It depends on the MSSQL version you are trying to connect to.
http://microsoft-sql-server-native-client.software.informer.com/versions/
For 2005 use 9.0, for 2008 - 10.5 and for 2012 - 11.0
It depends on the MSSQL version you are trying to connect to.
http://microsoft-sql-server-native-client.software.informer.com/versions/
For 2005 use 9.0, for 2008 - 10.5 and for 2012 - 11.0