question updating app pool on IIS web app

When I use set-item to change the app pool for an existing web app, I get path not found error.

However, when I use Set-ItemProperty to set the app pool it works just fine.  What am I doing wrong when setting app pool using Set-Item method?

    <# No workie!
    $virtualApp = Get-Item "IIS:\Sites\$iisSiteName\$iisAppRootFolderName\$iisAppName"
    $virtualApp.applicationPool = $iisAppPoolName
    $virtualApp | Set-Item 
    Set-Item : Item path \\ACME\api2\v6.7.1 cannot be found.
    #>    

#Workie 

Set-ItemProperty -path "IIS:\Sites\$iisSiteName\$iisAppRootFolderName\$iisAppName" -name applicationPool -value $iisAppPoolName -Force