An entry to mixed phosphine-osmium-NHC polyhydride, complexes is described, starting from the five-coordinate hydrido-alkylidyne compounds [OsHCl(equivalent to CPh)-(IPr)(PR3)]OTf (IPr = 1,3-bis(2,6-disopropylphenyl)-imidazolylidene; OTf = CF3SO3; PR3 = (PPr3)-Pr-i (1), PPh3 (2)). The experimental procedure involves the borylation of the Os-C triple bond of 1 and 2 with NaBH4 and the subsequent alcoholysis of the borylation products OsH2Cl(eta(2)-H-BCH2Ph)(IPr)(PR3) (PR3 = (PPr3)-Pr-i (3), PPh3 (4)) or OsH2(eta(2):eta(2):H2BCH2Ph)(IPr)-((PPr3)-Pr-i) (5). Stirring of 3 in 2-propanol affords the five coordinate chloride trihydride OsH3(kappa(2)-H2BH2)(IPr)((PPr3)-Pr-i)(2) (6), which reacts with NaBH4 to give OsH3(kappa(2)-H2BH2)(IPr)((PPr3)-Pr-i) (7). This trihydride-tetrahydrideborate derivative and its PPh3 counterpart OsH3(kappa(2)-H2BH2)(IPr)(PPh3) (8) can be also obtained in a one-pot procedure, starting from 1 and 2 and using methanol at -60 degrees C instead of 2-propanol as alcoholysis agent. The bonding situation in 7 and 8, analyzed by DFT calculations using AIM and NBO methods, resembles that found in B2H6 and contrasts with the bonding situation in the bis-sigma-borane derivative 5. Stirring of 7 and 8 in 2-propanol leads to the corresponding d(2)-hexahydride derivatives OsH6(IPr)(PR3) (PR3 = (PPr3)-Pr-i (9), PPh3 (10)), which reduce the C equivalent to N triple bond of benzonitrile and promote the subsequent chelate-assisted ortho-CH bond activation of the resulting phenylmethanimine, to forma the trihydride compounds OsH3{kappa(2)-N,C-(NH=CH-C6H4)}(IPr)(PR3)(2) (PR3 = (PPr3)-Pr-i (11), PPh3 (12)), containing a stabilized orthometalated aldimine.