-1 was assigned to anionic triangular Ti(02) (side-on coordination). 

In particular the last observation is easily understood by the molecular orbital picture given above. The 7z>orbitals of the monomers are required for a stabilization of the otherwise anti-bonding cluster orbitals of t2 symmetry which must accept six electrons. If this mixing is prevented because these orbitals adopt the electron density of the ligands, e.g., 7z>electrons of the side-on coordinated cyclopentadienyl groups, their contribution to the cluster stability is minimized or in particular cases the formation of clusters does not occur at all. Thus, the substituents attached terminally to the clusters strongly influence their stability by the different donor or acceptor capabilities. A further effect may result from the different steric demand of the substituents which will be discussed below. 

A) Side-on coordination of the nitrogen molecule bridging two iron atoms, as if it were substituting for the sulfur atom bridging the two subunits. 

The bonding situation in the stannanorbornyl cation 62 is reminiscent of the intriguing zwitterions of the types 63 and 64 in which triorganotin cations are intramolecularly stabilized by side-on coordination with an alkynyl group. The zwitterions 63 with a [3-l-1] coordination for the tin atom are characterized by a significantly deshielded Sn NMR resonance and sizeable j(SnC) couplings to the alkynyl carbon atoms (for = alkyl and R = Et 5" Sn = 160-215 J(SnC ) = 41 8Hz, J(SnC ) = 60-80 Coordination with the amino... 

The stannyl cation in zwitterion 80 is only stabilized by side-on coordination with the remote C = C triple bond. The crystal structure of 80 revealed a pyramidalized tin center with the sum of valence angles of 351.1° (see Table XIV). The coordination of the tin atom to the C = C triple bond is unsymmetrical and the SnC distances... 

The bicyclooctadienyl ligand in [(TMEDA)Li-fcicycZo-C8H9] ° (143) is coordinated to the (TMEDA)Li unit via an -coordination of the allyl residue and an rf- side-on coordination of the double bond (Figure 26). The first contact is 215 pm, while the distance of 232 pm to the center of the double bond is considerably longer (Table 16). 

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