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Using the 18-electron rule, identify the first-row transition metal M in each complex (treat all eta-x ligands as aromatic) and find the sum of their atomic numbers. (a) [M(CO)7]+ (b) [M(CO)2(CS)(PPh3)(Br)] (c) [(eta3-C3H3)(eta5-C5H5)M(CO)]- (d) [(eta4-C4H4)(eta5-C5H5)M]-

1. 96
2. 100
3. 102
4. 104

Correct answer: 102

Solution

(a) 7 CO = 14e, complex cation (+1) means metal contributes 18-14+1=... use: metal e + 14 - 1 (charge) = 18 => metal e = 5 => Mn (Z=25). (b) 2CO+CS+PPh3+Br = 10e, neutral: metal needs 8 => Fe (Z=26). (c) eta3(3e)+eta5(5e)+CO(2e)+1e(charge) = 11e, metal needs 7 => Mn (Z=25). Wait with ionic: eta3-allyl(4e)+Cp(6e)+CO(2e)=12, metal+charge: M⁰ has 6 => Cr? Let me use neutral count: eta3=3, eta5=5, CO=2, extra 1 (charge=-1): total ligand = 11, metal = 7 => Mn (Z=25). (d) eta4(4e)+eta5(5e)+1e(charge)=10, metal needs 8 => Fe (Z=26). Sum = 25+26+25+26 = 102.

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