Molecular orbital analysis of the trend in B-11 NMR chemical shifts for (Cp*M)(2)B5H9 (M = Cr, Mo, W; Cp* = eta(5)-C5Me5)

The B-11 NMR shifts of the two types of boron atoms directly bonded to the metal atoms in (CP*M)(2)B5H9 (M = Cr, Mo, W) experience a large systematic shift to higher field in going from Cr to Mo to W, whereas the shifts for t he boron atoms connected to the metal atoms via M-H-B bridge bonds are inva riant. The origin of this behavior is traced to two high-lying filled Mo's and two low-lying unfilled MO's, both of which have metal and boron charact er. The energy differences of these sets of MO's correlate well with the ob served chemical shifts, and the properties of these MO's provide an explana tion of the observations and a comment on the nature of the boron-metal clu ster bonding.