$(P)\, Fe(CO)_5\,\,\, (Q)\,CO\,\,\, (R)\, H_3B \leftarrow CO\,\,\,(S)\, [Mn(CO)_5]^-$
Correct sequence of $CO$ bond order:
$\underset{(CO\,bond\,order\,>\,3.0)}{\mathop{{{H}_{3}}B\leftarrow CO}}\,>\underset{B.O.=3.0}{\mathop{CO}}\,>\underset{CO\,bond\,order\,<3.0}{\mathop{Fe{{(CO)}_{5}}}}\,>\underset{(CO\,bond\,order\,<3.0)}{\mathop{{{[Mn{{(CO)}_{5}}]}^{-}}}}\,$
($en=$ એથીલીનડાયએમાઈન )
$(i)\, [Cr(NO_3)_3 (NH_3)_3]$ $(ii)\, K_3[Co(C_2O_4)_3]$
$(iii)\, K_3[CoCl_2(C_2O_4)_2]$ $(iv)\, [CoBrCl(en)_2]$
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