End-on coordination

The possibility of i) -H2 end-on coordination has also been mooted. For example, deposition of Pd atoms onto a krypton matrix doped with H2 at 12K apparently yields both Pd(t) -H2) and Pd( ) -H2) species, whereas with a Xe/H2 matrix only Pd(t) -H2) was obtained.Again, the complex [ReCl(H2)(PMePh2)4] appears to feature an asymmetrically-bonded H2 ligand which may well be ( ) -H2). ... 

It is not known whether there is any carbenoid chemistry of end-on coordinated manganese-diazoalkane complexes 420 410). However, it is known that diaryldiazo-methanes 408,411 and azibenzil411 yield carbene rather than diazoalkane complexes with (n5-C5H5)Mn(CO),THF. 

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). 

Examination of the azide bending-mode region (600-700 cm 1) in the SER spectra (Figure 2) is also instructive with regard to adsorbate orientation. Thus, the pair of bands (at ca. 610 and 670 cm"1) seen at the least negative potentials are characteristic of end-on coordinated azide (15) the loss of the lower-frequency partner for E < -0.15 V is therefore also indicative of the removal of azide bound in this adsorbate geometry, again in harmony with the interpretation of the infrared spectra (7). 

B) End-on coordination of the nitrogen molecule between two iron atoms of the Fe4S4 subunits. 

The preparation of a novel mononuclear iron(III)-t-butylperoxo-complex, end-on coordinated, with bis(6-pivalamido-2-pyridylmethyl)(2-pyridylmethyl)amine, bppa = (206), to Fe, " was followed by the isolation of a similar complex containing hydroperoxide. The latter reacts by O—O bond cleavage and a [(bppaH2)Fe —O ] intermediate." ... 

While most superoxo complexes—in contrast to peroxo compounds— have been assigned a bent, end-on coordination mode [9], the superoxide ligand of Tp Cr(02)Ph was suggested to exhibit the more unusual side-on (r] ) coordination [10]. The reactivity of the complex did not allow for the determination of its molecular structure however, close analogs could be isolated, crystallized and structurally characterized by X-ray diffraction. For example, the reaction of [Tp Cr(pz H)]BARF (pz H = 3-tert-butyl-5-methylpyrazole, BARF = tetrakis(3,5-bis(trifiuoromethyl)phenyl)borate) with O2 produced the stable dioxygen complex [Tp Cr(pz H)( ] -02)]BARF (Scheme 3, bottom), which featured a side-on bound superoxide ligand (do-o = 1.327(5) A, vo-o = 1072 cm ) [11]. Other structurally characterized... 

H NMR and IR spectra are employed to distinguish between the two coordination modes of RPhCHCN molecules. In the end-on coordination of RPhCHCN v(CN) is at lower frequencies than in the edge-on coordination and the a protons are markedly deshielded when the molecule is edge-on coordinated. 

In the case where the XY bond is a multiple one, binding and activation of the substrate leads to a reduction in the XY bond order (19). This is because the substrate orbitals involved in accepting electron density from the metal center are of the n type. For donation of electron density, the substrate employs either a nonbonding lone-pair orbital that gives end-on coordination (1) or a 7tb-type orbital that yields side-on or t 2 attachment. Since the oxides CO, C02, and NO all have multiple bonds, the notions outlined above will be important in considering their activation. 

Many diatomic ligands favor end-on coordination to a single metal atom. This mode of coordination is also known for several 02 complexes, although doubly bridging end-on coordination (structural types Ila and lib) is more common. For the ligand only type Ha and type lib structures are known. 

The variety of coordination modes for N2 defined in the simple metal complexes has given rise to various proposals of how the nitrogenases bind and activate N2. There are three models extant mononuclear end-on, mononuclear side-on, and bridging N2 species. In order to maximize the useful information from model systems it is imperative that the studies have a firm structural basis. In this context, the most well-defined model is that based on the mononuclear end-on coordination, and it is on this system that we will concentrate first. 

In view of the many approximations in these semi-empirical calculations their results can only be regarded as first hints for more elaborate methods. Such are DFT calculations on FeMoco-type clusters and model complexes. As it is discussed, for instance, in Ref. (31), N2 usually binds very weakly to transition metal complexes. It is thus not certain whether end-on coordination to one or two iron centers can lead to sufficiently strong coordination of dinitrogen. 

Orbital interactions of N2 and transition-metal M in end-on coordinated dinitrogen complexes. The dtt-ljt g overlap shown here also occurs in the yz plane. 

The lithium ions in these oligomers are differently side-on and end-on coordinated to the N—N unit. The degree of oligomerization decreases with increasing bulk of the silyl substituents. The chemistry of silylhydrazines and their lithium derivatives has been reviewed53. 

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