Group 2 elements complexity constants

The force constant analysis of 1 indicates that the H2 ligand here is further activated toward OA than may have been previously thought. It has been a paradox that the dHH in 1 or any of the group 6 complexes in Table 8.1 (0.85-0.89 A, solid state NMR) are not as stretched as some of those found in later transition element complexes (1.0-1.5 A), yet the H-H bond in 1 undergoes equilibrium cleavage in solution. Thus the observed dm may not always reflect the degree of readiness to break, i.e., a very late transition state may exist. At the other end of the spectrum, for W(CO)5(H2) and other complexes with extremely weakly bound H2, the T-shaped entity pictured above with one internal coordinate, the H-H stretch, may be a more appropriate model for vibrational analysis. However, it is difficult to determine which analysis should be applied because some minimal BD and incipient... 

Interesting prospects are to be expected in complex chemistry with main group elements as their number is increasing constantly. 

The activation of Si-H bonds by adjacent transition metals, or main group elements " is important for the transformation of silanes. In many cases, the Si nuclear shielding is markedly reduced (extremely so in the fluxional complex 32, Scheme 9), accompanied by smaller values of coupling constants 7( Si, H) (see also the silylene complex 14 ). However, the 8 Si values are not really a criterion for M-H-Si bridging, as can be seen from the data for 35 and 36 (Scheme 9). 

While the mechanism for the formation of the methoxy complex (14) is not established, it is significant that the dihydride Zr(C5Me5)2H2 is needed for the reduction of the CO coordinated in (13). A reasonable proposal for this reaction can be formulated if it is assumed that since complex (13) is formally d°, Zr—CO backbonding will not be of major importance, and that hydride complexes of the group 4 elements possess substantial hydridic character. The first assumption may lead to a more favorable equilibrium constant for carbonyl hydride formyl interconversion as in (5), while the second suggests H" attack in this sequence presumably on a coordinated formyl. If the latter results in Zr—H addition across C=0, then reductive elimination of a C—H bond leads to the observed product. This is shown in (21). 

---