$(P)\, Fe(CO)_5\,\,\, (Q)\,CO\,\,\, (R)\, H_3B \leftarrow CO\,\,\,(S)\, [Mn(CO)_5]^-$
Correct sequence of $CO$ bond order:
$\underset{(CO\,bond\,order\,>\,3.0)}{\mathop{{{H}_{3}}B\leftarrow CO}}\,>\underset{B.O.=3.0}{\mathop{CO}}\,>\underset{CO\,bond\,order\,<3.0}{\mathop{Fe{{(CO)}_{5}}}}\,>\underset{(CO\,bond\,order\,<3.0)}{\mathop{{{[Mn{{(CO)}_{5}}]}^{-}}}}\,$
$(i)\, [CoCl_3 (NH_3)_3]$ $(ii)\, [Co(en)_3]Cl_3$
$(iii)\, [Co(C_2O_4)_2(NH_3)_2]^-$ $(iv)\, [CrCl_2(NH_3)_2(en)]^+$
(પ. ક્ર.: $Sc = 21 , Ti = 22, V = 23, Zn = 30$)