Exchanging the methyl groups on the -diketiminate backbone to t-butyl was anticipated to change the rate of reaction with CO and possibly allow identification other reaction intermediates. Reaction of [Mg(-H){CH{C( t Bu)NDipp} 2 }] 2 (29) with CO at -90 °C yielded ethenediolate (32) as the sole product. Hill and co-workers also extended this work to the heavier group 2 metals. Reaction of the -diketiminate calcium hydride (37) was used for the calculation. The yet hypothetical diisopropylphenyl -diketiminate stabilised strontium (38) and barium (39) hydride dimers were also calculated for comparison. In all cases, coordination of CO to the hydride dimer and subsequent migratory insertion of CO into the M-H bond provided transient η 2 -C,O-formyl complexes (40-42). 40-42