Manganese carbonyl hydride preparation

The chemistry of the metal carbonyl hydrides and metal carbonylates remained the principal research topic for Hieber until the 1960s. He mentioned in his account [25], that it was a particular pleasure for him that in his laboratory the first hydrido carbonyl complexes of the manganese group, HMn(CO)5 and HRe(CO)5, were prepared by careful addition of concentrated phosphoric acid to solid samples of the sodium salts of the [M(CO)5] anions, giving the highly volatile hydrido derivatives in nearly quantitative yield [45, 46]. In contrast to HCo(CO)4 and its rhodium and iridium analogues, the pentacarbonyl hydrido compounds of manganese and rhenium are thermally remarkably stable, and in... 

Recently, polynuclear tetracarbonyls of the two heavier metals in the manganese sub-group have been discovered. The first was for technetium (2) as a major by-product during attempts to prepare the pentacarbonyl hydride, and more recently, for rhenium (3) by an analogous route. The dimetal deca-carbonyl is treated first with sodium amalgam in tetrahydrofuran solution. After a time, the solvent is evaporated and cyclohexane and phosphoric acid are added and the heterogeneous mixture is refluxed desired product is found in the cyclohexane layer. It was later discovered that the use of sodium boro-hydride in the first step instead of sodium amalgam led to the same products. 

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